Pairwise Velocity Dispersion of Galaxies at High Redshift: Theoretical Predictions

Abstract

We investigate the feasibility of determining the pairwise velocity dispersion (PVD) for Lyman break galaxies (LBGs) and of using this quantity as a discriminator among theoretical models. We find that within a given cosmological model, different schemes of galaxy formation lead to significant changes of the PVD. We propose a simple phenomenological model for the formation of LBGs, determined by the formation interval parameter Δz and the halo mass threshold Mh. With a reasonable choice for these two parameters, our model predicts an occupation number distribution of galaxies in halos that agrees very well with the predictions of semianalytical models. We also consider a range of galaxy formation models by adjusting the two model parameters. We find that model LBGs can have the same two-point correlation function (TPCF) over the range of observable separations, even though the cosmology and/or galaxy formation model are different. Moreover, with similar galaxy formation models, currently popular cosmologies can result in both the same TPCF and the same PVD. However, with the same cosmology, different galaxy formation models may show quite different PVDs, even though the TPCF is the same. Our test with mock samples shows furthermore that one can discriminate among such models already with currently available observational samples (if the measurement error of the redshift is negligible), which have a typical error of 80 km s-1. The error will be reduced by a factor of 2 if the samples are increased 4 times. We also show that an erroneous assumption about the geometry of the universe and different infall models only slightly change the results. Therefore, the PVD will become another promising statistic to test galaxy formation models with redshift samples of LBGs.