Public Data Access Overview / Discussion Forum

Goal: Finding Stellar Mass Ratios for a Specific Merger

I'm working with subhaloes that experienced a single merger event between z=1 and z=0. I already know the snapshot number where the merger occurred. My goal is to determine the stellar mass ratio between the galaxies involved in that specific event.

To do this, I'm tracking a given subhalo at z=0 (snap99, TNG50) back to the merger moment (e.g., snap 54) to identify the two progenitors. Then, I go one snapshot earlier (snap 53) to retrieve their stellar masses before the merger.

Here’s the current logic I’m using to identify the progenitors at the merger snapshot:

Current Logic for Identifying Progenitors

Here’s the logic I’m currently using to identify the two progenitors at the pre-merger snapshot (snapshot_pre_merger), using the SubLink merger tree:

# Get First and Next progenitors at the merger snapshot
prog1_id = tree['FirstProgenitorID'][remnant_idx]
prog1_idx = id_to_index.get(prog1_id, -1)

prog2_id = tree['NextProgenitorID'][prog1_idx] if prog1_idx != -1 else -1
prog2_idx = id_to_index.get(prog2_id, -1)

# Safety check
if prog1_idx == -1 or prog2_idx == -1:
    raise RuntimeError("Less than two progenitors found for this merger.")

# Ensure both are in the expected snapshot (before the actual fusion)
if tree['SnapNum'][prog1_idx] != snapshot_pre_merger or tree['SnapNum'][prog2_idx] != snapshot_pre_merger:
    raise RuntimeError("Progenitors not found in the expected snapshot.")

However, I'm not entirely sure if this correctly identifies the subhalo that merged with my target galaxy at z=0. I’d like to perform a sanity check to ensure the code is capturing the right objects involved in the merger.

Does anyone have suggestions on how to robustly identify the progenitor IDs involved in a specific merger event that results in a given descendant at z=0?

Thanks in advance!

Note that, during the merger process (that can span e.g. many Gyr), the stellar mass of the secondary (in particular) will tend to decrease substantially, due e.g. to tidal stripping.

So it is common practice to define the stellar mass ratios of mergers in terms of the "maximum past stellar mass", not the stellar mass in the snapshot immediately before the merger.

You can find related discussion, definitions in Rodriguez-Gomez+ 2015.