Publications OFTHE Astronomical Society OFTHE Pacific 100:1071-1075, September 1988

SUPPLEMENTAL TOPICS ON VOIDS

HERBERT]. ROOD P.O. Box 1330, Princeton, New Jersey 08542 Received 1988 April 9, revised 1988 May 16

ABSTRACT 1. In the spring of 1975 on Kitt Peak, the cosmological significance of superclusters and voids was clearly recognized by the small group of astronomers who were completing redshift surveys that first demonstrated the existence of the Coma supercluster and void. 2. A redshift survey to a faint magnitude limit over a large region of the sky can include, e.g., (a) all galaxies, (b) the galaxies in representative probes, or (c) randomly-sampled galaxies. Each of these map-making strategies has its own special virtues. 3. Redshifts for the Abell clusters and very distant objects are being measured from spectra of (a) individual galaxies recorded electronically and (b) several galaxies recorded simultaneously by means of (1) multiobject spectroscopy via multiaperture or fiber-optic coupling devices, (2) analysis of features on objective-prism spectral plates of Schmidt telescopes, and (3) computer synthesiza- tion of spectra from observed multicolor CCD images of a field. 4. A beautiful consistency now exists between the observed kinematics of the solar system and the predictions of Newtonian/general relativistic dynamics. However, a century ago serious discrepancies existed, and explanations were sought in terms of hypothesized missing mass and non-Newtonian dynamics. Today, the same approach is being applied toward resolving discrepancies apparent in extragalactic dynamics. 5. Future observational research on voids will include redshift surveys of galaxies and other objects to very faint magnitude limits. These objects will be selected from catalogs of data measured off direct-survey photographic plates by means of an automated plate scanner. Key words: galaxies-voids-superclusters

I. Preface servational efforts with strong theoretical connections. For a few days in 1975 the totality of active research on An article entitled "Voids ', scheduled for publication in voids was contained within a microcosm of two simulta- Volume 26 of Annual Review of Astronomy and As- neous observing runs by these five astronomers on Kitt trophysics (Rood 1988), reviews our current knowledge of Peak in Arizona. voids in the space distribution of galaxies. The length of Section III describes various strategies to obtain exten- the submitted manuscript was estimated to be 64 printed sive surveys of redshifts of galaxies that have been applied pages, more than twice the allotted 30 pages. By re- toward enhancing our knowledge of large-scale structure moving material judged to be somewhat parenthetical to in the cosmos. This section also provides references to the overall discussion, the editors were able to trim the galaxy redshift surveys in the constellations Perseus, final text by about 20% and obtain a more tightly-focused Pisces, Ursa Major and Lynx, Hydra-Centaurus, Pisces- review. The editorially-removed material is primarily Cetus, and Corona Borealis. (Rood (1988) concentrates seven self-contained sections (five are reproduced below) exclusively on galaxy redshift surveys in Coma, Hercules, that could interest researchers on voids and mass/time- and Bootes.) Finally, the researcher is cautioned to not scale problems in extragalactic dynamics. mistake an apparent void at small Galactic latitude (likely Section II is a historical tidbit of some interest. Today to be a dust void) for a physical void in the galaxy distribu- the existence and cosmological significance of superclus- tion. ters and voids is widely recognized. In 1975 the existence Section IV describes several techniques to determine of superclusters was controversial and, so far as I have redshifts of individual and especially ensembles of very been able to determine, only five astronomers knew that faint galaxies that could play significant roles in the red- both superclusters and voids exist and that their impor- shift mapping of ever-more-distant regions of the uni- tance for cosmology would soon cause a transformation verse. from modest observational studies to very extensive ob- Section V describes (a) the beautiful consistency that

