Do galactic potential wells depend on their large-scale environment?

Abstract

We study the dependence of the intrinsic velocities of galaxies on their large-scale environment, using a cross-correlation technique that provides an objective way of defining the local overdensity of 'trace' galaxies around 'target' galaxies. We use galaxies in optical (CfA and SSRS) and IRAS redshift surveys as tracers of the density field, and about 1000 spiral galaxies with measured circular velocities and elliptical galaxies with measured velocity dispersion as 'targets'. We find that the correlation function tends to increase with circular velocity, the trend being weak except in the case of cD-like elliptical galaxies with the highest velocity dispersions (σ ≳ 300 km s–1), where the effect is strong, possibly due to morphological segregations in clusters of galaxies. A fit to the mean overdensity δ(r < rp) of the trace galaxies (in spheres of radius rp) around target galaxies as a function of the circular velocities |$\upsilon_c$| shows a weak increase of δ with |$\upsilon_c$|⁠, with slope |$\Delta \delta(r\lesssim3.6\, h^{-1}\, \text {Mpc})/\Delta\upsilon_c\lesssim0.02$|⁠. The observed weak correlation is contrasted with the strong dependence of the correlation functions of dark haloes on their circular velocities predicted in some (e.g. high-biasing cold dark matter) models for galaxy formation. In particular, our results are inconsistent with the prediction of the 'natural' (high) biasing model at a high significance level. Comparison of our results with those of a simple biasing model suggests that either the observed circular velocities of galaxies are not simply related to the circular velocities of dark haloes, or most dark haloes were formed at high redshifts, or the galaxy distribution does not trace the matter distribution in a simple way.