[yt-users] cookbook_hop_mass_sum.py

Stephen Skory stephenskory at yahoo.com
Mon Nov 23 14:25:13 PST 2009


> This is the first line of hop_mass_info.txt...
> Total mass in HOP group 0 is 1.65889e+15 (gas = 2.61466e+14 / particles = 
> 1.39743e+15)
> Question : This is in units of solar masses (without any h factors). Right ?

Yes, Msol, not Msol/h.
> This is the first line of Hop_Analysis.out...
> Group Mass    # part  max densx       y       z       center-of-mass  x       y 
>       z       vx      vy      vz      max_r
>         0      1.009603589e+15       3202      6.803410660e+03 2.691574395e-01 
> 1.892197430e-01 8.293117285e-01 2.701045173e-01 1.889703433e-01 8.276973165e-01 
> -6.232934392e+06
> -2.872425257e+07        1.526929994e+06 1.886896417e-02
> Question : Why is 1.009603589e+15  different from 1.39743e+15  ? In fact, why is 
> it less ?

The mass provided by HOP is the simple sum of the mass of the particles that make up the halo as defined by HOP.

The halo profiler finds mass in a completely different way. It finds the mass inside of spherical shells centered on the halo, and adds that up out to the radius provided by HOP for that halo. This will include the dark matter particles in the halo, but also particles not in the halo (because the HOP halo isn't necessarily spherical), and baryons. The mass that is reported by the profiler is the virial mass of the halo at whatever value you set the virial threshold, which is probably the typical value of 200 times the average density. So the numbers are different because they are measuring different things.

Good luck!

sskory at physics.ucsd.edu           o__  Stephen Skory
http://physics.ucsd.edu/~sskory/ _.>/ _Graduate Student

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