D. Stoppacher's personal webpage

Personal research and project web page

View the Project on GitHub

A testrun on merger trees

—    —
Summary: In this post we study the redshift evolution of main progenitor halos in the Cholla simulation and show how we can easily construct merger trees of the same progenitors using two identification numbers provided by the halo finder as standard output.

Objectives of this test run

In this test run we aim to test if we can produce a valid merger tree of the main progenitor halos using only

  1. the ROCKSTAR halo finder (Behroozi et al. 2013a) to identify the halos
  2. the descendant identification numbers (IDs) -- descIDs provided by ROCKSTAR to link halos between snapshots. This ID represents the ID of the same halo in the next snapshot (moved forward in time).

These merger tree will only use the descID to make a connection between halos as a simple approach. Note that no tree builder algorithm was used therefore the merger tree was not fixed for e.g. missing links between snapshots or filling and correcting missing halos. See Behroozi et al. 2013b for details on that topic.

Task overview

  1. Run ROCKSTAR on the Cholla simulation (Schneider & Robertson 2015, Villaseñor et al. 2020)
  2. Convert ASCII output files of Rockstar which are named as out[snapshot].list into a custom HDF5 format.
  3. Use the haloid (ID of a certain halo in this snapshot) and the descIDs (the ID of the same halo in the next snapshot) to link a halo between snapshots.
  4. Introduce to more IDs which form together with the haloid and descIDs a set of IDs which identifies a certain halo uniquely within the entire merger tree and at every snapshot. Those are the predID (the ID of the same halo a snapshot one step back in time) and the treeID (an ascending counter assigned when the very first halo in the merger tree was detected. The lower the number the earlier the first progenitor was detected!) of the tree the halo sits on.
  5. Construct a merger tree using the set of four IDs [haloid,descID,predID,treeID].
  6. Validate the merger tree with comparing the number of particles found for a certain halo.

Task1: Find halos!

Run the ROCKSTAR halo finder catalog Behroozi et al. 2013a on the Colla particle simulation box with side-length 50 \(h^{-1}\)Mpc.

Status:  Task completed

Task2: Convert raw output from ROCKSTAR to custom file format

Thereby the units of kpc were converted to Mpc and the Hubble parameter absorbed in the value of the halo property.

Status:  Task completed

In this section we show that with the combination of haloid and descID which were provided by the ROCKSTAR as standard outputs the main progenitors (also the most massive progenitors) can be traced easily back and fourth in redshift.

The tables in this figure show lists of halo properties over four subsequent snapshots SN21 to SN24. In each table a particular halo is selected with the initial haloid of 3. This halo can be traced from one snapshot to another by comparing the haloid with the descID. Thereby the descID of e.g. SN21 for this halo is 29 which corrsponds to the haloid of the same halo in the next snapshot SN22. The treeID of the halo in this example is 3 which means that is was the third tree ever identified. This ID stays for all progenitors on the tree of this particular halo. If a new halo is identified in another snapshot with no progenitor, the treeID for this tree is simply assigned with +1 of the last identified progenitor tree.
In this cube visualize the location of the halos from Table 1 above for three subsequent snapshots. The halos in our initial snapshot SN21 are marked with a red cross. One can see that their progenitors can be clearly identified at the same location (blue circles for SN22 and green dots for SN23). If a halo was not already identified at SN21 or SN22, so its was newly found in the current snapshot it is only marked with a blue circle or a green dot, respectively.
Same as Figure 1 but the main progenitor halos at snapshot SN51 at z=0.81 are also visualized.

Status:  Task completed

Task3: Construct a merger tree of the main progenitors

After providing that we identify the correct halos when linking haloid and descID for each snapshot we can construct a simple merger tree considering the main progenitor halos.

The figure shows the location of all progenitors (progs) from selected trees (pink dots) from SN21 to SN51 at z=0.81 (the final snapshot for this test run was set arbitrarily). Various main progenitor are highlighted by their color coding corresponding to the treeID. Thereby our example form Figure 1 is represented by salmon dots. Some tree are marked extra with two color circles which represents their position at redshift z=2.03 (green circle) and z=0.81 (gray circle). If there are no circle in a certain tree that means that it did not exits at that snapshot anymore.

Status:  Task completed

Follow-up task & open questions

  1. Verify the approach of connection halos by the provided haloid and descID by comparing directly with the numbers of particle assigned to the halos between snapshots.
  2. How does ROCKSTAR assign the descID when running?
  3. Calculate halo mass function and accretion rates.
—    —