DM halo merger rate

Vivienne Langen
  • 20 Feb '23

Hello there,

I was wondering, is there a straight forward way of calculating the DM halo merger rate in function of redshift (for a given mass and mass ratio bin)
or the rate in function of mass ratio (for given redshift bin) ?

I looked into illustris_python.sublink.numMergers() but this seems to look into the merger history of a particular halo over its 'live' (not all mergers happening in a given time).
Or I am missing a functionality of this build-in function?

Thanks a lot for your help!
regards, Vivienne

Dylan Nelson
  • 22 Feb '23

Are you interested in the TNG answer, or just the answer in general? A classic reference for this is Fakhouri+2010.

The available LHaloTree and SubLink trees for TNG are based on subhalos, not halos. If you wanted to calculate the halo-halo merger rate, you would either want to make new merger trees based on (FoF) halos, or else to filter the existing subhalo-based trees to e.g. only consider central subhalo mergers.

Vivienne Langen
  • 22 Feb '23

Hey Dylan,

Great! In fact this is exactly the reference I am trying to reproduce! So I guess I am looking for the TNG answer.
Maybe it would already help if you could lead me to the right data files, like mergerTrees, snapshots or groupcats? That contain the right ID information.

Okey your answer confused me more, I thought I only have to look at subhalos to get to Fakhouri+10; as subhalos contain galaxies, and like so BHs in the end. (because in the very end I am doing a study on BH merger rates).

Best, Vivienne

Dylan Nelson
  • 22 Feb '23

Hi Vivienne,

Halo merger rates, subhalo merger rates, and SMBH merger rates are all different things.

You can use the merger trees to compute the subhalo merger rate. To compute the SMBH merger rate, you should look at the SMBH particles across snapshots, or possibly think about using the Black hole details and mergers catalog.

Vivienne Langen
  • 2
  • 23 Feb '23

Hi Dylan,

I wonder, is there a physical (in terms of what they scientifically represent) difference on halo merger / subhalo merger rate?
or are these terms purely due to different algorithms used as described in Srisawat+2013 (

I am a bit confused about how Fakhouri+2010 is using the terms subhalos/FoF halos and halo merger rate. In their work they use the LhaloTree-algorithm which is based on Subhalos (Srisawat+13) and when describing their merger tree construction they're referring to Subhalos (SH). However later on they use the terms FoF halo density (to obtain the mean merger rate per halo) and halo merger rate.
Should it not be subhalo density and (mean) subhalo merger rate (per halo) following Srisawat+13. ?

In any case, thanks for your patience to answer all our (maybe naive) Qs!

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