How "cold" is the flow of halos? Two panels on the right show halos in the 60Mpc/h simulation of a LCDM model. For each halo the color codes the rms velocity of dark matter particles inside a 3Mpc/h sphere centered on the halo. Average velocity of the sphere was subtructed.
6Mpc/h slice through the largest cluster. Xs show halos with the rms velocities less than 100 km/s.
The whole box. There are about 10,000 "galaxies" in the plot. The large cluster in the bottom-right corner ("Coma") has more than 250 halos; its rms 3D velocity is 1700 km/s
Pair-wise:very sensitive to spatial bias
Typically is small: 0.4-0.6
Explanation: merging in cluster center and formation of
central cD reduces the number of high-V pairs.
Not because halos move slower!
Single-point:less sensitive to spatial bias
-Historically was < 1.
-Recent results indicate small positive bias:
1.1 -1.3 (Diaferio et al.98, Okamoto & Habe 99, Colin et al. 99
Halos move faster in clusters.
Explanation: Dynamical friction and Jeans equation