Publications of the Astronomical Society of the Pacific 99:921-951, August 1987

THE REMARKABLE EXTRAGALACTIC RESEARCH OF ERIK HOLMBERG

A GLIMPSE FROM SANTA CRUZ

HERBERT J. ROOD School of Natural Sciences, Institute for Advanced Study, Princeton, New Jersey 08540 Received 1987 February 2, revised 1987 May 15

ABSTRACT The major extragalactic research contributions of Erik Holmberg are related to the present astronomical situation. Special note is made of the following contributions; (1) Holmberg (1946) presented the first strikingly clear demonstration that the size, shape, and integrated luminosity of a galaxy derived from measurements performed on its photographic image are much more accurately determined with a photoelectric microphotometer than by visual methods alone. This comparison was made from measurements of simulated photographic images of idealized galaxies with preset structural parameters. (2) With a microphotometer, Holmberg (1958) determined accurate magnitudes, colors, sizes, and shapes for 300 representative nearby galaxies, and he made the original derivations of precise analytical procedures which correct these observed parameters for effects of internal galactic extinction and absorption by dust in our Milky Way Galaxy. This data base even today largely constitutes our definitive knowledge of the global structural parameters of nearby galaxies. (3) Holmberg's extensive census of physical satellites of nearby spiral galaxies led to three major discoveries: (a) He found that the luminosity function (which he accurately derived over a baseline of ten magnitudes) depends on the morphological type of a galaxy, (b) He found that the surface density of satellites in polar sectors of a spiral galaxy is larger than in equatorial sectors at corresponding radial intervals. This result is present at the 4-sigma level of statistical significance, (c) Finally, he demonstrated that the absolute magnitude of a galaxy is correlated with its color and surface brightness so that these parameters can be combined with apparent magnitude to provide a distance determination to a galaxy; hence, Holmberg pioneered the current field which attempts to derive the distance to a galaxy by means of one or more of its global properties which correlate with absolute magnitude. (4) Holmberg (1941) constructed a photoelectric experimental apparatus which simulated the r~2 radial dependence of the gravitational force by means of the r~2 dependence of light intensity. His experiments with this apparatus were the first to clearly demonstrate effects of tidal interactions that occur during the encounter of two galaxies. It seems that these fascinating experiments have not been improved upon or even repeated to the present day. (5) Holmberg provided crucial initiative and general guidance which led to the creation of the very useful 1973 Uppsala General Catalogue of Galaxies and the 1982 ESO/Uppsala Survev of the ESO(B) Atlas. A complete bibliography of Holmberg's scientific publications is contained herein. This bibliography has been used to select a list of galaxy publications (Table I, pp. 948-49) which could provide a basis for a university course. The intent of this course would be to provide the student with a historical introduction to fundamental research technique in extragalactic astronomy. To the student who wishes to develop outstanding research capability, I suggest that he or she has much to learn from Holmberg's writings. This essay, although a result of the present writers initiative and efforts, has benefited in regard to some details from insights gained through correspondence with Professor Holmberg. Key words: Erik Holmberg-galaxies-research methods-history of astronomy

Preface It is concurrently an educational device for motivating It has recently become apparent to the author that the students, a contribution to the history of science, and a present essay is an unusual form of scholarly contribution. highly technical astronomical magazine article.

921 © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 922 HERBERT J. ROOD

Ever since the late 1960s I have been fascinated by cluded in this essay either in the third person with direct the intuitive wisdom, solid scientific methodology, and reference to this correspondence source or through direct charming literary style of Erik Holmberg. I have recog- quotes from the letters themselves, always with Holm- nized how pervasively his insights are ingrained within berg's permission. It is this inclusion which sets the essay the fabric of modern extragalactic astronomy and how his apart from a typical contribution in astronomical history research is viewed as "remarkable" by every extragalactic but which also tends to make the history more interesting astronomer that I have known who is knowledgeable of and revealing in a human sense. It is this inclusion and the his work. As my own research capabilities continually particular writing style adopted for the essay which also developed, I have come to realize ever more fully that the causes it to read somewhat like a very technical magazine caliber of my own work could have been significantly article. However, the article is written by a practicing enhanced if in graduate school I had only been aware of extragalactic research astronomer about the works of an- and studied Holmberg s works. It is apparent to me that other extragalactic research astronomer. This has sub- current graduate students as well could benefit signifi- stantially influenced the emphases parceled among the cantly from study of Holmberg s works. The present essay various scientific topics, and for a magazine article, the is, therefore, in part an educational device to alert the technical accuracy is especially high. student to these works of an elder astronomer who really knows how to do accurate and creative research. I. Introduction The present essay is also a document in the history of A workshop on "Nearly Normal Galaxies: From the science. Unusual care has been taken to compile an accu- Planck Time to the Present" was held at the University of rate and complete bibliography of Holmberg's scientific California in Santa Cruz from July 21 to August 1, 1986. research papers, to extract and describe a good deal of Sandra Faber provided the overall scientific organization their essence, and to illustrate how specific works of for the workshop which featured stimulating morning and Holmberg continue to contribute directly to the stimula- afternoon sessions on current topics in extragalactic as- tion and forward thrust of modern extragalactic research. tronomy contributed as a tribute to the distinguished Errors in comprehension which I originally made dur- elder American astronomer from Santa Cruz, Albert ing the compilation of the bibliography and extraction of Whitford. Whitford listened attentively with obvious in- results from the research papers were subsequently cor- terest to the many new developments derived from tradi- rected by Erik Holmberg himself, who was extremely tional photographic and photoelectric techniques, the kind (because he has been living in retirement for nearly a newer CCD detectors, radio and X-ray observations, and decade) to consent to my request that he check for factual computer technology. The dinner honoring Dr. Whitford accuracy. Consequently, the essay has benefited from the featured numerous entertaining speeches that docu- integrity, objectivity, and attention to detailed accuracy mented Whitford s exemplary contributions to American which have marked Holmberg s research contributions astronomy which ranged from his pioneering contribu- throughout his career. It is because initial errors in com- tions in the photoelectric determination of accurate mag- prehension by the historian have been mitigated signifi- nitudes and colors of stars and galaxies from direct obser- cantly through careful checking by the subject himself vations at the telescope (he used a photomultiplier with that the present document is unusually authoritative its linear response to measure the actual energy per unit among historical writings in astronomy. time received from each celestial source) to his skill as The historical context of this essay may be summarized chairman of the national committee which produced the as follows. The intellectual torch of optical extragalactic Whitford report, a document which contributed signifi- astronomy was fully ignited in the late eighteenth and cantly toward setting the stage for the great surge in early nineteenth centuries by the extensive observational astronomical exploration that has occurred over the last work of William Herschel. This occurred even before the three decades. Any astronomer who had arrived at the nature of what we now call "galaxies" was understood. The workshop not fully aware of the singular value of Whit- torch was eventually passed in the early twentieth cen- ford's work left the workshop with a much deeper admira- tury most notably to Knut Lundmark and Edwin Hubble, tion of his professional skills and accomplishments. and the nature of galaxies became fully recognized. Study The current article is intended to provide a glimpse of of the works of Holmberg transport the reader in a natural the equally remarkable work in extragalactic astronomy of manner from these fundamental early discoveries into the a retired contemporary of Whitford, his colleague from research concerns of the modern era. Sweden, the distinguished elder astronomer Erik Holm- Prompted by my questions in our correspondence, berg (Fig. 1). The information content of the article is Holmberg conveyed to me some information about his distributed about equally among the main text, the figure research motivations and environment which are outside captions (which usually closely represent the correspond- the domain of material contained within his published ing figure captions published by Holmberg), and the scientific papers. Some of these insights have been in- figures themselves; the latter (with the exception of Fig.

1) have been adapted directly from Holmbergs published papers. In photometry, Holmberg s work complements Whit- ford's (cf. Fig. 2) because Holmberg pioneered in the determination of accurate magnitudes and colors of galaxies with a self-recording microphotometer. (At one end this instrument consists of a source producing a narrow beam of light which passes through a photographic plate as it moves at a constant rate perpendicular to the beam, thereby scanning across the image of a galaxy. At the other end the instrument consists of a photomultiplier which measures the light flux of the beam as it emerges from the plate and a roll of paper which unwinds at a constant rate thereby measuring both the light flux at each instant and its location on the photographic plate.) The ratio of the light flux that emerges from a given location on the plate to the light flux incident on the plate determines its plate density. As the beam scans across the image of a galaxy, the distribution of plate density along the scanning strip is determined. Plate density is then converted into light intensity through a conversion curve obtained previously by scanning across a calibration image for which the distribution of light intensity is known. By obtaining a sufficient number of scans across the galaxy to determine the intensity over its entire surface, one is then able to extract considerable information about the galaxy; for example, one can numerically integrate the intensity over the surface area of the photographic image to derive the apparent magnitude of the galaxy (Fig. 10). Fig. 1—A 1965 photograph (taken in Uppsala, Sweden) of Erik Holm- berg, who was born on November 13, 1908, in Tofteryd, Sweden, to The experience that both Whitford and Holmberg Malcolm Holmberg and Anna Nilson. In 1947 Erik Holmberg married derived from applying the most advanced technology of Martha Asdahl and they have a daughter. Osa Holmberg, born in 1953. their day allows them to appreciate with exceptional per- In 1937 Holmberg obtained his Ph.D. at Lund University; his thesis advisor was Knut Lundmark; his thesis is entitled "A Study of Double spective the sophisticated technology applied in current and Multiple Galaxies" (Holmberg 1937α). Also in 1937 Holmberg astronomical research. became Assistant Professor at Lund Observatory; in 1951 he became Because Holmberg s research was not only pioneering Associate Professor at Lund Observatory, and from 1959 through 1975 he served as Professor and Director of the Astronomical Observatory at but definitive as well, the creative achievements of his Uppsala University. works pervaded the Santa Cruz workshop; they mani- Holmberg obtained observational material for his doctoral thesis in fested themselves in the following areas of current re- 1935-36 at Heidelberg Observatory. He visited astronomical institutes in the USSR in 1936, 1941, 1958, and 1977, and also visited many other search: (1) the structure and evolution of galaxies, (2) the European institutions. He was a guest investigator by invitation at the structure and evolution of systems of galaxies, (3) the Mount Wilson and Palomar Observatories in 1939-40, 1941, 1947-48, cosmography and kinematics of superclusters, (4) the cos- 1951-52, 1954-55, 1961, 1963, and 1968. He was a guest professor, at the invitation of the American Astronomical Society, at Wesleyan Uni- mic distance scale, and (5) the problem of the missing versity in the spring term of 1963. He was a guest lecturer on different mass. occasions at most of the larger observatories in the USA. Holmberg was To carry out his work, Holmberg demanded high-qual- president of Commission 28, "Galaxies", of the International Astronomi- cal Union in 1973-76; he and astronomical associates prepared a report ity data, and he obtained much of it himself. He pio- of the research on galaxies over that interval (Holmberg 1976). He was neered in the application of statistical techniques to first organizer of, and host to, IAU Symposium No. 44, External Galaxies and thoroughly understand the uncertainties in his data and Quasi-Stellar Objects, held in Uppsala, Sweden, 10-14 August 1970 (Holmberg 1972). From 1961 through 1976 he was a member of different then to derive basic information about the structural committees and the Council of the European Southern Observatory. He properties and evolution of objects in the cosmos. He is a member of several scientific and scholarly organizations (starting demonstrated that correct results can consistently be ob- date of membership is given in parentheses): the Royal Physiographic Society, Lund (1949); the Royal Society of Sciences, Uppsala (1959); the tained when statistics is applied with competence and Royal Academy of Sciences, Stockholm (1959); the American Astronom- good astronomical sense. Holmberg always analyzed his ical Society (1963); and the Swedish Astronomical Society, Stockholm data within a framework of cogent reasoning applied (chairman 1964-72). Erik and Martha Holmberg now live in retirement in Partille, Sweden. (The photograph and information for the caption to lucidly conceived fundamental concepts, many of were kindly provided by Professor Holmberg.) which he originated himself. And, finally, the publications

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Fig. 2-The upper panel demonstrates the excellent agreement between the apparent magnitudes of a nearby sample of galaxies derived by the photoelectric techniques of Whitford and the photographic/microphotometric techniques of Holmberg. The small scatter of the magnitude differences in the upper panel relative to that of either of the two lower panels illustrates the significant improvement of the innovative techniques by Whitford and by Holmberg in relation to the previous state-of-the-art technique, visual estimation from densities of images of cosmic objects on photographic plates, which was applied, for example, by researchers at Harvard and by Holetschek, and was in general use during the first half of the 20th century. (Adapted from Fig. 6 of Holmberg 1950¾.) which describe his work and results are masterpieces of Holmberg was a pupil of the great Swedish astronomer clarity and precise logic. (Holmberg was much impressed Knut Lundmark, who in his doctoral thesis in 1920 and probably influenced by the contracted and precise demonstrated that if (1) the intrinsic luminosities of the 22 style found in Hubble's papers.) Any graduate student or novae discovered in M 31 (the Andromeda galaxy) be- professional astronomer would treat himself to an excel- tween the years 1917 and 1919 were similar to the intrin- lent course on research methodology by studying the sic luminosities of the novae which have been observed in selected galaxy publications of Erik Holmberg which are our own galaxy, and (2) the one singularly bright explosive listed in Table I. The references cited in Table I have star, S Andromedae, observed in M 31 in the year 1883, is been extracted from the complete bibliography of Holm- a member of a different species (now known as "super- berg's major astronomical work located at the end of this novae"), then M 31 is located at such a large distance essay. that it would be a system of stars, gas, and obscuring

