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

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

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