Pairwise Velocity Dispersion Of Galaxies Research Articles

Galaxy and Mass Assembly (GAMA): small-scale anisotropic galaxy clustering and the pairwise velocity dispersion of galaxies

The galaxy pairwise velocity dispersion (PVD) can provide important tests of non-standard gravity and galaxy formation models. We describe measurements of the PVD of galaxies in the Galaxy and Mass Assembly (GAMA) survey as a function of projected separation and galaxy luminosity. Due to the faint magnitude limit ($r < 19.8$) and highly-complete spectroscopic sampling of the GAMA survey, we are able to reliably measure the PVD to smaller scales ($r_\bot = 0.01$ Mpc/h) than previous work. The measured PVD at projected separations $r_\bot <~ 1$ Mpc/h increases near-monotonically with increasing luminosity from $\sigma \approx 200$ km/s at $M_r = -17$ mag to $\sigma \approx 600$ km/s at $M_r \approx -22$ mag. Analysis of the Gonzalez-Perez (2014) GALFORM semi-analytic model yields no such trend of PVD with luminosity: the model over-predicts the PVD for faint galaxies. This is most likely a result of the model placing too many low-luminosity galaxies in massive halos.

Clear and measurable signature of modified gravity in the galaxy velocity field.

The velocity field of dark matter and galaxies reflects the continued action of gravity throughout cosmic history. We show that the low-order moments of the pairwise velocity distribution v_{12} are a powerful diagnostic of the laws of gravity on cosmological scales. In particular, the projected line-of-sight galaxy pairwise velocity dispersion σ_{12}(r) is very sensitive to the presence of modified gravity. Using a set of high-resolution N-body simulations, we compute the pairwise velocity distribution and its projected line-of-sight dispersion for a class of modified gravity theories: the chameleon f(R) gravity and Galileon gravity (cubic and quartic). The velocities of dark matter halos with a wide range of masses would exhibit deviations from general relativity at the (5-10)σ level. We examine strategies for detecting these deviations in galaxy redshift and peculiar velocity surveys. If detected, this signature would be a "smoking gun" for modified gravity.

The Pairwise Velocity Dispersion of Galaxies: Luminosity Dependence and a New Test of Galaxy Formation Models

We present the first determination of the pairwise velocity dispersion (PVD) for galaxies in different luminosity intervals using the final release of the Two-Degree Field Galaxy Redshift Survey (2dFGRS). We have discovered quite surprisingly that the relative velocities of the faint galaxies at small separation are very high, around 700 km s-1, reaching similar values as the brightest galaxies. At intermediate luminosities M* - 1 (M* is the characteristic luminosity of the Schechter function), the relative velocities exhibit a well-defined steep minimum near 400 km s-1. This result has been derived using a novel method to determine the real-space power spectrum and the PVD from the redshift-space power spectrum of the 2dFGRS. Combined with the observed luminosity dependence of clustering, our result implies that quite a fraction of faint galaxies, as well as the brightest ones, are in massive halos of galaxy cluster size, but most of the M* galaxies are in galactic halos. Our observed result is compared with the current halo model of galaxies of Yang et al., which was obtained by matching the clustering and luminosity functions of the 2dFGRS. With the model parameters they favored most, the halo model seems to be unable to reproduce the luminosity dependence of the PVD, because it predicts a monotonically increasing PVD with the luminosity. We discuss a possible solution to this model by raising the faint-end slope of the conditional luminosity function in rich clusters. The PVD luminosity dependence may also be an important constraint in general on theories of galaxy formation, such as semianalytical models and hydro/N-body simulations of galaxy formation.

Velocity of galaxies with different luminosity

We present the first determination of the pairwise velocity dispersion of galaxies at different luminosity with the final release of the Two-Degree Field Galaxy Redshift Survey (2dFGRS). Our result surprisingly shows that the random velocities of the faint galaxies are very high, around galaxies are in galactic halos.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Galaxy Clustering in Early Sloan Digital Sky Survey Redshift Data

We present the first measurements of clustering in the Sloan Digital Sky Survey (SDSS) galaxy redshift survey. Our sample consists of 29,300 galaxies with redshifts 5700 km s-1 ≤ cz ≤ 39,000 km s-1, distributed in several long but narrow (25-5°) segments, covering 690 deg2. For the full, flux-limited sample, the redshift-space correlation length is approximately 8 h-1 Mpc. The two-dimensional correlation function ξ(rp,π) shows clear signatures of both the small-scale, fingers-of-God distortion caused by velocity dispersions in collapsed objects and the large-scale compression caused by coherent flows, though the latter cannot be measured with high precision in the present sample. The inferred real-space correlation function is well described by a power law, ξ(r) = (r/6.1 ± 0.2 h-1 Mpc)-1.75±0.03, for 0.1 h-1 Mpc ≤ r ≤ 16 h-1 Mpc. The galaxy pairwise velocity dispersion is σ12 ≈ 600 ± 100 km s-1 for projected separations 0.15 h-1 Mpc ≤ rp ≤ 5 h-1 Mpc. When we divide the sample by color, the red galaxies exhibit a stronger and steeper real-space correlation function and a higher pairwise velocity dispersion than do the blue galaxies. The relative behavior of subsamples defined by high/low profile concentration or high/low surface brightness is qualitatively similar to that of the red/blue subsamples. Our most striking result is a clear measurement of scale-independent luminosity bias at r 10 h-1 Mpc: subsamples with absolute magnitude ranges centered on M* - 1.5, M*, and M* + 1.5 have real-space correlation functions that are parallel power laws of slope ≈-1.8 with correlation lengths of approximately 7.4, 6.3, and 4.7 h-1 Mpc, respectively.

