Proc. Natl. Acad. Sci. USA Vol. 93, pp. 14216–14220, December 1996 Colloquium Paper This paper was presented at a colloquium entitled ‘‘Symmetries Throughout the Sciences,’’ organized by Ernest M. Henley, held May 11–12, 1996, at the National Academy of Sciences in Irvine, CA. Mapping the universe in three dimensions (red shiftygalaxyycluster of galaxiesysuperclusterypeculiar velocity) MARTHA P. HAYNES Center for Radiophysics and Space Research and National Astronomy and Ionosphere Center, Cornell University, Ithaca, NY 14853 ABSTRACT The determination of the three-dimensional spectrometers at both optical and radio wavelengths have layout of galaxies is critical to our understanding of the made the redshift industry is a fast-paced one, showing a evolution of galaxies and the structures in which they lie, to greater than exponential growth. A recent more detailed our determination of the fundamental parameters of cosmol- review of red shift surveys was presented by Giovanelli and ogy, and to our understanding of both the past and future Haynes (2). In this discussion, I hope to convey some sense of histories of the universe at large. The mapping of the large the complexity of large-scale structure as well as the promise scale structure in the universe via the determination of galaxy of the next decade to advance our understanding of it. red shifts (Doppler shifts) is a rapidly growing industry thanks to technological developments in detectors and spec- Large-Scale Structure from Red Shift Surveys trometers at radio and optical wavelengths. First-order application of the red shift-distance relation (Hubble’s law) That galaxies tend to cluster has been recognized for more allows the analysis of the large-scale distribution of galaxies than a century, even before their extragalactic nature was on scales of hundreds of megaparsecs. Locally, the large- understood. The cosmological principle states that, on some scale structure is very complex but the overall topology is large scale, the universe is homogenous and isotropic. The not yet clear. Comparison of the observed red shifts with delineation of the topological description of the distribution of ones expected on the basis of other distance estimates allows galaxies in the local universe is an attempt to test whether, and mapping of the gravitational field and the underlying total on what scale, the cosmological principle holds. density distribution. The next decade holds great promise The Milky Way is one of two spiral galaxies (the other is the for our understanding of the character of large-scale struc- Andromeda galaxy, M31) that dominate the Local Group, a ture and its origin. loose aggregate of about 20 galaxies within a volume of radius about 1 Mpc (1 Mpc 5 3.1 3 1022 m or 3.26 3 106 light years). One of the most elusive tasks of astronomy is the development The Local Group itself lies on the outskirts of a flattened of a clear picture of how galaxies are distributed into clusters structure of a radius about 15 Mpc known as the Local and superclusters and at the same time avoid the large empty Supercluster, centered on a rich cluster of galaxies, the Virgo void regions. The difficulty in constructing such a picture rests cluster. A number of other superclusters are recognized within on our ability to interpret the observed projected distribution a few 100 Mpc, but the overall characteristic scale and topo- of galaxies on the sky and ultimately to measure the true logical description are still issues for debate. Here, we will location in space of large numbers of galaxies. In this paper, I simply examine the qualitative appearance of structures seen will adopt the standard view that the dominant motion in the in the local universe. universe is the smooth expansion described by Hubble’s law, Figs. 1–3 attempt to illustrate the kind of structure seen in relating a galaxy’s distance d to its observed recessional the red shift distribution of nearby galaxies. In each case, velocity cz by a simple constant of proportionality, the Hubble galaxies are projected onto an Aitoff equal area projection in constant (Ho). In fairness, it should be recognized that the celestial coordinates, right ascension (R.A.) and declination adoption of a simple red shift-distance relation is not univer- (Decl.), and centered on R.A. 5 6h, Decl. 5 08. Fig. 1 shows sally accepted (e.g., ref. 1). However, Hubble’s law arises as a the distribution of some 14000 galaxies with known recessional natural consequence of the expansion of a homogenous and velocities cz , 12,000 km s21. If the Hubble constant has a 21 21 isotropic universe, as predicted by the assumption that the value of Ho 5 65 km s zMpc , then to first order, the volume cosmological principle holds. Adopting the law, the