Gas masses in halos and subhalos

Farhanul Hasan
  • 10 Oct '22


In the documentation, it says that the SubhaloMassType field in the subhalo catalog represents the "Total mass of all member particle/cells which are bound to this Subhalo, separated by type". So for example, this field would give me the mass of all gas cells bound to a given subhalo. But is there a physical radius within which this is calculated? If I found that a given subhalo has, say a gas mass of 1e9, is there a defined radius from the center of the subhalo within which this is true?

Similarly, GroupMassType lists the "Sum of the individual masses of every particle/cell, split into the six different types, in this group". Is the gas mass then calculated within, say the virial radius, or some other radius from the center of a group?


Dylan Nelson
  • 11 Oct '22

Halos and subhalos will both have a maximum radius, within which all their members are found. This value is not stored anywhere, but you could calculate it.

However, such a radius is not used in their identification. Instead, halos are based on the friends-of-friends algorithm, while subhalos are based on the physical gravitational unbinding algorithm in Subfind. Neither uses a radius.

Farhanul Hasan
  • 12 Oct '22

Thank you. So if I wanted to calculate this maximum radius, would I have to look at particle data also?

Dylan Nelson
  • 13 Oct '22

Yes that's correct, but it is a fairly quick calculation, since you only need to consider the particles belonging to each (sub)halo, to calculate this radius.

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