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System EXTRAGALACTIC RESEARCH OF ERIK HOLMBERG 925 material with a size and structure quite comparable to our nitely the first to implement the premise that detailed and Galaxy (the Milky Way). Hence, Lundmark's study in accurate photometric information on a large sample of 1920 strongly anticipated the result achieved definitively representative galaxies is required to effectively reason in 1924 by Edwin Hubble from analyses of data obtained about the origin and evolutionary state of the stellar con- with the 60-inch and 100-inch telescopes at the Mount tent of galaxies. Visual estimates alone of structural Wilson Observatory. Also in 1920 Lundmark discussed parameters of galaxies from photographic images are in- the degree to which the observational evidence suggests a sufficient, suffering from both murky systematic and large correlation between the recessional velocity of a galaxy random errors. Holmberg demonstrated in an ingenious (derived from the observed Doppler redshift of spectral way that large systematic uncertainties are present in lines) and its distance; this study was done several years visual estimates of the apparent sizes and shapes of galax- before the redshift-distance relation was definitively de- ies. He first designed and constructed an instrument tected in the data by Hubble and Humason in the early (Figs. 3 and 4) which was able to produce photographic 1930s. (See Sandage (1961) and Hubble (1936ö) for inter- images of artificial galaxies! The artificial galaxies were esting discussions of these topics and for references to selected to have an elliptically symmetric light distribu- primary sources.) tion which falls off smoothly with distance from the galac- Finally, I mention two other of Lundmark's numerous tic center, r. The steepness of the falloff ranges between and diverse projects: gradual (r~0 5) to very steep (r-7 0). The artificial galaxies In a paper entitled "Studies of Anagalactic Nebulae" were also selected to contain a representative sample of (Lundmark 1927), where the wide scope of Lundmark's shapes specified by the ratio of minor to major diameter intellectual interests is reflected in an extensive historical (Fig. 5). Photographic images for 110 artificial galaxies introduction which mentions that William Herschel rec- covering a large range of intensity falloff and shape were ognized several distinctive concentrations of nebulae on thereby obtained (two exposures each for five different the sky and that Max Wolf discovered the Coma cluster shapes and ten different intensity falloffs, plus an extra (this sense of historical continuity of research from the pair of exposures for the r ~2 intensity distribution). The times of Tycho Brahe, to William Herschel, to the current major and minor diameters of the 110 photometric images workers continues to remain evident in the writings of Erik Holmberg), we learn that Lundmark examined the Coma cluster on a plate taken with the 100-inch telescope ι at the Mount Wilson Observatory. Included among his results are mention that (1) spiral structure is plainly visible in a few of the anagalactic nebulae (i.e., galaxies), (2) the diameters of the galaxies do not show any concen- tration toward the center in the sense that the largest objects crowd there, but (3) there is a definite crowding of luminous objects around the center of the cluster. Results (2) and (3) may be early detected observational effects camera caused by tidal stripping and dynamical friction in the gravitational interaction of galaxies in a cluster. During a visit at the Mount Wilson Observatory in 1929, Lundmark discovered a distant cluster of galaxies registered on a photographic plate that was on file at that observatory (Lundmark 1931). He then visually estimated structural parameters for a significant fraction of Fig. 3-A schematic drawing depicting the experimental arrangement the member galaxies. These included major diameter (a, used by Holmberg to produce photographic images of artificial galaxies. "size"), ratio of minor to major diameter (β/α, "diameter A quadrangular box is mounted on the axis of an electrical motor, which ratio," or, more loosely, "shape"), and apparent photo- generates the power to rotate the box. The inside of the box, which is graphic magnitude, m . Lundmark discovered among painted white, is illuminated by six electrical light bulbs. The front side pg of the box is covered by an exchangeable plate of milk glass. The plate is other interesting facts that the size and magnitude of a painted dark except for the region corresponding to a sector. The galaxy are strongly correlated. It was in the intellectual electrical motor with the box is mounted on a horizontal metal plate exuberance which flowed from Lundmark's research which rests on a table and can be turned horizontally around the point P, achievements that Erik Holmberg arrived on the scene at situated below the center of the sector. The rotating sector was pho- the Lund Observatory in the mid-1930s. tographed when the angle, φ, between the motor axis and the direction to the camera objective was set equal to a value corresponding to a Π. Galaxies preassigned ratio of minor to major diameter, b/a, of the photographic image of an artificial galaxy (note that cos φ = hi a). (Adapted from Fig. 1 Holmberg was probably the first to realize and defi- of Holmberg 1946.)

galaxies which could be obtained with a microphotometer. But, since very little good microphotometric data were available in 1946, he set out to obtain some. Through the generosity of Edwin Hubble and directors W. S. Adams and Ira S. Bowen of the Mount Wilson Observatory, it was possible for Holmberg to get access to the large telescopes on Mount Wilson whenever he wanted (from 1939 on). In fact, Bowen once said in public that Hohnberg was the ideal guest investigator who effi- ciently used the telescopes to collect a large amount of material for further analysis at the home institution. Holmberg was probably the first at Mount Wilson to work in this way on a large scale, and his pioneering success undoubtedly contributed to the situation we have today in astronomy where a large fraction of astronomical research is done in this manner. At Santa Cruz many results were reported that had been derived from observational data obtained through guest-investigator privileges at large national and private facilities which were then analyzed and interpreted at the home institution. At Mount Wilson, Holmberg had numerous discussions with Hub- ble from 1939 onward; Hubble was always very kind to Holmberg and seemed happy to analyze problems in his company. Holmberg felt that he was a pupil, perhaps the only pupil, of Hubble. (The historical information contained in this paragraph was derived from correspondence with Professor Holmberg.) Fig. 4-Reproductions of the ten light sectors on the ten exchangeable When Holmberg came to the Mount Wilson Observa- plates, respectively, constructed for use on the instrument sketched in Figure 3. The rotation axis of each sector is denoted by a white circular tory for the third time in 1947, there was felt a strong need spot. The sector in the upper-left corner of the figure gives a light for more reliable and homogeneous basic data for galaxies distribution proportional to r-0 5, whereas the sector in the lower-right such as magnitudes, colors, and angular diameters. -7 0 corner corresponds to r . (Adapted from Fig. 4 of Holmberg 1946.) Holmberg felt this need himself and was heavily supported by Hubble, Walter Baade, and others. The Mount were then measured both visually by five experienced Wilson Observatory had just got access to new Kodak observers at the Lund Observatory (Fig. 6) and by a plates which were five to ten times faster than the old ones self-recording microdensitometer (also called a "micro- (i.e., it took five to ten times less observing time to record photometer") (Fig. 7). The visual measurements were a given surface brightness at a given plate density). It shown to contain large random and systematic errors therefore seemed possible, if enough observing time (Figs. 6, 8, and 9) whereas the errors in the microphoto- could be obtained, to clean up the situation on a rather metric measurements were so small that Holmberg pro- large scale. The aim was put at about 300 nearby galaxies, ceeded to identify the microphotometric diameters with two photographic and two photovisual plates for each the true diameters a and b which were predetermined by object. Observing time was obtained for the project di- the laboratory setup. The plate densities in the outer vided into three periods, 1947-48, 1951-52, and 1954- parts of each galactic image which the microdensitometer 55, and the work was finished in 1958. According to detected were too faint to be perceived by the visual Holmberg, the project "took me ten years to do, with the inspections (Fig. 7). Holmberg (1946) concluded, "The help of a lot of assistants." It resulted in his "largest and great superiority of the photometer to the human eye is most important paper" (Holmberg 1958α). (The informa- clearly displayed." Holmberg realized that visual deter- tion contained in this paragraph was derived from corre- minations of major and minor diameters of galaxies in spondence with Professor Holmberg.) large and homogeneous samples are still valuable for The large material which Holmberg obtained mainly at studies such as the determination of orientation proper- the Mount Wilson Observatory with the 60-inch and ties of galaxies in space, but only to the extent that they 100-inch telescopes were photographic plates for 300 are corrected for the systematic effects which Holmberg galaxies distributed over the northern sky and selected to clearly demonstrated are present. Holmberg himself was be representative of nearby galaxies in general, i.e., the more inclined toward analyzing the accurate data on sample includes galaxies covering a large range of Hubble

Fig. 5-Reproduction of one of Kolmbergs plates showing five photographic images of an artificial galaxy corresponding to the ratios of minor to major diameter hla - 1.0, 0.8, 0.6, 0.4, and 0.2. (Adapted from Fig. 2 of Holmberg 1946.)

Fig. 6-Relations between diameter ratios as measured by five different observers; (I) K. Lundmark, (II) A. Reiz, (III) S. Cederblad, (IV) H. Kristenson, and (V) Ε. Holmberg, β/α is the ratio of visual minor to major diameter measured by a given observer, < β/α > is the average of the measured values of β/α for the five observers, and hla is the ratio measured with a microphotometer. Each point [hla, (β/α)/(< β/α >)] represents the average over the 22 photographic images with the indicated hla. The horizontal line represents an average over all five observers, which Holmberg calls "the mean observer". Note that the agreement between different observers increases with increasing diameter ratio. (Adapted from Fig. 6 of Holmberg 1946.)

morphological types and absolute luminosities. In gen- Fig. 7—Photometer tracings obtained by Holmberg along the major axes of three artificial galaxies. The light intensity falls off with distance eral, two pairs of plates were obtained for each galaxy, from the center of the galaxies in proportion to r 10, r ~ 5, and r . The each pair corresponding to an exposure in both the photo- straight horizontal lines indicate the minimum values of plate density graphic and photovisual region of the spectrum on the perceivable from a visual inspection of the plates. (Adapted from Fig. 7 international scale. The adopted standard exposure time of Holmberg 1946.)

Fig. 9-Relation between the ratio (β/α)/(έ>/α) and the power-law function which describes the light distribution. The different curves correspond to different values of the true diameter ratio hla. Note that, except for truly round images, visually measured shapes tend to be flatter (less round) than true shapes. (Adapted from Fig. 9 of Holmberg 1946.)

lence with which he collected his extensive and accurate data, but especially by the basic new structural information about galaxies which he realized he could derive from it, and then proceeded to derive very accurately usually by his own statistical methods. For some galaxies, Holm- berg used the individual photometric tracings to separate the light in the spiral arms from the light in the smooth underlying disk (Fig. 11). For the entire sample of 300 Fig. 8-The visually measured diameters α and β expressed in percent galaxies, he derived angular diameters, apparent magni- of the photometric diameters a and b. The full curves (corresponding to _n tudes, colors, morphological types, and quantitative in- different values of η in the relation light intensity r ) refer to the formation about absorption of light by interstellar obscur- major diameters, and the broken curves refer to the minor diameters. Note that the visual major diameter tends to be smaller for rounder ing material both in the galaxies and in our own Galaxy. photographic images, and the visual minor diameter tends to be slightly This information was then applied, for the first time ever larger for rounder images. (Adapted from Fig. 8 of Holmberg 1946.) by anyone, to derive the light and color output from the stellar component of each galaxy. The entire information was later found by microdensitometry to correspond to a was finally used to make inferences about the origin and limiting detectable surface brightness of 26.5 photo- evolution of stars in galaxies. Some details of this remark- graphic and 26.0 photovisual magnitudes per square arc able work are now described. second. Half of each plate was an in-focus exposure of a For each of the 300 galaxies, the angular major and galaxy and the other half was an extra-focus exposure of minor diameter at the limiting detectable surface bright- stars with known apparent magnitudes and colors in a ness of 26.5 photographic magnitudes per square arc comparison field (either Selected Area 57 or the north second (the limiting isophote) was derived. This surface polar region) which was used to obtain the photometric brightness is so small and the decline in surface bright- calibration curve to convert plate densities into intensi- ness at this isophote is so rapid that nearly all of the light of ties. This special method seemed to furnish magnitudes a galaxy is likely to be contained within it. At the Santa and colors of high accuracy and without any noticeable Cruz workshop, the term "Holmberg radius" was used to systematic errors. Back at Lund Observatory Holmberg signify the radius of the outer boundary of the light distri- and a group of assistants obtained about 20,000 microden- bution of a galaxy; the term "Holmberg radius" is pre- sitometer tracings across various paths on the plate im- cisely half the major diameter of the isophote of surface ages, or about 15 tracings across the four images of each of brightness 26.5 photographic magnitudes per square arc the 300 galaxies (Fig. 10). All calibration plates and about second (i.e., the limiting isophote measured by Holm- one-third of the exposures of galaxies were measured and berg). At the Santa Cruz workshop, several presentations reduced by Holmberg alone! were made of rotation curves (i.e., deprojected radial Holmberg s brilliance in carrying out astronomical re- velocity vs. distance from the center of a galaxy) that were search is demonstrated not only by the' technical excel- derived from (1) optical spectrograms measured to a large

100—

Fig. 10-Reproduction of 20 microphotometric tracings of the spiral galaxy NGC 2403 from a plate sensitive in the photographic spectral region. Each tracing represents the distribution of surface brightness in a cross section parallel to the minor axis of the galaxy. (Adapted from Fig. Fig. 11-Photographic surface-brightness distributions (solid curves) in 1 of Holmberg 1958α.) central cross sections parallel either to the right ascension or the declination circle for eight spiral systems of type Sc— with a face-on orientation. The vertical scale, the same for all the objects, gives the surface bright- fraction of one Holmberg radius and (2) 21-cm maps of ness in units of one star of magnitude 26.5 per square second. The neutral hydrogen measured to a few Holmberg radii. The numbers refer to the NGC designations of the galaxies. The broken latter rotational curves generally maintain a constant rota- curves, which for each galaxy have the same shapes on the left-hand and tional velocity in their outer parts which are usually inter- the right-hand sides, represent an attempt to determine the distribu- preted to indicated the presence of a dark and massive tions referring to the disk population; similar curves have been constructed for the photovisual distributions. From these data Holmberg galactic halo. found that the resulting integrated color of the disk population is practi- For all 300 galaxies, Holmberg summed up the light cally independent of the central distance. The difference between the within the limiting isophote in both the photographic and solid and broken curves represents the light within the spiral arms. photovisual regions of the spectrum; he thereby derived (Adapted from Fig. 5 of Holmberg 1975¾.)

