Are subhalos with no stars considered in the merger history?

Gain Lee
  • 1
  • 16 Oct '23


I am writing to request some clarification regarding the merger history data in TNG50-1.

As I understand, the merger history data is based on the "stellar mass ratio", categorizing mergers into 3 cases: major (stellar mass ratio > 1/4), minor (stellar mass ratio between 1/10 and 1/4), and all mergers (any stellar mass ratio).

Specifically, I have a couple of questions related to this data:

  • Does the merger history data include cases where the stellar mass ratio is 0, indicating that the secondary subhalo had no stellar particles?
  • Does the merger history data consider cases where the primary subhalo had no stellar particles? For example, resulting in a stellar mass ratio something like 0 / 0?

If you could answer these questions or provide guidance on how I can investigate these questions, I would greatly appreciate it.


Dylan Nelson
  • 17 Oct '23

Just to clarify, all mergers (and the merger trees) account for all subhalos, including dark subhalos i.e. those with no stars.

However, the "Merger History" catalog you link is, as you say, specifically about galaxy mergers. (A galaxy is a subhalo with non-zero stellar mass). So the answer to both questions is no. In particular, if either the primary or the secondary has zero stellar mass at the time when the mass ratio is evaluated, these mergers are not counted.

If you are interested in subhalo mergers, merger rates, merger mass ratios, etc, this information is all available in the tree. With a fairly simple script you can load the "full" SubLink tree for a given subhalo and make such statistics.

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