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© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 1072 HERBERT J. ROOD now exists between the observed kinematics of the solar tra and then deposits these data on magnetic tape for system and the predictions of Newtonian/general rela- computer utilization—a technique that was soon to revo- tivistic dynamics and (b) the serious problems that existed lutionize optical spectroscopy (Shectman and Hiltner a century ago in the dynamics of the solar system. The 1976). latter appear fundamentally very similar to problems that The midday discussion revealed that (a) Tifit, Gregory, exist today in extragalactic dynamics. The resolution of and Thompson, and (b) Chincarini and I were working on these mass/time-scale problems is prerequisite to a similar projects related to large-scale structure in the definitive theoretical understanding of voids. distribution of galaxies. (Tifft s entrance into this field is a Section VI provides references that discuss anticipated spinoff of his interest in properties of galaxies in a central future redshift surveys and the automated procurement region of the Coma cluster, and his discovery that the of data on galaxies and other objects from direct photo- initial plot of redshift vs. nuclear magnitude for these graphic plates. galaxies exhibits an unexpected band-like structure (Tifft The sections herein are supplements to sections in the 1972). Because (1) the statistical significance of this and review article by Rood (1988) according to the following related structure (Tifft 1982) has decreased with the appli- correspondences: (11:1.3), (111:2.2.1), (IV:2.2.1), (V:4), cation of more accurate data (Rood 1982; Schneider 1987), and (VI :5). and (2) the velocity dispersion (i.e., temperature) of the moving electrons required to produce the observed ther- Π. A Hidden Paradigm mal hremsstrahlung X-ray spectrum of a cluster is com- I believe that the year 1975, more than any other, parable to the velocity dispersion of its galaxies derived by marks the epoch when a basic enlightenment occurred the Doppler interpretation of redshift (Mushotzky 1984; concerning the space distribution of galaxies and when Rood and Dickel 1979; Rood 1982), the effect is unlikely interest shifted decisively away from surface distributions to signal "dramatic changes in our concepts of large-scale and toward the direct results provided by means of homo- gravitation" (Tifft and Cocke 1987), but it may contain geneous redshift surveys. Some evidence follows. information about substructure in systems of galaxies.) Ever since I had seen N. Mayall's plot of radial velocity Chincarini and I described our work on the structure of versus angular distance from the center of the Coma the outskirts of the Coma cluster (reported by Chincarini cluster for a homogeneous sample of 50 galaxies (Mayall and Rood 1975). Gregory and Thompson described the 1960, Fig. 2; depicted by Rood 1988, Fig. 1), I had evidence suggesting to them that the Coma cluster and assumed that the apparent field of galaxies in a surface A1367 are components of a common supercluster, which distribution is the result of a superposition of discrete prompted them to submit a proposal to KPNO for observ- groups; however, I did not realize (as G. Chincarini did) ing time to obtain spectrograms to construct a homoge- that it was widely believed that groups were superim- neous sample of redshifts that would test this hypothesis. posed on a uniform field of galaxies, so I passed up an (Results of this study are described by Gregory and opportunity to be a coauthor on the 1975 paper describing Thompson (1978) and reviewed by Oort (1983).) Tifft then the observational evidence for universal segregation of related that a flash of insight recently caused him to redshifts and the absence of a uniform field (Chincarini realize that the complete apparent-magnitude-limited and Martins 1975). Much later, Focardi, Maraño, and redshift survey of galaxies in a 36-square-degree region Vettolani (1983a) reached similar realizations. centered on the Coma cluster that he and Gregory were The scene shifts to midday following a clear night in completing is inconsistent with the uniform field of galax- spring 1975 at Kitt Peak National Observatory (KPNO), ies suggested by Hubble and indicates that virtually all Arizona, a night when the three largest telescopes on the galaxies occur in groups and clusters with a large fraction mountain were all pointing in the same direction, toward of space completely devoid of galaxies. (The latter fact Tifft the