I'm wondering why there are so many halos with zero mass. And there are 5 halos with zero mass but non-zero SFR. I will appreciate if you can help me on that. Thx.

Dylan Nelson

20 Mar '19

Hello,

I believe that the SO measurements (such as Group_M_Mean200) are not carried out in the case that a group has no subhalos. That is, when a group contains no collection of gravitationally bound elements numbering at least 20 (the minimum of Subfind).

So these should be very small (dark) halos and I expect not of interest for your analysis.

You could verify that they all have GroupNsubs == 0. Likewise if you ever make a selection of subhalos with stellar mass greater than zero, I think all of their parent groups should have non-zero SO quantities.

Dear Dr. Nelson,

I'm using illustris_python to load the halo mass of the group catalog from TNG100-1 at z = 0.

basePath = '../TNG100-1/output'

fields = ['GroupSFR','Group_M_Mean200']

halos = il.groupcat.loadHalos(basePath,99,fields=fields)

m_h = halos['Group_M_Mean200'] * 1e10

print len(m_h)

print len(m_h[(m_h ==0)] )

output:6291349

2861323

I'm wondering why there are so many halos with zero mass. And there are 5 halos with zero mass but non-zero SFR. I will appreciate if you can help me on that. Thx.

Hello,

I believe that the SO measurements (such as

`Group_M_Mean200`

) are not carried out in the case that a group has no subhalos. That is, when a group contains no collection of gravitationally bound elements numbering at least 20 (the minimum of Subfind).So these should be very small (dark) halos and I expect not of interest for your analysis.

You could verify that they all have

`GroupNsubs == 0`

. Likewise if you ever make a selection of subhalos with stellar mass greater than zero, I think all of their parent groups should have non-zero SO quantities.