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 930 HERBERT J. ROOD the apparent photographic magnitude and color index of in the Milky Way indicate that the ratio of total to selec- each galaxy. He noticed that the galaxies which Hubble tive absorption caused by its dust grains is A/E — 4, where (1926, 1936a) had classified as "irregular" are actually a A is the total absorption in magnitudes and Ε is the color mixture of objects with resolvable brightest stars and excess, i.e., the change in the color which results from the objects with irresolvable brightest stars; the former have reddening. Holmberg determined effects of absorption color indices C — 0.3 similar to the color indices of Sc caused by obscuring material both inside galaxies of dif- galaxies while the latter have color indices C — 0.8 similar ferent morphological types and inside our Galaxy. He to the color indices of luminous elliptical, SO, and Sa reasoned that the column length along the line of sight, of galaxies. Holmberg then pointed out that if one assumes the obscuring material in the plane of a galaxy of a given that the light from a galaxy originates simply from the morphological type, is larger (1) the smaller the inclina- mixture of stars of the two kinds of population, Baade s tion of the plane of the galaxy to the line of sight (i. e., the Population I and Population II (Baade 1951, 1963), then smaller the sine of the inclination, sin i ~ fc/α ,1 and (2) the the former is dominated by Population I stars and the smaller the angular distance of the galaxy from the Galac- latter is dominated by Population II stars. Accordingly, it tic plane (i.e., the smaller the Galactic latitude Β or the became evident to Holmberg that Hubble's "irregular" larger cosec B). morphological type must be replaced by two types which By plotting the observed color and surface brightness Holmberg designated Ir I (resolved, C — 0.3) and Ir II of galaxies, grouped according to morphological type, (unresolved, C — 0.8). Holmberg determined morpho- against hi a Holmberg derived empirical relations which logical types for all 300 galaxies on a system which refined describe the dependence of selective absorption, E, and Hubbie s original classifications through modifications total absorption. A, on inclination (Figs. 12 and 13). He that appeared necessary by the new data at hand. then demonstrated that when the ratio A/E is plotted The basic integral data which Holmberg had acquired against b/a (Fig. 14), its value is A/E = 4 for b/a > 0.6, in for his representative sample of 300 galaxies were tabu- agreement with the value found in the Milky Way from lated in a catalog (Holmberg 1958α) which includes photometric observations of stars which are partially ob- Galactic longitude (L), Galactic latitude (B), morphologi- scured by dust clouds, and A/E > 4 for b/a < 0.6, in cal type, major angular diameter (a), minor angular di- agreement with results of a theoretical analysis that is now ameter (b), apparent diameter ratio (fc/α), photographic described briefly. Holmberg considered a model of a magnitude {mpg), and color index (C = mpg — mpv, where spiral galaxy in which stars are located on either side of a mpv is the photo visual magnitude). All external checks central layer of absorbing material that is also mixed with with independent data obtained by others (for example, stars. The contribution to the observed light of a galaxy the comparison with Whitford's data shown in Fig. 2) that reaches an observer from its far side is substantially indicate that the accuracy of Holmberg's data remains reddened, that from the stars in the absorbing layer is less unsurpassed to this day. I now describe how Holmberg reddened, and that from its near side is essentially unred- used these data to obtain information about the absorp- dened. The smaller the inclination of a galaxy to the line of tion of light within galaxies in general and in our Galaxy in sight (δ/α), the smaller is the fraction of light that an particular, which he then applied to derive the light and observer receives from its far side (the substantially red- color output from the stellar component of galaxies. dened component). Therefore if A/E = 4 for galaxies with The layer of obscuring material in the plane of a galaxy b/a > 0.6, A/E > 4 for similar galaxies with b/a < 0.6. In absorbs light selectively; blue wavelengths in the spectral Holmberg s model, as the limit of an infinitely-thin opti- energy distribution of the stars of a galaxy are absorbed cally-opaque absorbing layer is approached, most of the more completely than are red wavelengths. Therefore, observed light originates from the near side of the galaxy the light which emerges from the stellar component of a so that the total absorption approaches 0.75 magnitude, galaxy is redder than the light which is emitted by the the reddening is negligible, and A/E approaches infinity. collective photospheres of the stars. Direct observations By plotting the observed color and surface brightness of of the spectral-energy distribution of individual stars and galaxies against cosec Β, Holmberg derived empirical aggregates of stars in nearby galaxies indicate that the relations which describe the dependence of selective and morphological type of a galaxy specifies its content of stellar and obscuring material to an accuracy set primarily taking into account the relative size ρ of the smallest axis of the by the uncertainty of the morphological classification it- three-dimensional galaxian disk (assumed to be an oblate spheroid), the self, i.e., the fraction of different types of stars and the inclination ¿ is, in fact, given by the formula amount of obscuring material per kiloparsec in the plane of a galaxy are very similar for galaxies of a given morphological type and differ significantly from type to type. a relation derived previously by Hubble (1926). In the Holmberg photo- Moreover, observations of absorption and reddening of metric diameter system the mean of the parameter ρ was found to be stars with known absolute luminosity and intrinsic color 1/5.

^Co Irl Sc+.Sc- * ♦αϊ- · ♦15- S -^ Γ"1 :—I— —ι r-r; 1— 0 10. 0.8 0.6 0Á ' 0.2 -Ql-J b/a Sc+ ♦ lo — - ~ r-r- * Γ π +05—

oo- Her 0.8 . . 0.6 0.4 0.2 -os— Sb+ Sb+Sb-(Sa : ■♦i r r-

+io— Sb-,Sa

+05— , —1 — T*"η · 1—;·· r «tf- Qo- Η 1 So 01 -I—: 1 T- -05—

Fig. 13-Total photographic absorption from obscuring material in Fig. 12-Selective absorption from obscuring material in galaxies of galaxies derived for two groupings of morphological types. If the surface different morphological types. The residual color index of a galaxy, ACq magnitude of a galaxy is defined as its average surface brightness (total = Cq — (where Cq is the observed color index reduced to B = photographic luminosity divided by total surface area transformed into 90°, and is the observed color index reduced to B = 90° and b/a magnitudes), then the surface magnitude residuals (observed, So, minus = 1.0 derived iteratively), is plotted against the diameter ratio, b/a. average (reduced to b/a = 1), , both reduced to Galactic latitude Note that selective absorption is evident (ACq increases as b/a decreases) 90°) are plotted against the apparent diameter ratio, b/a ; the open circles in spiral galaxies but not in SO and Ir I galaxies. The mean relations represent the average residuals for galaxies in different bins of diameter indicated by the dashed lines correspond to differential selective ab- ratio, and the solid curve is a fit to these points. The curve corresponds sorption of light of 0.049, 0.136, 0.128, and 0.251 for Sc+, Se—, Sb+, to a total photographic absorption for a face-on galaxy {b/a = 1) of 0.28 ± and Sb—, Sa galaxies, respectively, multiplied by (1 — b/a). (Adapted 0.07 (m.e.) magnitude for Sc+, Sc— galaxies and 0.43 ± 0.06 (m.e.) from Fig. 2 of Holmberg 1958α.) magnitude for Sb+, Sb—, Sa galaxies multiplied by (cosec i — 1), where cosec i — a/b. (Adapted from Fig. 4 of Holmberg 1958α.) total absorption on cosec Β (Figs. 15 and 16). The con- and representative sample of nearby galaxies, he applied stants of the linear analytical fits to the data imply a color these data to questions pertaining to the origin and evolu- excess Ε = 0.062 ± 0.007 magnitude and a total absorption of galaxies. Because he realized that the mass density tion A = 0.22 ± 0.05 (for 15° < B < 90°) or A = 0.26 ± of galaxies is likely to be an important ingredient for 0.07 magnitude (for 20° < ß < 90°) in the direction of the solutions to these problems, and because mass density is a north galactic pole. These values are consistent with the property which cannot be derived from his photometric ratio A/E = 4, and with the value A = 0.25 magnitude data alone (apparent magnitudes and colors) but also re- which Hubble (1934) had obtained from a study of the quires data from the literature (systemic and internal dependence of the observed surface density of galaxies, radial velocities of galaxies), Holmberg integrated all of brighter than a uniform apparent magnitude limit, on these data in the following remarkable way (Holmberg Galactic latitude. In 1974 Holmberg found that this value 1964). is also consistent with the variation over the sky of the By the year 1964, efforts which were begun at the observed surface density of the 9134 clusters of galaxies Mount Wilson Observatory by Edwin Hubble and Walter cataloged by Fritz Zwicky et al. (1960-68) (Fig. 17). Baade, and later carried on by Allan Sandage and cowork- Now that Holmberg had derived a diversified and accu- ers, to derive distances to very nearby galaxies by means rate collection of basic structural information for a large of the luminosities of their RR Lyrae, Cepheid variable,

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galaxy, M*, is related to its surface brightness defined as S* = m* + 5 log a (where m* is its total apparent photo- δ^Ε graphic magnitude, corrected for Galactic and internal 15- absorption, and a is its photometric major diameter in arc minutes, i.e., the Holmberg diameter) and to its absorp- 10- tion-free color index, C*, by the empirical equation, M* = 3 S* + 1.30 C* — 64.42. Holmberg realized that if we make the reasonable working assumption that this relation applies universally, then it could be used to obtain the distance to a galaxy from its apparent photographic o— I 1 1 1 1 1 1 1 1 1.0 0.8 0.6 OA 0.2 ^ magnitude and two distance-independent structural /a parameters, intrinsic surface brightness and intrinsic color index. Holmberg coined the word "photometric distance" for the distance derived in this way to distin- guish it from the more direct distance derived from prop- perties of stellar and gaseous components within galaxies. He derived photometric distances for 245 galaxies with radial velocities less than 2000 km s_1 to "secure a material that is to some extent representative of the nearer extragalactic space." From the radial velocities and photome- o- I 1 1 1 1 1 1 1 1 tric distances of these galaxies, he then derived a value for 1.0 0.8 0.6 OA 0.2 k -1 -1 the Hubble constant, H0 = 80 km s Mpc , which lies midway between the two values most often advocated today, 50 and 100 km s1 Mpc-1. At the Santa Cruz workshop, whenever reference was made to distances of Fig. 14-The ratio of differential photographic to selective absorption galaxies derived from (1) the Tully-Fisher relation which (inclination effects alone), ΔΑ/ΔΕ, and its variation with diameter ratio, relates the distant-independent parameter "line-width of b/a, as derived for spiral galaxies of types Sc+ to Sa (upper part). The corresponding ratio of total photographic to selective absorption (incli- the 21-cm profile of neutral hydrogen" to the absolute nation effects + absorption by a face-on galaxy), A/E, and its variation luminosity of a spiral galaxy, or (2) the Faber-Jackson with diameter ratio (lower part) according to the following adopted relation which relates the nuclear velocity dispersion of relations: A = ΔΑ + 0.28, Ε = ΔΕ + O.OTOfor Sc+, Sc— galaxies, and A an elliptical galaxy to its absolute luminosity, and when- = ΔΑ + 0.43, £ = ΔΕ + 0.107 for Sb-f, Sb—, Sa galaxies. Note that A/E ever attempts were described to improve upon these = 4 for b/a >: 0.6 (large inclinations, i, where cosec i — alb) and A/E > 4 for smaller inclinations {b/a <0.6). These values agree with the theoreti- relations by introducing another distance-independent cal estimates obtained by Holmberg from an analysis of absorption parameter such as the color index or the surface bright- effects caused by obscuring material mixed with stars in a plane-parallel ness of a galaxy, one could only recall Holmbergs remark- layer for which the total absorption in photographic and photovisual ably successful pioneering efforts in this field. magnitudes is proportional to cosec i. Moreover, the ratio of total to Direct distances, photometric distances, and/or dis- selective absorption determined observationally from studies of obscu- tances derived from radial velocities by assuming a Hub- ration effects on stars in local regions of the Milky Way is generally found 1 1 to be A/E — 4. (Adapted from Fig. 5 of Holmberg 1958α.) ble constant H0 = 80 km s Mpc were thereby available to renormalize masses published in the literature to cor- and most luminous stars, and the sizes of their ionized respond to a homogeneous and consistent system of dis- hydrogen regions had led to well-determined distances tances. Holmberg then performed an exhaustive exami- for eight spiral galaxies. By combining these distances nation of the literature on published determinations of with the accurate apparent photographic magnitudes of the masses of galaxies, from which he created a working galaxies that he had derived, Holmberg obtained numer- list of the galactic masses, SK, that he judged to have been ous accurate absolute photographic magnitudes. Because derived from acceptably extensive and accurate data. (1) several of the eight galaxies with well-determined Detailed explanations are presented for each individual distances lie within a group (and hence determine the galaxy with a published mass which was rejected from distance of the group), and (2) Holmberg had derived inclusion in his working list of galactic masses. The pub- accurate apparent magnitudes for several members in lished masses, SK, were generally determined by measur- many of these groups, he was able to determine absolute ing radial velocities in an inner region of a galaxy and then magnitudes directly for a large number of galaxies over a adopting an assumed nearly Keplerian falloff of the radial representative interval of the luminosity function. velocity, vr, with increasing radial distance, r, in the With these data Holmberg then discovered that the outer region. This was a generally-held assumption until absorption-free absolute photographic magnitude of a the mid-1970s, when it was found empirically that Vr

