The pair-wise velocity dispersion of galaxies: effects of non radial motions

Abstract

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) and taking account of non-radial motions by means of Del Popolo & Gambera (1998) model. I compare the results of the present model with the data from Davis & Peebles (1983), the IRAS value at 1 h{-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; Mo et. al 1993; Jing et al. 1998), taking account of non-radial motions produce smaller values for the PVD. At r <=1 h^{-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), 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, \xi_z(r_p,\pi), 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.