The pairwise velocity dispersion of galaxies: effects of non-radial motions

I discuss the effect of non-radial motions on the small-scale pairwise peculiar velocity dispersions of galaxies (PVD) in a CDM model. I calculate the PVD for the SCDM model by means of the refined cosmic virial theorem (CVT) (Suto & Jing 1997 (hereafter SJ97)) and taking account of non-radial motions by means of Del Popolo & Gambera (1998) (hereafter DG98) model. I compare the results of the present model with the data from Davis & Peebles (1983), the IRAS value at 1h 1 Mpc of Fisher et al. (1993) and Marzke et al. (1995). I show that while the SCDM model disagrees with the observed values, as pointed out by several authors (Peebles 1976, 1980; Davis & Peebles 1983 (hereafter DP83); Mo et. al 1993; Fisher et al. 1994b; SJ97; Jing et al. 1998 (hereafter J98)), taking account of non-radial motions produce smaller values for the PVD. At r � 1h 1 Mpc the result is in agreement with Bartlett & Blanchard (1996) (hereafter BB96). At the light of this last paper, the result may be also read as a strong dependence of the CVT prediction on the model chosen to describe the mass distribution around galaxies, suggesting that the CVT cannot be taken as a direct evidence of a low density universe. Similarly to what shown in Del Popolo & Gambera (1999, 2000) (hereafter DG99, DG00), Del Popolo et al. (1999) (hereafter D99), the agreement of our model to the observational data is due to a scale dependent bias induced by the presence of non-radial motions. Since the assumptions on which CVT is based have been questioned by several authors (BB96; SJ97), I also calculated the PVD using the redshift distortion in the redshift-space correlation function, �z(rp,�), and I compared it with the PVD measured from the Las Campanas Redshift Survey by J98. The result confirms that non-radial motions influence the PVD making them better agree with observed data.

Halo occupation numbers and galaxy bias

We propose a heuristic model that displays the main features of realistic theories for galaxy bias. We show that the low-order clustering statistics of the dark-matter distribution depend almost entirely on the locations and density profiles of dark-matter haloes. A hypothetical galaxy catalogue depends on (i) the efficiency of galaxy formation, as manifested by the halo occupation number -- the number of galaxies brighter than some sample limit contained in a halo of a given mass; (ii) the location of these galaxies within their halo. The first factor is constrained by the empirical luminosity function of groups. For the second factor, we assume that one galaxy marks the halo centre, with any remaining galaxies acting as satellites that trace the halo mass. These simple assumptions amount to a recipe for non-local bias, in which the probability of finding a galaxy is not a simple function of its local mass density. We have applied this prescription to some CDM models of current interest, and find that the predictions are close to the observed galaxy correlations for a flat $\Omega=0.3$ model ($\Lambda$CDM), but not for an $\Omega=1$ model with the same power spectrum ($\tau$CDM). This is an inevitable consequence of cluster normalization for the power spectra: cluster-scale haloes of given mass have smaller core radii for high $\Omega$, and hence display enhanced small-scale clustering. Finally, the pairwise velocity dispersion of galaxies in the $\Lambda$CDM model is lower than that of the mass, allowing cluster-normalized models to yield a realistic Mach number for the peculiar velocity field. This is largely due to the strong variation of galaxy-formation efficiency with halo mass that is required in this model.

The Caltech Faint Galaxy Redshift Survey. XII. Clustering of Galaxies

A clustering analysis is performed on two samples of ~600 faint galaxies each, in two widely separated regions of the sky, including the Hubble Deep Field. One of the survey regions is configured so that some galaxy pairs span angular separations of up to 1°. The median redshift is zmed ≈ 0.55. Strong clustering is obvious, with every pencil-beam field containing a handful of narrow redshift-space features, corresponding to galaxy structures with sizes of 5-20 Mpc. The structures are not obviously organized on planes, although one prominent, colinear triplet of structures is observed, spanning ~20 Mpc. This may be evidence of a filament. A galaxy-galaxy correlation function calculation is performed. No significant evolution of clustering (relative to stable clustering) is found in the redshift range 0.3 < z < 1.0. This is not surprising, since uncertainties in the correlation amplitude estimated from surveys such as these are large; field-to-field variations and covariances between data points are both shown to be significant. Consistent with other studies in this redshift range, the galaxy-galaxy correlation length is found to be somewhat smaller than that predicted from local measurements and an assumption of no evolution. Galaxies with absorption-line-dominated spectra show much stronger clustering at distances of <2 Mpc than typical field galaxies. There is some evidence for weaker clustering at intermediate redshift than at low redshift, when the results presented here are compared with surveys of the local universe. In subsets of the data, the measured pairwise velocity dispersion of galaxies ranges from 200 to 600 km s-1, depending on the properties of the dominant redshift structures in each subset.