constellation Coma Berenices. Photons were being believed especially interesting.) These results were soon gathered from galaxies by (a) the KPNO 2.2-m telescope published (Tifft and Gregory 1976). For the reasons de- equipped with a Carnegie image-tube spectrograph to scribed previously, the inference by Tifft that galaxies are complete Chincarini and Rood's (1975) homogeneous virtually all in groups and clusters was not new to Chin- redshift survey in a 30-sq-deg region west of the center of carini and me, but the significance of empty regions such the Coma cluster, (b) the new 2.3-m telescope of Steward as the void in front of the Coma cluster seems to have Observatory at the University of Arizona, equipped with been overlooked by all previous astronomers. Indeed, a new Carnegie image-tube spectrograph to complete virtually all previous observational and theoretical discus- Tifft and Gregory's (1976) homogeneous redshift survey sions of structure in the distribution of galaxies have in fields centered on the Coma cluster of radius 3° and 6°, focused on the groupings, without mention of the empty and (c) the 1.3-m telescope of the University of Michigan regions, neither of their significance nor of the possibility to test equipment near the final stages of development by that they, too, could be astronomical entities. This non- Shectman and Hiltner that electronically integrates spec- recognition was, in a sense, a hidden paradigm, uncon-

© Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System SUPPLEMENTAL TOPICS ON VOIDS 1073 sciously adopted as true knowledge by researchers on redshift surveys for other regions of the sky, e.g., (a) large-scale structure. The importance of paradigms in Perseus, Pisces, Ursa Major, and Lynx (cf. Focardi, scientific research is discussed by Kuhn (1962). Maraño, and Vettolani 1983^, 1984; Gregory, Thomp- son, and Tifft 1981; Giovanelli and Haynes 1982; Oort III. Galaxy Redshift Surveys 1983, pp. 394-400), (b) Hydra-Centaurus (Chincarini and The scene now shifts to a future date when there will Rood 1979; da Costa et al. 1986, 1987), (c) 1.75 steradians exist a high-quality galaxy catalog with locational coordi- of the southern sky (da Costa et al. 1988), (d) Pisces-Cetus nates and apparent magnitudes complete to (say) mp = (Burns, Moody, and Batuski 1987, preprint), and (e) 17.5 covering at least the entire northern sky. The goals Corona Borealis (Postman et al. 1986; Postman, Geller, are (a) to resolve questions concerning the Bootes void and Huchra 1988, the latter a very deep survey (Vlini — 0.1 (described by Kirshner et al. (1987) and reviewed by Rood c)), are also contributing significantly to our knowledge of (1988)), and (b) to map the galaxy distribution to a depth the existence and structure of superclusters and voids. corresponding to mp = 17.5 (limiting redshift velocity Vlim Regions west of 9h and east of 16h in the CfA 6-degree- 1 ~ 30,000 km s — 0.1 c). If the part of the sky covered by wide survey strip containing the Coma cluster are defi- the Center for Astrophysics (CfA) redshift survey contains cient in galaxies because of Galactic obscuration, as > 12,000 galaxies with mp < 15.5 0. Huchra 1987, pointed out by de Lapparent et al. (1986). Similarly, preprint), then this region contains > 200,000 galaxies Chincarini (1986) finds that two large voids discussed in with mp ^ 17.5. To complete both projects (a and b), the the previous literature (Einasto et al. 1983) are in the astronomical community could either (a) grit its collective direction of the Orion-Cygnus spiral arm and therefore teeth and measure redshifts for all > 200,000 galaxies may be caused by Galactic obscuration. One must always ordered, for example, in slices (approximately two- note the Galactic latitude of a newly-identified void to dimensional cross-sections) of the three-dimensional check whether it is a dust void caused by obscuration in region (cf. de Lapparent, Geller, and Huchra 1986; our Galaxy or a transparent extragalactic void of physical J. Huchra 1987, preprint); (b) measure redshifts for the > interest for studies of large-scale structure. 