Major mergers identifications

Neda Stojkovic
• 2
• 10 Dec '19

Hello,

In one paper (https://arxiv.org/pdf/1909.01033.pdf) I found statement that major merger identification is not possible just looking at the merger tree masses:

"We are looking for major mergers, and keep therefore only mergers with a total mass ratio higher than 0.25. However the masses provided by Illustris cannot be fully trusted, since when two galaxies are close, Illustris struggles to attribute particles correctly to each galaxy. Taking Illustris masses at the moment of the merger would often result in having one galaxy taking most of the mass, and the other one having very little mass left, making it very difficult to find a major merger (see Fig. 7 for an example). It is thus needed to compute the masses of the galaxies before the two merging galaxies get too close to know what is the mass ratio between the two progenitors. We dismiss the snapshots corresponding to the merger period, where the galaxies are too close, and keep only snapshots before the merger where the distance between the two progenitors is higher than 80 kpc. Even so, the total mass can sometimes vary due to the presence of other galaxies, so we choose to average the mass of each progenitor over 3 snapshots, to have a more reliable estimate of the real mass."

Is that necessary, looking for mass corrections?

Also, I have another question. In your (amazing) Example scripts page there is a little script that plots total subhalo mass of five most massive primaries through history (snapshots). There is mostly upward trend with some dips, and in text you say:
"Note that the single-snapshot dips seen in the cyan and red curves can sometimes occur due to the 'subhalo switching problem'. The downward trend in mass followed by the sudden increase in the cyan is a signature of a merger. "
There is a reference for subhalo paper, but I didn't find 'subhalo switching problem' explained in the paper. Can you tell me what is the cause of the dips?

Dylan Nelson
• 10 Dec '19

Hi Neda,

Yes the first quote is definitely correct, i.e. the merger ratio cannot be defined "at the moment" of the merger. I suggest to read Rodriguez-Gomez+ (2015), Sec 5.2 and Fig 3 in particular, which shows clearly why this is.

The "subhalo switching" or "flip-flopping" or "central-subhalo switching" problem is: The tree builder mixes up links between two halos but corrects it in subsequent snapshot(s), which leads to large change in the halo properties in the snapshots where it happens. Some description in Poole+ (2017) Fig 1, S3.1.1.

Neda Stojkovic
• 1
• 10 Dec '19

Just to be clear: masses of the subhalos in merger which I can get downloading the merger tree are already corrected to the one when the subhalos had maximum stellar mass, so there is no need for any corrections on my side - for the merger ratio I can use masses provided for the moment of merger? Or masses aren't updated and are taken in the moments of merger and I should find the moments with maximum stellar mass to calculate merger ratio?

Dylan Nelson
• 10 Dec '19

Hi Neda,

Can you clarify exactly what data you are looking at?

If you download the full merger trees, they have datasets like `SubhaloMass(t)`, where by `t` I mean time. These are not corrected in any way, they are the direct values from the group catalog.

Neda Stojkovic
• 10 Dec '19

Ok, I think I understand. Subhalo masses are directly taken from subhalo catalog, but if I want to define mass ratio of the merger I would have to look for maximum stellar masses earlier in the subhalos' histories. So it's not enough to look for subhalo masses in the snapshot of the merger and calculate mass ratio of the merger, as that masses are unstable. In other words, we can't find out mass ratio of the merger at one snapshot using masses from the group catalog of that snapshot. Am I correct?

Dylan Nelson
• 10 Dec '19

Hi Neda,

Yes that is correct.

You can see an example of what you might want to do in the "numMergers" example function in the example scripts. There, a particular kind of mass ratio (defined as the maximum mass of some type, for both objects) is computed.

Neda Stojkovic
• 10 Dec '19

Thank you so much, Dylan!

Neda Stojkovic
• 28 Jan '20

Hello Dylan,

I have one more question about merger ratio. Your choice was to take stellar masses in the moment when secondary subhalo reaches its maximal stellar mass.
Is there some kind of reason why we don't use total mass (dm+stellar+gas) in mass ratio, and take total masses at the moment of secondary subhalo reaching maximal total mass?

Dylan Nelson
• 1
• 28 Jan '20

Both "stellar" and "total" mass ratios are definitely interesting for mergers. The former can, in theory, be measured observationally, so often this makes sense to investigate. But you're right that the subhalo (or, in essence, halo) mass ratio of a merger is also interesting. This has more to do then with the growth of dark matter halos in LCDM and the hierarchical assembly of structure, rather than details of the baryonic physics and the properties of galaxies which form in those halos.

Neda Stojkovic
• 28 Jan '20

So it depends on problem we investigate. Thank you very much!

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