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~~c'-crn~~

*0.2- - ·>·#»> ·_j>. — — c- · 0.0 - ·νΓ· ι — *;"··*·; · 2.0 3.0 4.0 ^ 5.0 cosec Β - 0.2 —

~~I I 90ΐ·e 50ί 40^ 30 25 20 15 12· Β~~

Fig. 15-Selective absorption from obscuring material in our Galaxy derived from galaxies of types Ir I to Sa. The residual color index of a galaxy, C ' — (where C ' is the observed color reduced to b/a = 1, and is the average color reduced to b/a = 1 and Β = 90°), is plotted against the cosecant of Galactic latitude, cosec B. The mean relation, indicated by the dashed line, corresponds to a selective absorption at the Galactic pole of 0.062 ± 0.007 (m.e.) magnitude multiplied by cosec B. (Adapted from Fig. 3 of Holmberg 1958α.)

tematically too small by a factor less than ~ 2. And the absorption-free photographic luminosity of a galaxy, L* (which corresponds to the absolute photographic magnitude, M*) is an excellent estimate of the luminosity within a Holmberg radius. The ratio SOÎ/L* which Holmberg + f " adopted for his analyses is therefore, to within a factor ~ 0.0— 1 1 1 1 1 2, a characteristic ratio of the mass to the luminosity of a 10 20 30 cosec Β galaxy. Holmberg then discovered that Sft/L* of a galaxy is an increasing function of its intrinsic color, C* (Fig. 18). By applying this empirical relation to the intrinsic luminosity, L, and linear major diameter, A, of each galaxy for Fig. 16-Total absorption from obscuring material in our galaxy, ΔΑ = which he had obtained microphotometric data. Holm- S ' — , as derived from the photographic surface magnitudes, S ' berg derived its characteristic mass density, D. The mass (reduced to b/a = 1), and the average value, (reduced to b/a = 1 density is an astrophysical parameter which is basic for and Β = 90°), of spiral galaxies with Hubble types Sc+ to Sa. The theories of star formation, galaxy formation, and dynami- half-length of each vertical line is the rms error of the average surface cal evolution within galaxies. Holmberg then showed that magnitude residual. Hubble's cosecant law, ΔΑ = 0.25 cosec Β magnitudes (where Β is Galactic latitude) is represented by the broken line. the mass density, D, is an increasing function of the Hubble's cosecant law was obtained in the year 1934 from an analysis of intrinsic color, C* (Figs. 19 and 20). Because morphologi- surface number densities of faint galaxies as a function of Galactic cal type is strongly correlated with intrinsic color. Holm- latitude and applies only in the range of Galactic latitudes larger than the berg had found that the mass density of a galaxy increases limit set by the "zone of avoidance" where obscuration blots out galaxies nearly completely. By the method of least squares, Holmberg fitted his along the Hubble sequence from late to early-type spiral new accurate data on surface magnitudes of galaxies to a cosecant law galaxies to SO + elliptical galaxies. If the mass density is a and derived a Galactic pole photographic absorption of 0.22 ± 0.05 robust parameter not easily subject to change during the (m.e.) magnitude from all 119 data points, and 0.26 ± 0.07 (m.e.) stellar and dynamical evolution of a galaxy, then Holm- magnitude from the 113 points with |ß | > 20°. The agreement between bergs density-color relation for a representative sample Hubble's results derived from galaxy counts and Holmberg's results derived from surface magnitudes of individual galaxies is excellent. of galaxies strongly suggests that, in general, the morpho- (Adapted from Fig. 8 of Holmberg 1958α. ) logical type of a galaxy is determined at the time that it is formed, i.e., in general, initial conditions determine the Hubble type of a galaxy (Fig. 20). remains essentially independent of r in the outer parts of a A problem that Holmberg has pursued for most of his galaxy. Nevertheless, still provides an estimate of the active life has been the derivation of a reliable luminos- mass contained within a Holmberg radius which is sys- ity function for galaxies from the observational data. He

~~© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 934 HERBERT J. ROOD~~

Fig. 17-Deviations ΔΑ (in units of 0.01 photographic magnitude) from Bubble's cosecant law, A = 0.25 cosec Β magnitude, as derived from the surface number density of the 7753 distant, very distant, and extremely distant clusters of galaxies cataloged by Zwicky and collaborators. The horizontal axis is Galactic longitude and the vertical axis is Galactic latitude. Each dot denotes the center of a field from the Palomar Observatory Sky Survey (which is a photographic reproduction of the entire sky accessible on Palomar Mountain obtained with the 48-inch Schmidt telescope). The full curves outline Hubbie s zone of avoidance and the dashed curves outline areas of exceptionally large or small absorption. The area enclosed by open circles is the region of the very nearby Virgo cluster. The mean deviations scatter as a function of cosec ß in a manner consistent with the Hubble cosecant law. The deviations themselves are determined by effects of patchiness in the local distribution of dust in the Milky Way, superclustering of galaxies, and uncertainties inherent in the identification of rich clusters of galaxies. (Adapted from Fig. 3 of Holmberg 1974fl. )

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~~♦012 0Λ 0.6 »0.8~~

Fig. 18—Relation between the logarithm of the mass-luminosity ratio and intrinsic color index (the color index corrected for internal and Galactic absorption) for the sample of galaxies with direct measurements of mass. The derived values for (a) a sample of stars in the solar neighborhood and (b) a cylinder with an axis perpendicular to the Galactic disk extending through the Sun to beyond the Galaxy on either side are denoted by filled circles. The probable position of our Galaxy is indicated by a cross. (Adapted from Fig. 1 of Holmberg 1964.)

0.0 0.2 0.4 0.6 +0.8 Fig. 19-Relation between the logarithm of the mass density (in solar masses per cubic parsec) and intrinsic color index, derived for 160 galaxies of types Sa-Sb-Sc. The probable position of our Galaxy is indicated by a cross. (Adapted from Fig. 4 of Holmberg 1964.) derived φ(Μ), the number of galaxies per cubic Mpc in a galaxies. Holmberg started on this problem in 1947 when one-magnitude interval centered on absolute photo- he studied the Messier 81 and Messier 101 groups of graphic magnitude M as a function of M. The luminosity galaxies along with our own Local Group (Holmberg function is a fundamental input for theories which at- 1950¾). From visual inspection of photographic plates tempt to describe the formation and the evolution of which he took with the 10-inch astrographic refractor at

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~~logD~~

-2

-3

~~-0.2 0.0 +0.2 *0Λ*0.6 *08 ♦l.O~~

Fig. 20-Relation between the logarithm of the mass density (in solar masses per cubic parsec) and the intrinsic color index for galaxies of all Hubble morphological types. The detailed sequence for types Sa-Sb-Sc galaxies is shown in Figure 19. Type Ir I galaxies are represented by open circles, Ir II galaxies by crosses, and SO galaxies by filled circles. The solid curve is a universal relation in that it appears to be applicable to all types of galaxies. The dotted lines indicate the evolutionary tracks that correspond to the assumption of a time-independent mass density. (Adapted from Fig. 6 of Holmberg 1964.) the Mount Wilson Observatory, Holmberg tried to find as on the photographic plates that he had obtained with the many new members as possible. He had some success. 10-inch refractor. Ho I, Ho II, and Ho IV denoted in Figure 21 are three of Later on, Holmberg decided to push this type of inves- these newly discovered galaxies (The designation "Ho" tigation for determining the luminosity function for galax- denotes the discoverer of the galaxy without introducing ies in loose groups as far out into space as possible. The possible confusion because of the designation "H" used to truly remarkable paper Holmberg (1969) resulted from denote the element hydrogen). The great project which this goal. By means of the first-edition prints of the Palo- culminated in the microphotometric determination of mar Observatory Sky Survey which cover square regions apparent magnitudes, colors, and major and minor angu- of the sky 6.6 degrees on a side to a limiting photographic lar diameters for about 300 representative galaxies began magnitude — 21, and by means of the plates which he with the measurements from plate material secured secured with the 60-inch and 100-inch telescopes of the mainly with the 60-inch telescope at the Mount Wilson Mount Wilson Observatory, Holmberg derived the com- Observatory for members of the Local Group, the M 81 posite luminosity function of galaxies in survey fields with group, and the M 101 group. The data for these three a radius of 50 kpc centered on the 174 galaxies of types groups gave a material of galaxies corresponding to a S0-Sa-Sb-Sc with Holmberg diameters > 5 arc minutes. given volume of space. In order to fulfill this critical The print and plate material were first inspected in an condition (given volume of space) it seemed to Holmberg attempt to pick up all galaxies with major diameter ^ 1.0 then (and still does today) that the only way to obtain a kpc located both within each survey area and in two reliable result is to use physical groups with the distance comparison fields of equal area located typically 112 arc supplied by a dominating group member. The loose phys- minutes on either side. (The limiting angular diameter ical groups, in contrast to the rich clusters, rather well needed for the observation of each field was derived by represent the "general field" of galaxies in cosmic space. converting 1 kpc into angular units by means of the radial Holmberg s study succeeded in describing the bright end velocity of each dominant galaxy transformed into dis- -1 -1 of the luminosity function, but the description of the faint tance via the Hubble relation with H0 = 80 km s Mpc .) end suffered from incompleteness because he did not find The survey fields were divided into three classes accord- a sufficient fraction of the low-luminosity dwarf galaxies in ing to whether the central galaxy has an apparent diame- the groups, many of which would be too faint to register ter ratio ^ 0.53 (called "edgewise orientation") or > 0.53

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Fig. 21-Sky map of the Messier 81 region (top panel) and the Messier 101 region (bottom panel). The coordinate grid specifies right ascension (in hrs) and declination (in degrees) for epoch 1950. The galaxies accepted as group members are denoted by open circles. Galaxy designations, when available, are NGC numbers or Ho I, Ho II, and Ho IV. The latter refer to galaxies that Holmberg discovered on photographic plates which he took with the 10-inch astrographic refractor at the Mount Wilson Observatory. (Adapted from Fig. 1 of Holmberg 1950¾. )

(called "face-on orientation"), or whether it appears to be linear diameter > 1 kpc in each of the survey and com- disturbed by nearby large galaxies. From this print and parison fields. Because the data for the survey fields refer plate material, Holmberg determined the morphological to physical satellites plus foreground/background galaxies type and apparent major axis of all identified galaxies with ("optical galaxies") projected onto each satellite system

