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 dis- crepancies existed, and explanations were sought in terms of hypothesized missing mass and non-Newtonian dynamics. Today, the same approach is being applied toward resolving discrepan- cies 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 1071 © 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.