The Three‐Point Correlation Function of Galaxies Determined from the Las Campanas Redshift Survey

We report the measurement of the three-point correlation function (3PCF) of galaxies for the Las Campanas Redshift Survey (LCRS). We have not only measured the 3PCF in redshift space, but we have also developed a method to measure the projected 3PCF that has simple relations to the real-space 3PCF. Both quantities have been measured as a function of triangle size and shape with only a fractional uncertainty in each individual bin. Various tests derived from mock catalogs have been carried out to assure that the measurement is stable and that the errors are estimated reliably. Our results indicate that the 3PCFs both in redshift space and in real space have small but significant deviations from the well-known hierarchical form. The 3PCF in redshift space can be fitted by Qred(s, u, v) = 0.5×10$^{{\0.2 + 0.1{[}s{/}{(}s + 1{)}{]}2{\}}v2}$ --> for 0.8 −0.3p for 0.2 1 h-1 Mpc is noted. The real space Q(r, u, v) for 0.2 r12 3 h-1 Mpc and r31 6 h-1 Mpc can be well described by half the mean 3PCF predicted by a cold dark matter (CDM) model with Ω0h = 0.2. The general dependence of the 3PCF on triangle shape and size is in qualitative agreement with the CDM cosmogonic models. Quantitatively the 3PCF of the models may depend on the biasing parameter and the shape of the power spectrum, in addition to other model parameters. Taking our result together with the constraints imposed by the two-point correlation function and the pairwise velocity dispersion of galaxies also obtained from the LCRS, we find that we have difficulties in producing a simple model that meets all constraints perfectly. Among the CDM models considered, a flat model with Ω = 0.2 meets the 2PCF and PVD constraints, but gives higher values for the 3PCF than observed. This may indicate that more sophisticated bias models or a more sophisticated combination of model parameters must be considered.

Spatial Correlation Function and Pairwise Velocity Dispersion of Galaxies: Cold Dark Matter Models versus the Las Campanas Survey

We show, with the help of large N-body simulations, that both the real-space two-point correlation function and pairwise velocity dispersion of galaxies can be measured reliably from the Las Campanas Redshift Survey. The real-space correlation function is well fitted by the power law ξ(r) = (r0/r)γ with r0 = (5.06 ± 0.12) h-1 Mpc and γ = 1.862 ± 0.034, and the pairwise velocity dispersion at 1 h-1 Mpc is 570 ± 80 km s-1. A detailed comparison between these observational results and the predictions of current cold dark matter (CDM) cosmogonies is carried out. We construct 60 mock samples for each theoretical model from a large set of high-resolution N-body simulations, which allows us to include various observational selection effects in the analyses and to use exactly the same methods for both real and theoretical samples. We demonstrate that such a procedure is essential in the comparison between models and observations. The observed two-point correlation function is significantly flatter than the mass correlation function in current CDM models on scales 1 h-1 Mpc. The observed pairwise velocity dispersion is also lower than that of dark matter particles in these models. We propose a simple antibias model to explain these discrepancies. This model assumes that the number of galaxies per unit dark matter mass, N/M, decreases with the mass of dark haloes. The predictions of CDM models with σ8 Ω0.60~0.4-0.5 and Γ ~ 0.2 are in agreement with the observational results, if the trend of N/M with M is at the level already observed for rich clusters of galaxies. Thus CDM models with Γ ~ 0.2 and with cluster-abundance normalization are consistent with the observed correlation function and pairwise velocity dispersion of galaxies. A high level of velocity bias is not required in these models.

The Galaxy Pairwise Velocity Dispersion as a Function of Local Density

The standard method of measuring the galaxy pairwise velocity dispersion on small scales via the anisotropy in the two-point correlation function in redshift space suffers from the fact that it is a pair-weighted statistic, and thus is heavily weighted by the densest regions in a way that is difficult to calibrate. We propose a new statistic, the redshift difference of close projected pairs of galaxies as a function of local density, which is designed to measure the small-scale velocity dispersion as an explicit function of density. Computing this statistic for a volume-limited subsample of the Optical Redshift Survey, we find that the small-scale velocity dispersion rises from 220 km s-1 in the lowest density bins to 760 km s-1 at high density. We calculate this statistic for a series of mock catalogs drawn from a hydrodynamic simulation of an Ωh = 0.5 cold dark matter universe (standard CDM) and find that the observed velocity distribution lies 1 σ below the simulations in each of eight density bins; this measurement formally rules out this model at the 7.4 σ level and quantifies the well-known problem that this model produces too high a velocity dispersion. This comparison is insensitive to the normalization of the power spectrum, although it is quite sensitive to the density and velocity bias of galaxies relative to dark matter on small scales.

Pairwise Velocities of Galaxies in the CfA and SSRS2 Redshift Surveys

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 |