200,000/s galaxies contained in numerous small individual probes chosen to cover in a representative manner the IV. Clusters and Large-Redshift Surveys fraction/ of the entire survey area (e.g., in the Kirshner s Will the apparent void of characteristic dimension ~ et al. Bootes survey, f = 0.02) (cf. Kirshner et al. 1987; s 300 Mpc in the space distribution of Abell clusters (Bah- Postman, Huchra, and Geller 1986); (c) first construct a call and Soneira 1982; Batuski and Burns 1985) break up list of all > 200,000 galaxies ordered randomly in location and apparent magnitude, and then determine redshifts into smaller voids as more complete redshift data become for galaxies selected by descending the list one galaxy at a available? This question provides one of several astro- time (Chincarini, Rood, and Thompson 1981) (because nomical stimuli toward the direct measurement of red- large-scale structure appears to be insensitive to the abso- shifts for all 2712 Abell clusters. To date 578 are published lute luminosity of a galaxy (evidence reviewed by Rood and cataloged (Struble and Rood 1987). The means to (1988), Sections 2.1.2 and 3.1.2), as more and more red- complete the job are available thanks to modern technol- shifts are determined, this procedure creates statistically ogy. Redshifts of individual galaxies in a cluster can be unbiased successive representations of large-scale struc- obtained with electronic spectrographs (cf. Shectman and ture with greater and greater detail (somewhat analogous Hiltner 1976) and modern data-reduction techniques (cf. to a Polaroid photographic print developing before our Tonry and Davis 1979; Schechter 1980) (e.g., by applying eyes)); or (d) select some combination of the above an intensified Reticon detector and data reduction with stratagems, or something else. It is left to the reader to cross-correlation techniques, Beers, Geller, and Huchra -1 ponder the relative advantages of these various strategies (1982) achieved uncertainties σν — 50 km s for measure- as a function of the research goal to be achieved. (After the ments of the Doppler velocities of 24 galaxies in the foregoing was written, my attention was directed to a cluster A98, redshift ζ = 0.1); redshifts for — 30 galaxies paper by Kaiser (1986), who provides a cogent analytical can be obtained simultaneously by multiobject spec- analysis that supports the choice of a sparse-sampling troscopy via multiaperture or fiber-optic coupling devices strategy for studies of superclusters and voids and, more (e.g., Ellis et al. (1984) achieved uncertainties σν — 100 generally, for large-scale structure in the linear regime km s-1 for measurements of Doppler velocities of 21 (i.e., structure for which the crossing time exceeds the galaxies in the IC 282 cluster, redshift 2; = 0.04); currently Hubble time scale).) less-accurate redshifts are obtained simultaneously for an Although the emphasis in the review by Rood (1988) on even larger number of galaxies by (a) measuring the dis- the observational descriptions of the Coma, Hercules, placement of spectral features relative to an emulsion and Bootes voids is consistent with the great amount of cutoff wavelength of an objective-prism Schmidt photo- research that has been devoted to these voids, galaxy graphic plate (from Curtis Schmidt plates, Cooke et al.

(1977) achieved a preliminary redshift measurement er- solve these discrepancies fell into two categories: (a) Un- ror σζ — 0.02 for galaxies with ζ > 0.8 and smaller errors seen matter was hypothesized, and discrepancies were for galaxies with smaller redshifts) and (b) computer syn- then used to predict its location and mass. The planet thesizing spectra from observed multicolor CCD images Neptune was predicted from anomalies in the kinematics of a field. (Baum (1962) implemented the fundamental of Uranus (cf. Smart 1953; Turner 1963). On the other ideas 25 years ago; Koo (1985), with four color filters, hand, various types of unseen matter, including a hypo- achieved a typical redshift measurement error σζ — 0.04 thetical planet Vulcan, were predicted from anomalies in for galaxies with ζ < 0.35, and σζ — 0.06 for galaxies with the kinematic parameters of Mercury; despite extensive 0.35 ^ ζ < 0.6; Loh and Spillar (1986), with six filters, visual searches, the predicted matter was not detected, measured the redshifts of about 30 galaxies in the 2; = 0.4 (b) Gravitational laws different from Newton's were hy- cluster 0024+1654 with an uncertainty σζ 0.05; and pothesized. Early attempts to simply modify or replace Hickson (1987, private communication) has developed Newton's law of gravitation were unsuccessful. However, and used a detector with 45 filters obtaining