Galaxy Zoo Starburst Talk

negative log mass

  • wassock by wassock moderator

    HI A simple scattergram of log mass vs redshift for the quench control data shows a lot of galaxies with a negative mass - all on the -0.5 mark which is where the scale stops, the quench sample has some also but only a few. So 2 questions what causes a negative mass to show and why are there so many more of these for the control group?

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

    I think this was addressed in the GZ (Tools) Hangout, i.e. in the video embedded in this GZ blog post: Next GZ (Tools!) Hangout: Wednesday 24th July, 20:30 GMT: estimates for the stellar masses of these galaxies was not loaded into the QS or QC database at the time it was created.

    There's more on this topic in the QT (Quench Talk) thread "Reliable masses"?

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  • wassock by wassock moderator

    Thanks Jean, had that wrong any way, the sample group has more negatives. Just to clarify, the sample group has the post quenched stuff and the control is mostly still star forming?

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

    I knew I'd seen it somewhere ...

    For each post-quenched galaxy, we identified a mass and redshift-matched galaxy. By mass-matched, I mean a galaxy with a total stellar mass within a factor of a few of the post-quenched galaxy. And my redshift-matched, I mean a galaxy within a redshift of 0.02.

    That's Laura, who is the PI (Principal1 Investigator) of the Quench project (at least, I think that's the case). The "somewhere" isn't so obvious to find; it's in a thread entitled Characterising classification biases, and in the second post in that thread.

    What's odd is that if the galaxies are "mass-matched", how come so many galaxies - in QS and also in QC - do not have masses? In QC, I count just four without masses (i.e. log_mass = -1), but in QS 112. What are these 112 (and 4) matched with?

    1 In this particular project, I suspect she's also the Principle Investigator too 😉

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  • trouille by trouille scientist, moderator, admin

    To follow up on this, the post-quenched galaxy sample definitely has 112 galaxies whose mass is equal to '-1'. We just used '-1' as a placeholder for these sources. We aren't able to determine reliable stellar masses for them. For any plots or analysis we do that depends on mass, we'll need to filter these sources out.

    Even if we couldn't determine reliable masses for these sources, we were able to see that their emission and absorption lines cause them to fall into the post-quenched category. So that's why they're included in the sample.

    In order to find a matching control galaxy for each of these 112 sources, we only used their redshift (since they don't have a mass to fix on!).

    In my program that finds control galaxies (written in IDL, a common language that astronomers use), I didn't require that these control galaxies (chosen only based on redshift) have a reliable mass. It just so happened that 5 of the 112 do not. That's why you see 5 control galaxies with mass equal to '-1'.

    So for both the sample and control galaxies, for any plot that depends on mass, be sure to put a filter that only uses sources with masses greater than 0.

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

    In my program that finds control galaxies (written in IDL, a common language that astronomers use), I didn't require that these control galaxies (chosen only based on redshift) have a reliable mass. It just so happened that 5 of the 112 do not. That's why you see 5 control galaxies with mass equal to '-1'.

    Thanks Laura.

    For the 112 QS objects with unreliable (or no) estimates of stellar mass, there surely must be hundreds (or more) SDSS galaxies with reliable redshifts that are within the selection criterion (±0.02, if memory serves), right? So did your IDL routine choose one at random? If not, how did you make the selection?

    In a nice piece of work, mlpeck produced estimates of stellar mass for 109 (110?) of these 112, and showed that these estimates are likely as robust as those for the 2890 other QS objects, or at least to within a 0.3 dex or so (again, I'm going by memory; I'll check up on this later). With these mlpeck stellar mass estimates in hand, how well do the corresponding selected QC objects match? Specifically, are they as close as for the remaining 2890 QS-QC pairs?

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