Concordance between the Dynamics of Satellite Galaxies in the 2 dFGRS and Λ CDM

We compute the velocity dispersion profile, σ v (r p), for the satellites of host galaxies in the Two Degree Field Galaxy Redshift Survey (2dFGRS) and in the ΛCDM GIF simulation. The host–satellite selection algorithm yields 1345 host galaxies in the 2dFGRS with luminosities in the range 0.5 L * b J ≤ L ≤ 5.5 L * b J , for which a total of 2475 satellite galaxies is found. The magnitudes of the galaxies in the GIF simulation are converted to the b J band pass, and hosts and satellites are selected in the same manner as in the 2dFGRS. On average, ∼ 1200 hosts and ∼ 4100 satellites are found in the GIF simulation, where the precise number depends upon the angle from which the simulation is viewed. Overall, there is excellent agreement between σ v (r p) for the satellites in the 2dFGRS and the GIF simulation. On large scales, the velocity dispersion profiles for the complete samples decrease with projected radius, in good agreement with the expectations of a CDM universe. Further, there is a marked dependence of the velocity dispersion profile on both the host spectral type and the host luminosity. In particular, σ v (r p) has a substantially higher amplitude and steeper slope for satellites of early–type hosts than it does for satellites of late–type hosts. In addition, both the amplitude and slope of σ v (r p) increase with host luminosity. The velocity dispersion of satellites located within small projected radii from the host (r p ≤ 120 kpc) is only marginally consistent with the local B–band Tully–Fisher relation (σ v ∝ L 0.3) and is fitted best by a relationship of the form σ v ∝ L 0.45±0.10. Subject headings: dark matter — galaxies: fundamental parameters – galaxies: halos — galaxies: kinematics and dynamics – surveys – 2 –