redshifts for general relativity achieved a striking success by fully galaxies in the range ζ — 0.2-0.8 with estimated uncer- explaining the anomaly in the perihelion precession of tainty σζ ~ 0.01. The multifilter redshifts are currently Mercury. Currently, the major aspects of the observed useful for determinations of very-large-scale structure.) kinematics of the solar system are explained by Newto- nian mechanics, and residual aspects are fully explained V. Solar-System and Extragalactic Dynamics by general relativity. Complete consistency exists be- The masses of the planets and the Sun deduced from tween the observed kinematic properties of the solar the observed kinematics (i.e., instantaneous motions and system and the predictions of Newtonian/general rela- positions) of the planets by means of Newtonian/general tivistic mechanics. Any researcher who embarks on re- relativistic theory have been integrated consistently search directed toward the resolution of any of the mass/ within the framework of an overall physical understand- time-scale problems in extragalactic astronomy discussed ing of these objects. For example, the derived mass of the by Rood (1988, Section 4) could gain perspective by first Sun is consistent with the gravitational acceleration reading the book by Roseveare (1982) on analogous work within the solar atmosphere, as inferred from its observed for the solar system. The parallelism between early solar- spectral properties analyzed with laws of thermodynam- system and recent extragalactic problems in gravitational ics and hydrodynamics. And the derived mass of the Sun, mechanics is discussed in more detail elsewhere (e.g., combined with its intrinsic luminosity and radius, and Rood 1969; Rekenstein 1987). with the spectrographically-deduced composition and temperature of its atmospheric surface, has been applied VI. Directions for Future Research to the equations of hydrostatic equilibrium, thermal equi- The reader undoubtedly has his or her own ideas about librium, and energy transport, to derive the structure of directions for future research. In addition, previous arti- the solar interior. This theoretical exercise demonstrates cles that emphasize the necessity for obtaining large ho- that the deep interior is a source of nuclear energy that is mogeneous collections of redshifts of extragalactic objects continually releasing radiant energy at the rate required include Huchra and Geller (1987) and Ostriker (1986). A to maintain the intrinsic luminosity and other structural sparse-sampling strategy for analyzing large-scale cluster- properties of the Sun. Moreover, this model of the Sun ing from redshift surveys is discussed by Kaiser (1986). and corresponding models for other stars necessitates The current status of the automated plate scanner at the their time evolution, and the physical theory of stellar University of Minnesota is described by Humphreys et al. evolution constructed from these models predicts conse- (1987). Finally, the review article by Rood (1988) and the quences from which the observed statistical properties of present text are sprinkled throughout with (a) inferred stars are well understood. This wonderfully consistent research directions and (b) references to a representative picture, free from apparent observational and theoretical selection of papers that provide additional references and discrepancies, attests to the validity of Newtonian/gen- information toward effecting this research. eral relativistic mechanics within the domain of parame- This manuscript was written during a 1987-88 visit by ter space occupied by the solar system. H. J.R. at the Institute for Advanced Study; the hospital- This was not always the case. During the last century, ity of the faculty in the School of Natural Sciences is the observed kinematics of the discrete masses that com- gratefully acknowledged. The work is supported in part prise the solar system were believed to be governed by the National Science Foundation under grant AST exclusively by Newtonian gravitational mechanics. But as 85-13087. observational data accumulated it became apparent that some observed kinematic properties deviated from theo- REFERENCES retical predictions by amounts that were too large to be Bahcall, Ν. Α., and Soneira, R. M. 1982, Ap. /., 262, 419. caused by observational uncertainties. Attempts to re- Batuski, D. J., and Burns, J. O. 1985, A./., 90, 1413.

