mass versus subhalo velocity dispersion

The zeroth order question is the following:  If you observe the velocity dispersion of galaxies in a halo, how tightly does that correlate to the halo mass?  To answer this I've taken halos in the lcdm_pe simulation, and run subhalo finders on each of them.  Using the mean velocities of each of the subhalos in a halo, I've computed a velocity dispersion.  I have plotted this against the host halo mass.  Here is a plot, using all identified subhalos.

The top panel is the total velocity dispersion, and the bottom panel is the radial velocity dispersion, the quantity that can potentially be observed.   There is a noticeable correlation, with a lot of scatter.

Next, I thought it would be interesting to place mass thresholds on the subhalos used.  This is interesting because we believe that different morphological types of galaxies live in dark matter halos of different masses, e.g. big old red dead ellipticals can be expected to have a higher characteristic dark halo mass than young star-forming spirals.

Below is a sequence of subhalo thresholding, ranging from 1e10 to 1e12.  Note that the y axis on the 5e11 and 1e12 plots have been significantly expanded.

What we see is that as we go to larger subhalos, the velocity dispersion is a much less accurate tracer of host halo mass.  Assuming the brightest galaxies live in the largest subhalos, this means it's unlikely that choosing the brightest few galaxies to compute the velocity dispersion will yeild a result that correlates at all tightly with the mass.

Originally we were thinking of generating light cylinders and computing the velocity dispersion among all galaxies along the line of sight, but to me, the scatter for the bright guys looks large enough that there may not be a point.

Notes to own self:  Procedure for getting these plots:
1) modified tree19/apps/etc/haloproc.c to dump a separate file for every halo in the .find file of lcdm_pe simulation
2) ran isodensity subhalo finder tree16/vfind64/pos/vfind.x86_64-mpi on each halo from step 1
3) ran original tree19/apps/etc/haloproc.corei7 on the .find files from step 2
4) to get subhalo masses ran ~mswarren/tree16/apps/hmassr/hmassr.x86_64 to generate .halos files
5) code to make these plots lives in projects/velfunc/hproc/shvdvsmass

Halo Gallery: Corrected

The bug in Mike's halo files has been corrected, and these are the updated plots of the highest velocity particles in the halo, at early times. 

Halo Gallery II

The previous halo gallery was annoying because it was impossible to see structure, because the plots wer saturated.  I thinned the particles out by a factor of 50...  Here are the same set of halos, with slightly better contrast.

Halo Gallery

I have automated plot making, and have generated a gallery of 7 halos, that exist in mikes directory path = '/u/msw/lcdm_me/4-512/halos/'

The plots show 6 snapshots of the locations of particles whose velocities were within 100km/s of the fastest particle in each radial bin at z~0.  

You can click on the plots to make them larger. I didn't find these super interesting, honestly.  There seem to be two classes, round, and not round... and the round ones have a tendency to have a ring of higher v particles around the outside... In all cases it seems like bulk motion happens more than evolution in the shape of the distribution of high v particles since z~1, which continues to surprise me.

Incidentally, I was using the halo files in the halo directory (on which the subhalo finder has not yet been run) and it makes me nervous that halo 4 does not seem to agree with the plots I made using the halo_iso_004 file (which should have the same particles, just with subhalo labels). Another worrysome thing, it looks like several of these halos are in roughly the same spot, and the particles they are built out of all overlap, does this bother anybody else??