Galaxy Zoo Starburst Talk

Companions: how to tell when they're part of a merger?

  • JeanTate by JeanTate

    As opposed to being a satellite or not-really-interacting group/pair members?

    Somewhat related: if we had M31 (with its companions M32 and M110/NGC 205) as a control (QC) object, at what z would we both see either one or both companions (assuming favorable projection)? Ditto the Milky Way (and the LMC and SMC as companions)?

    The analysis (ses) we plan to do, involving 'merger fraction', does not attempt to distinguish between companions vs interacting/merging, but it does imply all companions are merging/interacting. The fact that we have a very good control sample means that the distinction is rather moot (i.e. we'll report a considerably, and significantly, higher 'merger fraction' for the QS objects), but in our paper, I think we should at least mention (if not discuss) the distinction.

    I tried to find a good paper on this, but failed (probably didn't look long, or carefully, enough). Of course, it's fairly straight-forward to estimate the z at which companions such as LMC/SMC/M32/M110 would become undetectable in the SDSS images we used; I'll have a go at that later.

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  • mlpeck by mlpeck in response to JeanTate's comment.

    ElizabethB consistently advises people to look for signs of morphological disturbance in deciding if there's evidence for a merger. If everyone followed her advice and we were 100% reliable at picking out morphologically disturbed galaxies we should have a high purity sample of disturbed galaxies. Whether we should call those mergers is maybe an interesting question.

    I'll try to dig up a reference for this, but from memory interactions between ∼equal mass galaxies produce tidal disturbances that are visible for ∼1 Gyr, which happens to be about the same timescale as spectroscopic indicators of a post-starburst population.

    I've been digging a bit into the work of Joshua Barnes, who's the world expert on N body simulations of interacting galaxies. He even gives away software for people who want to do their own. I'm not sure my several generations old Linux PC is up to that level of number crunching, but I might give it a try someday anyway.

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  • JeanTate by JeanTate

    It's actually a very interesting question, and a lot more complicated than I initially imagined.

    In terms of completing the analyses for the Quench paper, we have both the 'merger fraction' results, and the 'asymmetry fraction' ones (done by Chris Molloy). We can also download value-added data from sources such as Meert+ (2014)* and use TOPCAT (or similar) to show how zooites' 'merging' and 'asymmetry' classifications align with CAS (concentration asymmetry clumpiness) values (for example)^.

    If we run with "observational constraints on the role of mergers and AGN activity in quenching star formation", from Trouile+ (2014), as being a key result we present in our Quench paper, we surely do need to buttress our 'merger fraction' (etc) classification results with at least literature references, but better, some sort of matching, shouldn't we?

    *which includes "the CASGM measurements" of Conselice (2003) and Lotz+ (2013); I have the Meert+ (2014) catalogs, and have begun familarizing myself with them

    ^"There is also a strong relationship between the dynamical state of a galaxy and the presence of a merger. Generally, merging galaxies are asymmetric, while non-mergers are not" (source)

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  • mlpeck by mlpeck in response to JeanTate's comment.

    This review article is kind of interesting: Struck, C. 2005, "Galaxy Collisions - Dawn of a New Era." His research interests are actually in how (and where) galaxy interactions produce enhanced star formation.

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  • mlpeck by mlpeck

    This is unrelated (probably) to the Quench project but it does relate to the review article I linked the other day:

    Renaud, Bournaud & Duc 2014: A parsec-resolution simulation of the Antennae galaxies: Formation of star clusters during the merger.

    Follow the link to the movies. They're pretty neat!

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  • JeanTate by JeanTate in response to mlpeck's comment.

    Thanks for this! 😃

    I'm slowly my way through it (very interesting). One thing that struck me - nothing to do with mergers, per se - is that Bill Keel, who we know by the handle, NGC3314, is the author of quite a few of the cited papers; cool! 😄

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