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 938 HERBERT J. ROOD whereas the data for the comparison fields refer to optical galaxies alone, Holmberg statistically corrected the data M for the survey fields to obtain a frequency distribution of linear diameter which corresponds to the physical satel- -20 lites alone (Fig. 22). For his representative sample of about 300 galaxies with accurate photometric data, Holm- berg then demonstrated that absolute diameter. A, is tightly correlated with absolute magnitude, M (Fig. 23). This empirical relation could then be used as a calibration curve to convert the data on diameters into absolute —15 magnitudes and thereby to construct the luminosity function for galaxies (Fig. 22 and Fig. 24). By also applying the data on galactic morphological types, Holmberg was able to construct both a total luminosity function and a lumi- L*ol0 . nosity function for E-S0-Ir galaxies alone. The latter in- χ --10/ creases monotonically toward fainter magnitudes, while 0i*>n the total curve displays a prominent hump near M = —18 I I I I I I I ) I I I I I I I I I I ι ι ι which reflects an approximately bell-shaped luminosity 310 3.5 4.0 45 (og Α function for spiral galaxies. Holmberg has the feeling that the double luminosity curve which he arrived at cannot be much improved, that is, it would be difficult to collect a Fig. 23-Relation between the absolute photographic magnitude and the logarithm of the absolute major diameter (in parsecs) as obtained for galaxies of types E-S0 (open circles), Sa-Sb-Sc (filled circles), and Ir I - log Ν (crosses). These physical quantities were derived from the apparent magnitude corresponding to the luminosity contained within the isophote of surface brightness 26.5 photographic magnitudes per square -♦2 arc second and its angular major diameter, where the transformation from the observed to the physical quantities was obtained by applying distances derived from the Hubble relation, distance = observed -1 Doppler velocity/Ho, where Η0 = 80 km sec Mpc, a value derived from the tight correlation which Holmberg (1964) discovered between the absolute magnitude of a galaxy and its surface brightness and color index: This empirical relation anticipates the present-day Tully-Fisher and Faber-Jackson relations for obtaining distances to galaxies. (Adapted from Fig. 9 of Holmberg 1969.)

more complete material representing a given volume of space in the "general field". Indeed, the dependence of the luminosity function on Hubble morphological type which Holmberg derived for small groups has more recently been repeated for rich clusters only. At Santa Cruz, as Bruno Binggeli presented new results obtained with Allan Sandage and Gustav Tammann which demonstrated that galaxies of different morphological types in Fig. 22-Frequency distribution of logarithm of the diameter in parsecs (and absolute photographic magnitude) as obtained for the 274 satellite the Virgo cluster have different luminosity functions, he galaxies in the 160 survey areas centered on the spiral galaxies with mentioned that more than a decade ago Holmberg ob- angular diameter > 5 arc minutes. In each area the adopted distance of tained similar results for small groups of galaxies. the central spiral supplies the distance of the physical satellites. The At the workshop in Santa Cruz, Peter Quinn delivered large open circles refer to the entire material, and the small circles to a lecture about the "Holmberg effect", a term which types E-S0-Ir; the filled circles represent those satellites for which the absolute photographic magnitudes are derived from directly measured Quinn used to refer to the following discovery that Holm- apparent magnitudes. The excellent agreement between the frequency berg made with the data just described: The combined distributions of absolute magnitude and the logarithm of the diameter distribution on the sky of the 218 physical and optical for the overlapping data set of larger and more luminous galaxies is a galaxies within 50 kpc of each of the 58 undisturbed consequence of the strong correlation between the absolute photo- edge-on galaxies in his sample revealed to Holmberg that graphic magnitude and the diameter of a galaxy discovered by Holm- berg and reproduced in Figure 23. The dotted line labeled "Opt" the average number of satellites per square degree is represents the distribution of the subtracted optical (background/fore- significantly larger in the two polar sectors than in the two ground) galaxies. (Adapted from Fig. 8 of Holmberg 1969.) equatorial sectors (Fig. 25). In a reexamination of Holm-

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~~- 0.06 f(M)~~

~~■\ - 0.06 \~~

~~-0.(K~~

~~0\ - 0 02 E-So-IrN \ \ " ^ 0 ·?~~

O.ooι : ι ι ι ι ,ι ι ^ ,' -¾ -16 -IB -20 M -22 Fig. 25-The combined distribution of the 218 galaxies with a diameter ^ 1 kpc in the 58 survey areas centered on the undisturbed spiral galaxies with an angular diameter > 5 arc minutes and edgewise orienta- Fig. 24-The brighter end of the luminosity function for the entire tion. The ellipse shows the average size of the central system. Note that material and for E-S0-Ir galaxies (corresponding to the right-hand part of the surface number density of satellites in the two polar sectors is larger Fig. 22 normalized to a volume in space of 1 cubic Mpc). The filled than that in the two equatorial sectors, an observational property which circles represent the luminosity function derived for galaxies with has come to be known as the "Holmberg effect". (Adapted from Fig. 3 of known redshifts north of Galactic latitude +30 degrees {B > 30°). The Holmberg 1969.) squares refer to the Local Group and the M 81 group. (Adapted from Fig. 10 of Holmberg 1969.) new approach); a later more definitive presentation is given in Fig. 27(b)). Moreover, this study provided a clear berg data, Quinn and Goodman (1986) calculated that this quantitative demonstration that single galaxies tend to be result is statistically significant at the 4 σ level. They then spirals, whereas double and multiple systems and the rich attempted to explain the Holmberg effect in terms of a Virgo cluster all tend to contain a larger fraction of ellipti- gravitational drag effect, dynamical friction, experienced cals (Fig. 28). The pronounced dumpiness in the distribu- by satellites orbiting in the mass distribution of a spiral tion of galaxies was further revealed in the sky map of the galaxy. The equatorial satellites, which experience a 827 double and multiple galaxies which Holmberg con- greater amount of dynamical friction than do the polar structed as part of his doctoral thesis (Fig. 29). These satellites, are expected to spiral into the central regions of results made a profound impression upon Holmberg, for a spiral galaxy faster. Quinn and Goodman's extensive he wrote in his doctoral thesis and also in later writings theoretical calculations showed that only about 10% of the that, "Among the bright galaxies every second forms part Holmberg effect is accounted for by dynamical friction, of a double or multiple system. . . . There is an unbroken so another physical or statistical selection mechanism is line of transition: double galaxies-multiple galaxies- more dominant and still remains to be identified. metagalactic clusters-metagalactic superclusters or clouds." In the course of his meditations on how this ΠΙ. Systems of Galaxies dumpiness might have been created and of the depen- As we have seen, Holmberg originated several broad dence of the multiplicity of a system on the morphology of ideas about galaxies that are of fundamental importance. a member galaxy, Holmberg very probably became the The first of these came in his doctoral thesis (Holmberg first extragalactic astronomer to fully realize that when 1937α) where he demonstrated statistically that most of two galaxies pass by each other in an initially unbound the close pairs of galaxies (Fig. 26) are physical binaries hyperbolic orbit, translational kinetic energy of the two- (close together in space, rather than chance lineups) and body system is converted into internal energy of the noted that the masses of these physical binaries can be individual galaxies through the action of tidal forces. estimated from their orbital motions. In a later study Hence, the relative velocity of the two galaxies decreases Holmberg (1940α) demonstrated quite generally that a as a result of the encounter, and if the transfer from significant fraction of luminous galaxies tend to be mem- translational to internal energy is sufficiently large, the bers of double or multiple systems, i.e., groups of galaxies final relative velocity could be reduced to such an extent (Fig. 27(a) (which is of historical interest as representing a that the galaxies would capture one another to become a

~~© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 940 HERBERT J. ROOD~~

~~No 215. (N(tC 3395, m)~~

Fig. 27(a)-The frequency distribution of the observed number of galaxies listed in John Herschel's General Catalogue (as revised by Karl Reinmuth) as a function of the angular distance from each of the galaxies listed in the Shapley-Ames catalog within the intervals of apparent photographic magnitude 10.1-12.0 and 12.1-13.0. The straight lines No 520. represent random distributions of optical companions. The figure sug- (NGC 5194, 95) gests that many galaxies are members of multiple systems. (Adapted Fig. 26-Photographic reproductions of double galaxies No. 215 and from Fig. 2 of Holmberg 1940α. ) No. 526 from the atlas of double and multiple galaxies prepared by Erik Holmberg (1937α) as part of his doctoral dissertation. The photographs were selected by T. L. Page to illustrate physical binary galaxies with Ν different probable orbital inclinations. The orbit of No. 526 may be in 40- the plane of the sky, whereas the orbit of No. 215 is probably greatly inclined to the plane of the sky. Discussion of pioneering attempts by Holmberg and by Page to determine the average mass of binary galaxies from the radial-velocity differences and separations (in the plane of the sky) of their components is included in a popular article on Holmberg's research prepared by Page and Rood for the magazine Mercury of the Astronomical Society of the Pacific. Holmberg defined a double galaxy as a pair of galaxies which satisfy the criterion ^/(Ηχ + fy — 4, where is the angular distance between the components of the double system, and Κχ and fí2 are the observed radii of the components. In a multiple system this criterion is valid for each of the components taken together with each of the others. A single galaxy is one which does not satisfy this criterion and is also not a member of a rich cluster. (The Virgo cluster is the only rich cluster Fig . 27(b)-The frequency di stribution of the observed number of galax- which is located so near us that many of its members are listed in A ies listed in the source described in Figure 27(a) as a function of the Revised Shapley-Ames Catalog of Bright Galaxies (Sandage and Tam- linear distance from each of the 111 galaxies with an apparent photographic magnitude ^ 13.0 and an available redshift (from which the mann 1981).) (Adapted from Plates 2 and 6 of Holmberg 1937α. ) _1 distance is derived from the Hubble relation with H0 = 80 km s Mpc1). The solid points represent the observed counts, the dashed physical binary system. Continual multiple captures straight line represents a random distribution of optical companions, would result in the creation of groups with ever larger and the histogram refers to the physical companions. (Adapted from multiplicity. Moreover, if the encounter were actually so Fig. 1 of Holmberg 1969.)

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discussed by Holmberg more than four decades ago. 30 SlHQLt' NCBULAC To demonstrate in a clear way that a hyperbolic encounter between two galaxies converts translational energy into internal energy resulting in structural distor- tions of the individual galaxies, Holmberg (1941) / performed a very ingenious experiment. On a work- / · bench, he constructed two plane-parallel artificial galaxies each consisting of 37 light bulbs corresponding to 37 mass elejnents (Figs. 30 and 31). The candlepower of each light bulb is proportional to a given fraction of the mass of DOUBLE ANO MULTIPLE NEBULAE the galaxy; the candlepowers of the array of light bulbs could be adjusted to simulate any desired galactic mass distribution. At each given location, the perceived gravitational attraction from a given point source is proportional to the perceived light intensity, because both the light intensity and the gravitational force decrease as the square of the distance from the source. At the start of a simulated encounter between two galaxies, the radial mass distribution in the plane of each galaxy was chosen to VIRGO CLUSTER be independent of azimuthal angle such that the centripetal acceleration of each mass element could be derived analytically. Holmberg then simply replaced each light bulb, in turn, by a photoelectric cell which was turned successively in the directions +x, — x, +y, and —y to obtain the light intensity of the radiation from these directions. The output from the photocell was recorded by a galavanometer calibrated by comparing the com- Sb puted centripetal accelerations in one of the galaxies with the corresponding measured values. At later instants in the encounter, when each galaxy has a tidally deformed Fig. 28-The solid circles represent the observed frequency distribution of Hubble morphological type for (a) the single galaxies listed in the geometry with a gravitational field that is too complicated Shapley-Ames catalog within the interval of apparent photographic to derive analytically and too impractical to calculate with magnitude 10.1-12.0, (b) double and multiple galaxies, and (c) mem- the primitive calculating tools that were available at that bers of the Virgo cluster. The dashed-line fits to the data indicate that time, the galavanometer measurements alone were used there is a greater tendency for a galaxy to be a late-type spiral (Sb or Sc) to derive the χ and y components of the net gravitational and a smaller tendency for it to be an elliptical if it is single than if it is a member of a double or multiple system, ((a) The fit for the single galaxies force per unit mass or, correspondingly, the net accelera- represents the observational data directly; (b) the fit for double and tion of the mass element. By multiplying the acceleration multiple galaxies contains a correction for foreground/background galax- by a unit of time chosen to be small compared to an orbital ies derived by assuming that the types of optical companions have the period of either galaxy, the change in the velocity and same distribution as that for single galaxies (the open circles represent location of the mass element over this time interval could data corrected in this way); and (c) the fit for the Virgo cluster assumes that a large fraction of the Sc galaxies are temporary cluster members.) be calculated. Because these steps were performed for (Adapted from Fig. 7 of Holmberg 1940α. ) each of the 37 mass elements in both galaxies, their locations at the end of the chosen time interval were close as to be interpenetrating, then two spiral galaxies determined and all 74 light bulbs could then be placed at could merge and the captured internal energy could these locations, and the experiment could then be re- transform the newly bound system into an elliptical peated again and again until the end of the encounter. In galaxy. When Holmberg had these ideas, the age of the this way the orbits of each of the 37 mass elements in the universe (~ Hq'1) was still an open question. Today there two galaxies could be traced out during the encounter and is general agreement that Hq'1 lies in the range of 10 to 20 the effect of the encounter on the distribution of masses in billion years, and given the observed relative velocities each galaxy could be determined directly. The results of and separations of galaxies in general, very few would be two such experiments are shown in Figure 32. We ob- captured by this mechanism in the age of the universe, serve that the galaxies are considerably distorted as a except possibly within dense groups and clusters. For result of the encounter, and spiral arms are even created. these systems the current literature contains many theo- A significant effect of the tidal transfer of translational retical studies of effects of the tidal capture mechanism energy into internal energy has been demonstrated. Fi-

