[Illustris-1] Subhalos with mass_dm = 0.0

mauro sciarratta
  • 7 Sep '17


Is it possible (read: is it right) that subhalos are recognised as such even if, with non-zero stellar mass, they have mass_dm = 0.0? For example, looking at subhalos of parent halo id:0 at redshift z=0, I see that subhalo with id: 372 has:

"mass_dm": 0.0,
"mass_stars": 0.218179,

the list counts 38 subhalos over 16396 inside the whole cluster, I could provide the other 37 ids if necessary. What I am specifically asking is: in a spectrophotometric analysis of those subhalos should I consider those subhalos as galaxies (i.e. galaxies not embedded in dark matter haloes)?

Thanks for reading, best regards.


Dylan Nelson
  • 7 Sep '17

Hi Mauro,

This is the correct information for this subhalo yes, as you point out, 10^9.5 Msun in stars, no gas, no DM.

As to whether you should treat it as a real galaxy, it is a difficult question, and will depend on your science goal. Probably to make the best answer, you will want to look at the merger tree of this object, and figure out something about its origin. Once you know more information about its origin (and properties in the past), probably it will be clear.

It could be e.g. a stellar clump or star cluster which has been ejected from a central galaxy, or stripping off of an incoming satellite due to some sort of dynamical interaction. It could also be an infalling satellite which has been, by z=0, completely stripped of its gas and also become disassociated with its DM component due to dynamical friction. it could also be a substructure (i.e., still at z=0) of a galaxy -- the results of Subfind aren't necessarily guaranteed to be galaxies, they are rather hierarchical substructures which are gravitationally bound. You may check that e.g. SubhaloParent is the central subhalo of the group. And you may check the distance from the center of the halo -- if it is within 1 or 2 times the stellar half mass radius, for instance, then this is likely a substructure of a larger galaxy (i.e. an overdensity in a stellar disk) and not a galaxy in its own right.

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