Hello, I'm trying to understand how the locations of TNG galaxies are typically determined. In TNG I can get the coordinates of all star particles assigned to a particular subhalo, which I understand is taken to be a galaxy. In the data specifications, I see that for FoF groups (DM halos) the overall location is listed as the most bound particle. I don't see an equivalent overall location for subhalos.
For many purposes, we want a single galaxy location. I understand this may depend on the science case, but I don't see it written in the PNG papers or others that use it (e.g. https://arxiv.org/pdf/1905.08799.pdf). Is it typically taken to be the location of the most bound star particle in a subhalo? Or the center of mass of the star particles?
There is GroupPos for halos, and SubhaloPos for subhalos, they are similarly based on the minimum of the gravitational potential.
Hi Dylan, thanks for the quick response! I see SubhaloPos now. It says it is the "Spatial position within the periodic box (of the particle with the minium gravitational potential energy)." Is this for the most bound dark matter particle though? Is the location of the galaxy in the subhalo defined by the same position, or is it rather the most bound star particle?
Whenever we refer to a "most bound particle" or "particle with the minimum gravitational potential", it could be of any type, e.g. a DM particle, star, or gas cell.
Perhaps it is more clear to imagine that "the location of the galaxy in the subhalo" is always the center of the subhalo, i.e. the location of the subhalo and the location of the galaxy are the same.
This is only not true if you are thinking about a halo, rather than a subhalo, and a satellite galaxy. In this case, the location of a satellite galaxy is certainly not the location of its host halo, but rather the location of its parent subhalo.
I see, thanks so much!