ts) Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System Fig. 29-The apparent distribution on the sky of the 827 double and multiple galaxies cataloged by Holmberg (1937α). The points representing the galaxies are distributed in equatorial coordinates on the celestial sphere reproduced in AitofF's equal-area projection. The dark areas represent Bubble's "zone of avoidance," and the dotted lines indicate the boundary of the apparent Milky Way. (Adapted from Fig. 3 of Holmberg 1937α. ) nally, an accounting of energies of the final configuration showed that, as expected, the translational kinetic energy of the two-body encounter had been decreased by an amount equal to the total increase of the internal energy of the two galaxies. As described above, by the year 1941 Holmberg had built an apparatus that automatically performed parts of a calculation which would have been tedious and exorbi- tantly time-consuming with the standard tools available at that time: tables of functions such as logarithms and rela- tively primitive devices for performing simple arithmetic calculations. Holmberg had in fact built an analog computer to perform calculations which today could be and are done with electronic computers. Alar and Juri Toomre (1972) and later others have simulated in great detail Fig. 30-The cross section of a light bulb and a photoelectric cell used in galaxy-galaxy encounters with modern digital computers Holmberg's analog computer simulation of two gravitationally interact- that incorporate setup conditions and reach conclusions ing galaxies which move past each other. (Adapted from Fig. 1 of Holmberg 1941.) which are remarkably similar to those obtained decades earlier by a pioneering Erik Holmberg. With Professor Holmberg s permission, this section of the first computer). The results show tidal distur- concludes with two quotes from a letter which I received bances that may lead to the formation of spiral arms; from him dated 24 August 1986. Professor Holmberg a table gives the resulting loss in kinetic energy, a wrote. loss that may change the encounter into a capture. I Among my smaller papers I especially like 1941.1 was very happy with these results. My happiness think it was the first time that tidal disturbances in returned when I read the paper by Toomre and galaxies at close encounters were studied by means Toomre from 1972 (Ap. /., 178, pp. 662-663): ". . . of laboratory models; it certainly was the first (and the ingenious and still astonishingly relevant opti- only?) time that gravitation was substituted by light cal-numerical Ν-body computations of Holmberg (we must remember that this was before the arrival (1941). .

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~~Í ν •s έ c0 1 « o~~

~~^1 Light intensity~~

Fig. 31-Further information on Kolmberg s analog computer simulation of two gravitationally interacting galaxies which move past each other. The top left-hand panel displays the empirical relation between galvanometer deflection and light intensity. The bottom left-hand panel displays the empirical relation between galvanometer deflection and the angle of incidence of the light from a light bulb onto a photoelectric cell. The dotted line is the theoretical cosine law. The top right-hand panel displays the coupling scheme for the experiment. The bottom right-hand panel displays the arrangement of the 37 light bulbs (comprising groups A and B) used to represent the mass distribution of a galaxy. (Adapted from Figs. 2 and 3 of Holmberg 1941.)

The second quote underlines the practical context in essentially complete to the limiting diameter of one arc which Holmberg did his work: minute on the blue prints. The newly derived data in- In the early days we usually had to work on a very clude the locational coordinates, major and minor diame- small budget. We certainly were very poor. In the ter, position angle of the major axis, and the Hubble 1941 paper, for instance, I had to do all the work in morphological type for each galaxy. The Lauberts survey person, including the rebuilding of the laboratory was also carried out at the Uppsala Observatory and room and making all the electrical installations. It includes corresponding data for galaxies and other objects was time-consuming, but gave some additional in the Southern Hemisphere derived from visual exami- spices and satisfaction when you found that you nation of a glass copy of the blue sensitive plates which could handle everything yourself. The scientist to- comprise the ESO(B) Atlas obtained with the 1-meter day is certainly a rather spoiled person, especially in Schmidt telescope of the European Southern Observa- the rich countries. tory at La Silla, Chile. The Nilson catalog contains data for 12,921 galaxies and the Lauberts catalog contains data for IV. Galaxy Catalogs 18,438 objects, primarily galaxies. The Nilson catalog has Two of the most useful catalogs of observational proper- been used extensively, for example, by radio astronomers ties of galaxies are the Uppsala General Catalogue of who observe the 21-cm line of neutral hydrogen, to select Galaxies by Peter Nilson (1973) and The ESO/Uppsala especially late-type galaxies for observational study. The Survey of the ESO (Β) Atlas by Andris Lauberts (1982). Lauberts catalog is presently being used, for example, The data contained in the Nilson catalog, which describe by Marc Davis and others who are obtaining an exten- galaxies primarily in the Northern Hemisphere, were sive optical survey of the redshifts of southern galaxies obtained from a visual examination of the first-edition for application to studies of the large-scale structure and prints of the Palomar Sky Survey and were designed to be kinematics of galaxies in space.

Fig. 32-The left-hand panel displays the tidal deformations corresponding to parabolic orbital motions, clockwise galactic rotations, and a distance of closest approach equal to the diameters of the two gravitationally interacting simulated galaxies moving past each other. Holmberg notes that the spiral arms point in the direction of the rotation. The right-hand panel displays the tidal deformations corresponding to parabolic orbital motions, counterclockwise rotations, and a distance of closest approach equal to the diameters of the two gravitationally interacting simulated galaxies moving past each other. Holmberg notes that the spiral arms point in the direction opposite to the rotation. (Adapted from Fig. 4 of Holmberg 1941.)

Erik Holmberg initiated the catalog work that culmi- responsibility of constructing suitable survey machines nated in both the northern survey of galaxies by Nilson and hiring suitable help. All of the plates have by and by and the southern survey by Lauberts. been checked by Holmberg, but the major part of the At the Moscow meeting of the International Astronom- work was done by his pupil Andris Lauberts, who proved ical Union in 1958, Holmberg was given the responsibil- himself a very reliable worker. At Holmberg's suggestion, ity (as chairman of a group also including Zwicky and the final catalog, with objects listed in order of right Vorontosov-Velyaminov) to examine the possibility of ascension, was published by ESO in 1982 under the starting work on a comprehensive galaxy catalog. During single name of Lauberts. (The two ESO astronomers on most of his years at Uppsala, Holmberg had an average of the project, Schuster and West, were not involved in the about 16 students working toward the Ph.D. He sug- gathering of galaxy data from the survey plates.) gested the project of constructing a homogeneous and extensive galaxy catalog to several of his students but V. Perplexing Problems warned them at the same time that they might not be able The clarity which Holmberg brought toward the reso- to carry it through. Peter Nilson, a very eificient and lution of questions such as "What are the kinds and sizes of prominent student of astronomy, was the only one who systematic errors in the visual measurement of the diame- volunteered. After several years of extremely painstaking ters and shapes of elliptical images?" and "What are the and apparently very homogeneous survey work, Nilson effects of the tidal transfer of translational kinetic energy was able to publish his finished catalog in 1973. into internal energy as a result of a close encounter be- In that same year (1973), the survey of galaxies on the tween two galaxies?" he also brought to the resolution of glass copies of the ESO Schmidt blue plates from Chile other more perplexing problems. was started by Holmberg at the request of A. Blaauw, By the late 1930s it was clear that the observed ra- then the general director of the European Southern Ob- dial velocity within the nuclear bulge of a spiral galaxy servatory. A formal written contract gave Holmberg the increases in rough proportion to the distance from the

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galactic center (Vr ~ r). Attempts in the literature to galaxy, mt, and the average apparent magnitude of its five explain this unexpected result included (1) a linear law of brightest stars, ms, is a constant for each sample of galaxies gravitational attraction, (2) a constant mass density, and selected according to morphological type: Sb, Sc, and Ir. (3) a large internal gravitational viscosity. None of these The constant, which is different for each of the three hypotheses was particularly appealing. (1) One does not morphological types, was derived by Hubble from obser- abandon Newtonian gravitation lightly. (2) The observed vational data. This result opened up the possibility of surface brightness of a nuclear bulge decreases with in- using ms as a distance indicator for a galaxy. Holmberg creasing radial distance, which is in conflict with a con- realized that Sb, Sc, and Ir galaxies have different average stant mass density if the ratio of mass to luminosity is absolute· luminosities such that < ms — mt > (which approximately independent of location (i.e., constant) corresponds to the average difference in absolute magni- within a nuclear bulge. (3) The theory of gravitational tudes, < Ms — Mt >) increases with increasing average viscosity, a concept taken by analogy from fluid dynamics, absolute luminosity of the sample. If this phenomenon was still in a preliminary stage, and it seemed question- occurs in general, then it could be understood as an effect able whether the mass density of a nuclear bulge is suffi- of random sampling from a universal frequency distribu- ciently large for gravitational viscosity to become nonneg- tion of absolute magnitudes of stars (a universal stellar-lu- ligible. Holmberg presented a fourth explanation. He minosity function), where the pool of stars being sampled demonstrated that when one pays sufficient attention to is proportional to the absolute luminosity of the galaxy. the relation between observed radial velocity and corre- Holmberg tested this idea by examining whether other sponding orbital velocity (Fig. 33) and the absorption of more local samples of stars of similar stellar population light within a galaxy, then Newton's law of attraction (Walter Baade's Population I) also follow the relation results in more-or-less linear relations which within the discovered for spiral and irregular galaxies (Holmberg uncertainties are consistent with the observed relation Vr 1950&). The local samples which Holmberg considered ~ r (Holmberg 1939d). were open star clusters and spherically concentric vol- Hubble (1936fc) discovered that < ms — mt >, the umes of stars of increasing radius centered on the Sun. In average difference between the apparent magnitude of a Figure 34 he demonstrated that all of these samples of stars (local and extragalactic) fall on a universal relation, so that its nature can be understood as an effect of random sampling from a universal luminosity function of Popula- tion I stars. Perhaps the most perplexing astrophysical problem of our time concerns the gravitational dynamics of individual and systems of galaxies. It is variously known as the "mass discrepancy", "virial mass discrepancy", "missing mass", "missing light", "hidden mass", "dark matter", or "Tl/L" problem. Historically, this problem was first recognized in a study of the Coma cluster of galaxies by Zwicky (1933) (see also Zwicky 1957); since then, its presence became revealed for numerous systems of galaxies and more recently for individual galaxies through the unexpected discovery that their outer rotation curves are essentially flat. The following discussion refers to clusters of galaxies. In brief, the mass of a cluster that is calculated by applying Newtonian dynamics to the observed kinematical data is typically ~ 10 times larger than the sum of the masses of the individual galaxies and known inter- galactic material. The kinematical data corrected for opti- Fig. 33-Schematic representation of the nuclear bulge of a spiral cal companions consist of the unweighted or luminosity- galaxy. The plane of the figure is assumed to coincide with the principal plane of the galaxy. An x,y coordinate frame is shown, where the x- axis is weighted radial velocities and separations between assumed to coincide with the line of sight. The radius of the circle is put cluster members seen projected on the sky. The mass of equal to unity. A cylinder is drawn (shaded area) with its axis along the each individual galaxy is estimated from its luminosity line of sight. The observed radial velocity is the integration over the combined with a ratio of mass to luminosity that is gener- cylinder of the radial component of the galactic rotational velocity of ally inferred from its color (cf. Fig. 18) or morphological each mass element at polar coordinates r, β weighted by its luminosity density multiplied by the fraction which remains unabsorbed by the type calibrated through a sample of galaxies whose masses obscuring layer that lies between the mass element and the observer. have been derived directly from internal kinematical (Adapted from Fig. 1 of Holmberg 1939íi. ) data.

