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[(m_h ==0)] )
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.
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.
GroupNsubs == 0