Pairwise Velocities of Galaxies in the CfA and SSRS2 Redshift Surveys

Abstract

view Abstract Citations (127) References (68) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Pairwise Velocities of Galaxies in the CfA and SSRS2 Redshift Surveys Marzke, Ronald O. ; Geller, Margaret J. ; da Costa, L. N. ; Huchra, John P. Abstract We combine the CfA Redshift Survey (CfA2) and the Southern Sky Redshift Survey (SSRS2) to estimate the pairwise velocity dispersion of galaxies σ_12_ on a scale of ~1 h^-1^ Mpc. Both surveys are complete to an apparent magnitude limit B(0) = 15.5. Our sample includes 12812 galaxies distributed in a volume 1.8 x 10^6^ h^-3^ Mpc^3^. We conclude: (1) The pairwise velocity dispersion of galaxies in the combined CfA2+SSRS2 redshift survey is σ_12_ = 540 +/- 180 km s^-1^. Both the estimate and the variance of σ_12_ significantly exceed the canonical values σ_12_ = 340 +/- 40 measured by Davis & Peebles (1983) using CfA1. (2) We derive the uncertainty in σ_12_ from the variation among subsamples with volumes on the order of 7 x 10^5^h^-3^ Mpc^3^. This variation is nearly an order of magnitude larger than the formal error, 36 km s^-1^, derived using least-squares fits to the CfA2+SSRS2 correlation function. This variation among samples is consistent with the conclusions of Mo et al. (1993) for a number of smaller surveys and with the analysis of CfA1 by Zurek et al. (1994). (3) When we remove Abell clusters with R > 1 from our sample, the pairwise velocity dispersion of the remaining galaxies drops to 295 +/- 99 km s^-1^. (4) The dominant source of variance in σ_12_ is the shot noise contributed by dense virialised systems. Because σ_12_ is pair weighted, the statistic is sensitive to the few richest systems in the volume. This sensitivity has two consequences. First, σ_12_ is biased low in small volumes, where the number of clusters is small. Second, we can estimate the variance in σ_12_ as a function of survey volume from the distribution of cluster and group velocity dispersions n (σ). For either a COBE-normalized CDM universe or for the observed distribution of Abell cluster velocity dispersions, the volume required for σ_12_ to converge (δσ_12_/σ_12_ < 0.1) is ~5 x 10^6^h^-3^ Mpc^3^, larger than the volume of CfA2+SSRS2. (5) The distribution of pairwise velocities is consistent with an isotropic exponential with velocity dispersion independent of scale. The inferred single-galaxy velocity distribution function is incompatible with an isotropic exponential. Thus the observed kinematics of galaxies differ from those measured in the hydrodynamic simulations of Cen & Ostriker (1993). On the other hand, our observations appear to be consistent with the velocity distribution function measured by Zurek et at. (1994) using collisionless simulations with much higher resolution than Cen & Ostriker (1993). The large dynamic range in the simulations by Zurek et al. (1994) affords a more accurate treatment of galaxy interactions, which dramatically alter the dynamical evolution the galaxy distribution (Couchman & Carlberg 1992; Zurek et al. 1994). The agreement between the observed velocity distribution and the one predicted by these high-resolution simulations may be another clue that mergers play an important role in the evolution of galaxies and large-scale structure. Publication: The Astronomical Journal Pub Date: August 1995 DOI: 10.1086/117536 arXiv: arXiv:astro-ph/9504070 Bibcode: 1995AJ....110..477M Keywords: SURVEYS; GALAXIES: DISTANCES AND REDSHIFTS; Astrophysics E-Print: 61 pages uuencoded, compressed postscript in 5 pieces. Also available in one piece at http://www.dao.nrc.ca/DAO/SCIENCE/science.html full text sources arXiv | ADS |