Fig. 34-The relation between the average absolute photographic magnitude of the five most luminous observed stars in an extended object, Ms, and the absolute photographic magnitude of the extended object, Mt. The open and closed points in the right-hand part of the diagram refer to galaxies and the points in the left-hand part refer to open clusters; from left to right, the triangles refer to the stars in nearby spheres of increasing effective radius centered on the Sun. (Adapted from Fig. 9 of Holmberg 1950¾. )

In a paper (Holmberg 1961α) presented at the Confer- redshifts that Holmberg discovered. Through an exten- ence on Instability of Systems of Galaxies in Santa Bar- sive analysis of the observed redshift, V, and estimated bara, California, in 1961, Holmberg presented a detailed surface brightness of the nuclear bulge, m', of galaxies in analysis of the observational material available for the the Virgo cluster, small groups, double systems, and even Virgo cluster of galaxies which led him to conclude that single galaxies in the "general field", Holmberg (1961&) the large virial mass of the cluster can be satisfactorily concludes that the observed redshift of a galaxy in a explained as a result of orbital motions within the sub- multiple system tends to depend on the surface bright- groups of the cluster, systematic errors in the measured ness of its nuclear bulge (Fig. 35). On the average, the redshifts, and disturbance by optical (background/fore- change in V, AV, is related to the change in m', Am', by ground) members. Before the presentation of this paper, ΔΎΙΔητ' = 79 ± 12 km s-1, and the confidence level of the tremendous potential importance of the first two of this result corresponds to the 6.6 level of statistical signifi- these effects was not commonly appreciated. More recent cance. Because for the Virgo cluster the observed km' ~ studies take account of all of these effects, but even when 7.5 magnitudes, the range of the systematic effect for this this has been properly done the problem of the "missing system is 600 km s_1, equal to the rms deviation of the mass" remains unresolved. To my knowledge, no one has radial velocity of a member galaxy from the systemic ever reexamined Holmberg's arguments in order to iden- radial velocity of the cluster. In trying to find an explana- tify the source or sources of the different conclusions tion for this empirical result with its very large statistical reached. weight, Holmberg (1961¾) concludes ". . . we have a Related to the above discussion is a systemic effect in priori to consider two different possibilities, (1) the exis-

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~~V -905~~

Fig. 35-Relation between V — 905 km s_1 (where V is the radial velocity derived from the observed redshift) and the estimated surface brightness of the nuclear bulge, m', as derived for 76 galaxies in the field of the Virgo cluster. Twelve suspected optical (background/foreground) members are denoted by crosses. The average radial velocity in each of five equally populous classes are represented by open circles, where the radius of a circle is equal to its rms error. (Adapted from Fig. 1 of Holmberg 1961¿>. ) tence of a hitherto unknown magnitude effect (or possibly VI. Epilogue mass effect), referring to the basic parameters of the To summarize this essay I present a quote from Doppler law, or (2) the presence of systematic errors in Mitchell Struble (with his kind permission) which he the observational results." Possibility (2) seemed to spontaneously contributed when I mentioned to him the Holmberg to offer the most reasonable explanation. At present work. Struble remarked: "Erik Holmberg cer- the 15th symposium of the International Astronomical tainly ranks with Edwin Hubble and Fritz Zwicky as one Union held at the Santa Barbara Campus of the Univer- of the distinguished extragalactic researchers of the cen- sity of California from August 10 to 12, 1961, Holmberg tury. He invented, from the ground up, techniques and presented his empirical result and suggested that it might possibly be a consequence of a heretofore unsuspected measures that have become standards in the profession, systematic error in the optical determination of radial which he has consistently and systematically applied to velocities from spectrographic measurements. This sug- his own data as well as to the data of others. We are gestion immediately prompted Nickolas U. Mayall, Mar- indebted to Holmberg for his systematic measurements garet Burbidge, Ch. Fehrenbach, Rudolf Minkowski, of galaxy diameters, magnitudes, and inclinations of spiral Thornton Page, Morton Roberts, and D. S. Evans to galaxies; separations and position angles of the compo- describe results of extensive checks that had been made nents of binary galaxies; and concepts like mass/luminos- which established the absence of any large systematic ity ratio, obtaining absolute luminosity estimates for effect in published optical radial velocities, and to provide galaxies from the correlation between the magnitude of a other evidence which suggested that possibility (2) was parent galaxy and the magnitude of its brightest stars, not viable. If that is indeed the case, then possibility (1) finding masses of double galaxies from orbital motions, must be examined more carefully, and this in fact has and estimating the internal reddening of spiral galaxies been done over the last two decades, but the published using our Galaxy (the Milky Way) as a model. He pio- results have been bewilderingly confusing. Other possi- neered many original ideas and inventive analyses which ble explanations should also be examined. What Holm- were often not within the general trends of extragalactic berg's result signifies is still unclear. However, it would research, but now are accepted as viable, such as galaxy make excellent sense for a skilled researcher to repeat his mergers (1937, 1940) and effects of subclustering on virial entire analysis both with the data that Holmberg applied masses of clusters of galaxies (1961). One can confidently and with the entire data base available to present-day take his results as correct because of the care invested in researchers. Such an investigation is likely to provide the the data collection and methods of analysis; his papers are needed clarification. models to follow."

Accordingly, it is suggested that the university curricu- REFERENCES lum in extragalactic astronomy for advanced undergradu- Baade, W. 1951, Michigan Obs. Pub., 10, 7. ate and graduate students would be significantly im- 1963, in Evolution of Stars and Galaxies, ed. C. Payne- Gaposchkin (Cambridge, MA: Harvard University Press), pp. 96- proved if (preferably in the fall semester) a course were 185. taught directly from the research papers of Erik Holm- Holmberg, Ε. 1934-81¾ (listed below under "Publications of Erik berg (Table I, pp. 948-49) supplemented by readings Holmberg"). Hubble, E. 1926, Ap. /., 64, 321. selected from the other references in the present essay. 1934, Ap./., 79, 8. The instructor would provide a reading assignment, and 1936a, The Realm of the Nebulae (New Haven: Yale University each student would be required to study it sufficiently Press). (Also, 1958 (New York: Dover).) thoroughly so that he or she would be able to deliver a 19366, Ap./., 84, 158. Lauberts, A. 1982, The ESO/Uppsala Survey of the ESO (Β) Atlas lecture or lead a discussion on it. In keeping with an (Munich: European Southern Observatory). educational practice sometimes used today, at the start of Lundmark, K. 1927, Uppsala Medd., No. 30. each class, the name of each student and the instructor 1931, Lund Obs. Cire., 2, 30. Nilson, P. 1973, Uppsala General Catalogue of Galaxies {Uppsala Astr. would be entered into a hat from which a random drawing Obs. Ann., Vol. 6). would provide the name of the lecturer or discussion Quinn, P. J., and Goodman, J. 1986, Ap. /., 309, 472. leader. The readings, the lectures, and the discussion Sandage, A. 1961, The Hubble Atlas of Galaxies (Washington, DC: Carnegie Institution of Washington). generated from them would provide the students with Sandage, Α., and Tammann, G. Α. 1981, A Revised Shapley-Ames abundant educational material. Any student with a good Catalog of Bright Galaxies (Washington, DC: Carnegie Institution understanding of undergraduate statistics, calculus, and of Washington). physics would benefit from such a course. A historical Toomre, A., and Toomre, J. 1972, Ap. /., 178, 623. Zwicky, F. 1933, Helv. Phys. Acta, 6, 110. introduction to (1) the forward flow of increased under- 1957, Morphological Astronomy {ΒβτΜη: Springer-Verlag). standing in extragalactic astronomy and (2) the basics of Zwicky, F., Herzog, Ε., Karpowicz, M., Kowal, C. T., and Wild, P. quality research technique would thereby be presented 1960-68, Catalogue of Galaxies and of Clusters of Galaxies, Vols. to the student in a meaningful way. Ideas generated in the 1-6 (Pasadena: California Institute of Technology). course could lead (beginning in the spring term) to timely research which could include practical theoretical and TABLE I observational projects. The latter could make use of direct Selected Galaxy Publications of Erik Holmberg observations of cosmic entities as registered on photographic reproductions of the Palomar Observatory Sky (Chronological Order) Survey and the European Southern Observatory (B) Atlas of the Southern Sky. 1937a, Lund Obs. Ann., 6, 1-173, "A Study of Double and Multiple Galaxies Together with Inquiries I am grateful to Drs. Jeremy Goodman and George Into Some General Metagalactic Problems with Lake for the discussion at the Institute of Advanced Study an Appendix Gontaining a Gatalogue of 827 Dou- which stimulated the undertaking of this project; to Dr. ble and Multiple Galaxies '. John Bahcall for his consistently strong efforts over more 1939c, Lund Obs. Medd. II, 100, 1-19, = Lunds Univ. than a decade toward maintaining and enhancing the Ârsskr., 35, Nr. 6, "On the Existence of a System- quality of the astrophysical research environment at atic Error in the Estimated Magnitudes of Galax- Princeton; to Drs. Peter Quinn, Mitchell Struble, and ies". Rene Walterbos for helpful discussion; to Dr. Thornton 1939d, M.N.R.A.S., 99, 650-661, = LundObs. Medd. I, Page for coauthoring with me the popular article 154, "On the Interpretation of the Spectroscopi- "Galaxies and Erik Holmberg" to appear in the magazine cally Observed Rotations of Galaxies". Mercury of the Astronomical Society of the Pacific; and to 1940a, Ap. /., 92, 200-234, = Mt. Wilson Obs. Gontr. 633, "On the Glustering Tendencies Among the Dr. Erik Holmberg for kindly sending me illuminating Nebulae". information not found in previous literature. Both Drs. 1940^, Festschrift für Elis Strömgren (Kopenhagen, Ε. Holmberg and Page carefully read prior versions of this Munksgaard), 114-122, "On the Relation Be- manuscript and contributed valuable corrections and sug- tween Luminosity and Mass for Stellar Systems". gestions. This study was done during a 1985-86 visit by 1941, Ap. J., 94, 385-395, "On the Clustering Tenden- HJ.R. at the Institute for Advanced Study; the hospital- cies Among the Nebulae. II. A Study of Encoun- ity of the IAS faculty and members is gratefully acknowl- ters Between Laboratory Models of Stellar Sys- edged. This work is supported in part by the National tems by a New Integration Procedure". Science Foundation under grant AST 85-13087. 1945, Lund. Obs. Medd. II, 114, 1-27, = Lunds Univ.

~~© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System EXTRAGALACTIC RESEARCH OF ERIK HOLMBERG 949~~

~~TABLE I (Continued)~~

Ärsskr., 41, Nr. 6, = Kungl. Fysiografiska Säll- Galactic Research, Aug. 10-12, 1961, ed. G. C. skapets Handlingar, 56, Nr. 6, "A Photographic McVittie (Macmillan: New York), 187-193, Photometry of the Spiral Nebulae NGC 891 and "Statistics of Double and Multiple Systems". NGC 3623". 1962&, in I.A.U. Symposium 15, Problems of Extra- 1946, Lund Obs. Medd. II, 117, 1-82, = Lunds Univ. Galactic Research, Aug. 10-12, 1961, ed. G. C. Arsskr., 42, Nr. 2, = Kungl. Fysiografiska Säll- McVittie (Macmillan: New York), 401-410, "A skapets Handlingar, 57, Nr. 2, "On the Apparent Systematic Effect in Measured Red-shifts". Diameters and the Orientation in Space of Extra- 1964, Arkiv för Astronomi, 3, 387-438, = Uppsala Obs. galactic Nebulae". Medd., 148, "A Study of External Galaxies". 1947, Lund Ohs. Medd. II, 120, 1-62, = Lunds Univ. 1969, Arkiv för Astronomi, 5, 305-343, = Uppsala Obs. Arsskr., 43, Nr. 3, = Kungl. Fysiografiska Säll- Medd., 166, "A Study of Physical Groups of skapets Handlingar, 58, Nr. 3, "On the Absorp- Galaxies". tion in the Spiral Nebulae". 1971, in ESO Symposium 1969, The Magellanic Clouds, 1950a, Lund Obs. Medd. I, 170, 1-11, = Kungl Fys- ed. A. B. Muller (Dordrecht, Holland: D. iografiska Sällskapets Förhandlingar, 20, Nr. 12, Reidel), 109-113, "Satellites of Stellar Systems in "A Study of the Absorption in NGC 5195". General". 1950fc, Lund Obs. Medd. II, 128, 1-56, - Lunds Univ. 1972, in I.A.U. Symposium 44, External Galaxies and Arsskr., 46, Nr. 5, = Kungl. Fysiografiska Säll- Quasi-stellar Objects, Aug. 10-14, 1970, ed. D. skapets Handlingar, 61, Nr. 5, "A Photometric S. Evans, (Dordrecht, Holland: D. Reidel), vii, Study of Nearby Galaxies". "Introductory Remarks". 1952a, Lund Obs. Medd. I, 180, 1-13, = Kungl. Fys- 1974a, Astr. Αρ., 35, 121-141, "Distribution of Clusters iografiska Sällskapets Förhandlingar, 22, Nr. 5, of Galaxies as Related to Galactic Absorption". "On the Masses and Luminosities of Extragalactic 1974&, Astr. Ap. Suppl, 18, 463-489, "The ESO/Upp- Nebulae". sala Survey of the ESO (Β) Atlas of the Southern 1954, Lund Obs. Medd. I, 186, 1-20, = Kungl. Fys- Sky. Γ (with A. Lauberts, Η.-Ε. Schuster, andR. iografiska Sällskapets Förhandlingar, 24, Nr. 11, M. West). "On the Masses of Double Galaxies". 1975fc, in Stars and Stellar Systems, Vol. 9, Galaxies and 1958a, Lund Obs. Medd. II, 136, 1-103, = Lunds Univ. the Universe, ed. A. Sandage, M. Sandage, and J. Ârsskr., 54, Nr. 1, = Kungl. Fysiografiska Säll- Kristian (Chicago: University of Chicago Press), skapets Handlingar, 69, Nr. 1, "A Photographic 123-157, "Magnitudes, Colors, Surface Bright- Photometry of Extragalactic Nebulae. I. A Study ness, Intensity Distributions, Absolute Lumi- of Integrated Magnitudes and Colors of 300 nosities and Diameters of Galaxies". Galaxies". 1976, in Reports on Astronomy (Transactions of the 1961a, A.J., 66, 620-628, "On the Dynamics of the Virgo International Astronomical Union, 16A, Part 3), Cluster", in Conference on the Instability of Sys- ed. G. Contopoulos (Holland-Boston: D. Reidel), tems of Galaxies, Santa Barbara, CA, Aug. 8-9, 1-35, "Galaxies" (with P. Nilson, G. A. Tam- 1961, ed. J. Neyman, T. Page, and E. Scott, A.J., mann, H. Arp, Β. A. Vorontsov-Velyaminov, Β. 66, 533-636, - Van Vleck Obs. Pub., 4. Ε. Westerlund, Η. D. Abies, and P. Pismis). 1961fc, Arkiv för Astronomi, 2, 559-582, = Uppsala 1981fc, Astr. Ap. Suppl, 46, 311-346, "The ESO/Upp- Obs. Medd., 138, "A Systematic Eifect in the sala survey of the ESO (Β) Atlas of the Southern Redshifts of Extragalactic Nebulae". Sky, IX" (with A. Lauberts, Η.-Ε. Schuster, and 1962a, in I.A.U. Symposium 15, Problems of Extra- R. M. West).