Baum, W. A. 1962, in IAU Symposium 15, Problems of Extragalactic Humphreys, R. M., Pennington, R., Ghigo, F., and Dickey, J. 1987, Research, ed. G. C. McVittie (Dordrecht: D. Reidel), p. 390. Visitor Use of the Automated Plate Scanner at the University of Beers, T. C., Geller, M. J., and Huchra, J. P. 1982, Ap./., 257, 23. Minnesota (3d ed.; Minneapolis, MN: University of Minnesota As- Bekenstein, J. D. 1987, in Proceedings of the Second Canadian Con- tronomy Department). ference on General Relativity and Relativistic Astrophysics, ed. Kaiser, N. 1986, M.N.R.A.S., 219, 785. C. Dyer (Singapore: World Scientific), in press. Kirshner, R. P., Oemler, Α., Schechter, P. L., and Shectman, S. A. Chincarini, G. 1986, Astr. Αρ., 154, 376. 1987, Ap./., 314, 493. Chincarini, G., and Martins, D. 1975, Ap. /., 196, 335. Koo, D. C. 1985, A./., 90, 418. Chincarini, G., and Rood, H. J. 1975, Nature, 257, 294. Kuhn, T. S. 1962, The Structure of Scientific Revolutions (Chicago: The 1979, Ap./., 230, 648. University of Chicago Press). Chincarini, G., Rood, H. J., and Thompson, L. A. 1981, Ap. /. (Let- Loh, Ε. D., and Spillar, Ε. J. 1986, Αρ. /., 303, 154. ters), 249, L47. M ay all, N. U. 1960, Ann. d'Ap., 23, 344. Cooke, J. Α., Emerson, D., Nandy, K., Reddish, V. C., and Smith, Mushotzky, R. F. 1984, Phys. Scripta, T7, 157. M. G. 1977, M.N.R.A.S., 178, 687. Oort, J. H. 1983, Ann. Rev. Astr. Αρ., 21, 373. da Costa, L. N., Nunes, Μ. Α., Pellegrini, P. S., Willmer, C., Chincar- Ostriker, J. P. 1986, in Galaxy Distances and Deviations from Universal ini, G., and Cowan, J. J. 1986, A./., 91, 6. Expansion, ed. B. F. Madore and R. B. Tully (Dordrecht: Reidel), da Costa, L. N., Pellegrini, P. S., Sargent, W. L. W., Tonry, J., p. 273. Davis, M., Meiksin, Α., and Latham, D. W. 1988, Ap. /., 327, 544. Postman, M., Huchra, J. P., and Geller, M. J. 1986, A./,, 92, 1238. da Costa, L. N., Willmer, C., Pellegrini, P. S., and Chincarini, G. 1987, Postman, M., Geller, M. J., and Huchra, J. P. 1988, A./., 95, 267. A./., 93, 1338. Rood, H. J. 1969, Sky and Tel., 37, 152, 225. de Lapparent, V., Geller, M. J., and Huchra, J. P. 1986, Ap. /. (Let- 1982, Ap./. Suppl., 49, 111. ters), 302, LI. 1988, Ann. Rev. Astr. Αρ., 26, 245. Einasto, J., Corwin, H. G., Huchra, J., Miller, R. H., andTarenghi, M. Rood, H. J., and Dickel, J. R. 1979, Ap. /., 233, 418. 1983, Highlights Astr., 6, 757. Roseveare, Ν. T. 1982, Mercury's Perihelion from Le Verrier to Ein- Ellis, R. S., Gray, P. M., Carter, D., and Godwin, J. 1984, M.N.R.A.S., stein (Oxford: Clarendon Press). 206, 285. Schechter, P. L. 1980, A./., 85, 801. Focardi, P., Maraño, Β., and Vettolani, G. 1983a, Asir. Express, 1(2), Schneider, S. E. 1987, Sky and Tel., 73, 469, 476. 75. Shectman, S. Α., and Hiltner, W. A. 1976, Pub. A.S.P., 88, 960. 1983¾, in IAU Symposium 104, Early Evolution of the Universe Smart, W. M. 1953, in Celestial Mechanics (New York: Longmans, and Its Present Structure, ed. G. O. Abell and G. Chincarini (Dor- Green, and Co.), p. 247. drecht: D. Reidel), p. 295. Struble, M. F., and Rood, H. J. 1987, Ap. /. Suppl, 63, 543. 1984, Asir. Αρ., 36, 178. Tifft, W. G. 1972, Ap./., 175, 613. Giovanelli, R., and Haynes, M. P. 1982, A./., 87, 1355. 1982, Ap./., 257, 442. Gregory, S. Α., and Thompson, L. Α., 1978, Αρ. /., 222, 784. Tifft, W. G., and Cocke, W. J. 1987, Sky and Tel., 73, 19. Gregory, S. Α., Thompson, L. Α., and Tifft, W. G. 1981, Αρ. /., 243, Tifft, W. G., and Gregory, S. Α., 1976, Αρ. /., 205, 696. 411. Tonry, J., and Davis, M. 1979, A./., 84, 1511. Huchra, J. P., and Geller, M. J. 1987, in The Proceedings of the 13th Turner, Η. H. 1963, Astronomical Discovery (Berkeley and Los Ange- Texas Symposium on Relativistic Astrophysics, ed. M. Ulmer, in les: University of California Press). press.