distance to contained extends to 184 Mpc. Dotted lines denote the locus a galaxy can be estimated to first order (see the discussion of of points at galactic latitudes b 52208, 0, and 1208, delin- peculiar velocities below) simply by measuring its doppler shift eating the Zone of Avoidance, within which distant galaxies z 5 dlyl. For discussions of structural characteristics, the are obscured at optical wavelengths by the dust and gas within Hubble constant is only a scale factor, and is not critical to the the Milky Way. The distribution of galaxies within this volume present discussion. Note that we astronomers use the terms is not random; galaxies tend to cluster. Two major structures ‘‘recessional velocity’’ and ‘‘red shift’’ interchangeably, and are easily seen. To the left of the map, the strong concentration have the bad habit of talking about distances in velocity units. of galaxies in the central region of the Local Supercluster in the The fundamental objective of surveys of galaxy red shifts is Virgo region is clearly noticeable. On the upper right side, the to provide a first-order measurement of galaxy distance: d 5 linear string of galaxies running diagonally (and almost per- czyHo. Beyond the Local Supercluster at least, recessional pendicular to the b 52208 line) is the Pisces–Perseus velocities dominate peculiar ones, and thus observed red shifts supercluster (PPS). can be used to trace large-scale structure. Recent advances in Fig. 2 shows galaxies with velocities lower than 3000 km s21: the Local Supercluster. It is clear in this representation that there are more galaxies on the left side of the map than on the The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked ‘‘advertisement’’ in accordance with 18 U.S.C. §1734 solely to indicate this fact. Abbreviation: PPS, Pisces–Perseus supercluster. 14216 Downloaded by guest on September 26, 2021 Colloquium Paper: Haynes Proc. Natl. Acad. Sci. USA 93 (1996) 14217 FIG. 1. The sky distribution of 14,000 galaxies with known recessional velocities cz , 12,000 km s21. The projection is an Aitoff equal area one in celestial coordinates. The continuous lines that rise diagonally through the center of the map show the locus in celestial coordinates of the lines of constant galactic latitude at 2208,08, and 1208. right side. This asymmetry occurs because the Local Group is Large-scale clustering is quite easily visible also in Fig. 3, located on the edge of the Local Supercluster. When we look which displays objects in the red shift range 3000 , cz , 6000 towards the center, we see lots of galaxies; when we look away km s21. The PPS is visible in the northwest quadrant. The from the center, we see many fewer. Using more imagination, adopted centering tends to deemphasize the overdensity in the one can trace a continuous line through the concentration in Cen-Hyd-Pavo-Ind region in the southern hemisphere, al- Virgo, across the zone of avoidance, through the right side and though it is still visible. Numerous other structures are also back again through the north pole. This continuous distribu- seen. Published datasets currently sample both hemispheres tion is the Supergalactic plane. Although less well-defined than quite well to this depth; at larger red shifts, however, the the galactic plane, its presence indicates that the Local Super- asymmetry in the distribution of telescope sites favoring the cluster, like the Milky Way, is a flattened structure. Within the northern hemisphere becomes critical. Local Supercluster, galaxies tend to be found in groups and Noticeable in Fig. 1 despite its distance, the PPS is one of the clouds, with other regions being relatively empty. most prominent structures in the local extragalactic sky. It must FIG. 2. Similar to Fig. 1, but showing only the sky distribution of galaxies with known recessional velocities cz , 3000 km s21. Downloaded by guest on September 26, 2021 14218 Colloquium Paper: Haynes Proc. Natl. Acad. Sci. USA 93 (1996) FIG. 3. Similar to Fig. 1, but showing only the sky distribution of galaxies with known recessional velocities 3000 , cz , 6000 km s21. be emphasized that the PPS is not a random volume of the We can examine this structure by looking at the two- universe. As discussed by Giovanelli et al. (3), the optical dimensional distributions of galaxies in the region of the catalog used to select targets for the red shift surveys of this supercluster. Fig. 4 shows the distribution on the plane of the region is particularly suited to tracing volume density enhance- sky of galaxies in a 908 by 308 slice across the supercluster. The ments with characteristic sizes of 5 to 20 Mpc if they are main ridge is roughly outlined by straight lines. This is the same situated at a distance of about 5000 km s21.