~~PUBLICATIONS OF ERIK HOLMBERG (Chronological Order)~~

1934, Lund Obs. Medd. II, 71, 1-25, = Kungl Svenska and Multiple Galaxies Together with Inquiries Vetenskapsakademiens Handlingar III, 13, Nr. 7, into Some General Metagalactic Problems with "Some Statistical Investigations of Eclipsing Bi- an Appendix Containing a Catalogue of 827 Dou- naries". ble and Multiple Galaxies". 1937a, Lund Obs. Ann., 6, 1-173, "A Study of Double 1937fc, Lund Obs. Medd. I, 146, 1-7, "Eine Bemerkung

zu den Spektralvergleichungen in der Bergedor- and Statistical Investigation of 1119 Stars in the fer Spektral-Durchmusterung". Hyades Region". 1938a, Lund Obs. Medd. II, 92, 1-23, "Invisible Com- 1945, Lund. Obs. Medd. II, 114, 1-27, = Lunds Univ. panions of Parallax Stars Revealed by Means of Arsskr., 41, Nr. 6, = Kungl. Fysiografiska Modern Trigonometric Parallax Observations". Sällskapets Handlingar, 56, Nr. 6, "A Photo- 1938&, Lund Ohs. Medd. II, 97, 1-68, "Accidental and graphie Photometry of the Spiral Nebulae NGC Systematic Errors of Modern Trigonometric Par- 891 and NGC 3623". allaxes". 1946, Lund Obs. Medd. II, 117, 1-82, = Lunds Univ. 1938c, Lund Obs. Medd. II, 98, 1-15, "Positions of 110 Arsskr., 42, Nr. 2, = Kungl. Fysiografiska Sälls- Stars Mainly Eclipsing Binaries and Reference kapets Handlingar, 57, Nr. 2, "On the Apparent Stars of Ν ebulae". Diameters and the Orientation in Space of Extra- 1939a, Cassiopeia (Lund), 1939, 40-52, "Pâ jakt efter en galactic Nebulae". förmörkad sol" ("Hunting for an Eclipsed Sun"). 1947, Lund Obs. Medd. II, 120, 1-62, = Lunds Univ. Report by the writer as leader of the Swedish Ârsskr., 43, Nr. 3, = Kungl. Fysiografiska Sälls- solar eclipse expedition to Caucasus, USSR, in kapets Handlingar, 58, Nr. 3, "On the Absorption 1936. in the Spiral Nebulae". 1939b, Lund Obs. Medd. I, 152, 1-13, "A Statistical 1948, Lund Obs. Medd. I, 164, 1-11, "The Mass Ratios Problem of Integration". of Castor and 70 Ophiuchi". 1939c, Lund Obs. Medd. II, 100, 1-19, = Lunds Univ. 1949, Cassiopeia (Lund), 1949, 15-54, "Lundensisk As- Arsskr., 35, Nr. 6, "On the Existence of a System- tronomi Under ett Sekel" ("Astronomy at Lund atic Error in the Estimated Magnitudes of Galax- During the Last Century"). Review of the astro- ies". nomical research work at the Lund observatory 1939á, M.N.R.A.S., 99, 650-661, = Lund Obs. Medd. since 1850. I., 154, "On the Interpretation of the Spectro- 1950a, Lund Obs. Medd. I, 170, 1-11, = Kungl. Fys- scopically Observed Rotations of Galaxies". iografiska Sällskapets Förhandlingar, 20, Nr. 12, 1939c, Zeitschrift für Astrophysik, 18,132-139, = Lund "A Study of the Absorption in NGC 5195". Obs. Medd. 1,151, "The Problem of Star Chains". 1950¿, Lund Obs. Medd. II, 128, 1-56, = Lunds Univ. 1940α, Αρ. /., 92, 200-234, = Mí. Wilson Obs. Contr., Ârsskr., 46, Nr. 5, = Kungl. Fysiografiska Sälls- 633, "On the Clustering Tendencies Among the kapets Handlingar, 61, Nr. 5, "A Photometric Nebulae". Study of Nearby Galaxies". 1940fc, Festschrift für Elis Strömgren (Kopenhagen, Ε. 1951, Lund Obs. Medd. I, 175, 1-13, = Kungl. Fys- Munksgaard), 114-122, "On the Relation Be- iografiska Sällskapets Förhandlingar, 20, Nr. 18, tween Luminosity and Mass for Stellar Systems". "On the Luminosity Function of Type I Stars". 1941, Ap. J., 94, 385-395, "On the Clustering Tenden- 1952a, Lund Obs. Medd. I, 180, 1-13, = Kungl. Fys- cies Among the Nebulae. II. A Study of Encoun- iografiska Sällskapets Förhandlingar, 22, Nr. 5, ters Between Laboratory Models of Stellar Sys- "On the Masses and Luminosities of Extragalactic tems by a New Integration Procedure". Nebulae". 1942, Pub. A.A. S., 10, 258-259, "On the Existence of a 1952b, Kungl. Fysiografiska Sällskapets Förhandlingar, Third Component in the System 70 Ophiuchi" 22,1-4, "Walter Gyllenberg 1886-1952". Biogra- (with D. Reuyl). phy in Swedish. 1943a, A.J., 50, 100-105, "A Determination of the Mass 1954, Lund Obs. Medd. I, 186, 1-20, = Kungl. Fys- Ratios and Parallaxes of Castor and 70 Ophiuchi iografiska Sällskapets Förhandlingar, 24, Nr. 11, from Photographs taken with the Leander Mc- "On the Masses of Double Galaxies". Cormick 26-inch Refractor". 1955, Populär Astronomisk Tidskrift, 36, 1-10, "Om 1943b, Ap. /., 97, 41-45, = McCormick Obs. Pub., 9, den Selektiva Absorptionen i Vintergats Sys- Part 8, "On the Existence of a Third Component temet" ("On the Selective Absorption in the in the System 70 Ophiuchi" (with D. Reuyl). Milky Way"). 1943c, Ciel et Terre, 59, 284-287, "Sur le Problème des 1958a, Lund Obs. Medd. II, 136, 1-103, = Lunds Univ. chaines d'étoiles". Ârsskr., 54, Nr. 1, = Kungl. Fysiografiska Sälls- 1944a, Ciel et Terre, 60, 49-50, "Sur le Problème des kapets Handlingar, 69, Nr. 1, "A Photographic Nébuleuses Extragalactiques Doubles et Multi- Photometry of Extragalactic Nebulae. I. A Study ples". of Integrated Magnitudes and Colors of 300 1944&, Lund Obs. Medd. II, 113, 1-107, = Lunds Univ. Galaxies". Ârsskr., 41, Nr. 2, = Kungl. Fysiografiska Salis- 1958fc, Kungl. Fysiografiska Sällskapets Förhandlingar, kapets Handlingar, 56, Nr. 2, "A Photometric 28, 1-8, "Knut Lundmark 1889-1958", Biogra-

~~© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System EXTRAGALACTIC RESEARCH OF ERIK HOLMBERG 951~~

phy in Swedish. 1974c, Astr. Ap. Suppl., 18, 491-509, "The ESO/Upp- 1959a, Nordisk Astronomisk Tidsskrift 1959, 1-7, 37- sala Survey of the ESO (Β) Atlas of the Southern 41, "Universums Expansion" ("The Expansion of Sky. Π" (with A. Lauberts, Η.-Ε. Schuster, and the Universe"). R. M. West). 1959¿, Populär Astronomisk Tidskrift, 40, 81-91, "Om 1975a, Astr. Ap. Suppl, 22, 327-402, "The ESO/Upp- Universums Uppbyggnad" ("On the Structure of sala Survey of the ESO (Β) Atlas of the Southern the Universe"). Sky. Ill" (with A. Lauberts, Η.-Ε. Schuster, and 1961a, A.J., 66, 620-628, "On the Dynamics of the Virgo R. M. West). Cluster", in Conference on the Instability of Sys- 1975b, in Stars and Stellar Systems, Vol. 9, Galaxies and tems of Galaxies, Santa Barbara, CA, Aug. 8-9, the Universe, ed. A. Sandage, M. Sandage, and J. 1961, ed. J. Neyman, T. Page, and E. Scott, A./., Kristian (Chicago: University of Chicago Press), 66, 533-636, = Van Vleck Obs. Pub., 4. 123-157, "Magnitudes, Colors, Surface Bright- 1961&, Arkiv for Astronomi, 2, 559-582, = Uppsala ness, Intensity Distributions, Absolute Lumi- Obs. Medd., 138, "A Systematic Effect in the nosities, and Diameters of Galaxies". Redshifts of Extragalactic Nebulae". 1976, Reports on Astronomy ( Τ ransactions of the Inter- 1962a, in I AU Symposium 15, Problems of Extra-Galac- national Astronomical Union, 16A, Part 3), ed. tic Research, Aug. 10-12,1961, ed. G. C. McVit- G. Contopoulos (Holland-Boston: D. Reidel), tie (Macmillan: New York), 187-193, "Statistics of 1-35, "Galaxies" (with P. Nilson, G. A. Tam- Double and Multiple Systems". mann, H. Arp, Β. A. Vorontsov-Velyaminov, Β. 1962b, in 1AU Symposium 15, Problems of Extra-Galac- Ε. Westerlund, Η. D. Abies, and P. Pismis). tic Research, Aug. 10-12, 1961, ed. G. C. McVit- 1977, A^r. Ap. Suppl, 27, 295-342, "The ESO/Upp- tie (Macmillan: New York), 401-410, "A System- sala Survey of the ESO (Β) Atlas of the Southern atic Effect in Measured Red-shifts". Sky. IV" (with A. Lauberts, Η.-Ε. Schuster, and 1964, Arkiv for Astronomi, 3, 387-438, = Uppsala R. M. West). Obs. Medd., 148, "A Study of External Galaxies". 1965, Populär Astronomisk Tidskrift, 46, 24-40, "Om 1978a, Astr. Ap. Suppl, 31, 15-54, "The ESO/Uppsala Survey of the ESO ( ) Atlas of the Southern Sky. Galaxernas Utveckling" ("On the Evolution of Β V" (with A. Lauberts, .- . Schuster, andR. M. Galaxies"). Η Ε 1969, Arkiv for Astronomi, 5, 305-343, = Uppsala West). Obs. Medd., 166, "A Study of Physical Groups of 1918b, Astr. Ap. Suppl, 34, 285-340, "The ESO/Upp- Galaxies". sala Survey of the ESO (Β) Atlas of the Southern 1971, in ESO Symposium 1969, The Magellanic Clouds, Sky. VI" (with A. Lauberts, Η.-Ε. Schuster, and ed. A. B. Muller (Dordrecht, Holland: D. Rei- R. M. West). del), 109-113, "Satellites of Stellar Systems in 1980, Astr. Ap. Suppl, 39, 173-195, "The ESO/Upp- General". sala Survey of the ESO (Β) Atlas of the Southern 1972, in 1AU Symposium 44, External Galaxies and Sky. VII" (with A. Lauberts, Η.-Ε. Schuster, and Quasi-stellar Objects, Aug. 10-14, 1970, ed. D. R. M. West). S. Evans (Dordrecht, Holland: D. Reidel), vii, 1981a, Astr. Ap. Suppl, 43, 307-329, "The ESO/Upp- "Introductory Remarks". sala Survey of the ESO (Β) Atlas of the Southern 1974a, Astr. Αρ., 35, 121-141, "Distribution of Clusters Sky. VIII" (with A. Lauberts, Η.-Ε. Schuster, of Galaxies as Related to Galactic Absorption". and R. M. West). 19742?, Astr. Ap. Suppl, 18, 463-489, "The ESO/ Upp- 1981&, Astr. Αρ. Suppl, 46, 311-346, "The ESO/Upp- sala Survey of the ESO (Β) Atlas of the Southern sala Survey of the ESO (Β) Atlas of the Southern Sky. I" (with A. Lauberts, Η.-Ε. Schuster, andR. Sky, IX" (with A. Lauberts, Η.-Ε. Schuster, and - M. West). R. M. West).