Galaxy Zoo Starburst Talk

Galaxies which are too big (Petro_R50 >> fiber aperture)

  • JeanTate by JeanTate

    A thread devoted to exploring whether the QS and QC galaxies for which Petro_R50 is considerably greater than 3" should be excluded.

    Or analyzed separately.

    The background is in this thread, specifically a post by jtmendel (moderator, scientist):

    ... the fraction of total galaxy light observed by the SDSS fibers -- sometimes called the fiber covering fraction -- can have a significant effect on the observed spectrum. This is actually one thing that control galaxies can be very helpful for. Because there is a relatively good correlation between galaxy stellar mass and physical (as opposed to angular) size, comparing to galaxies at fixed stellar mass and redshift often does a reasonable job of controlling for variations in covering fraction. It might be interesting to see if something like this is feasible for the QS and QC samples.

    Let's start with the data; what are the objects - in QS and QC - for which Petro_R50 is > 20"? How many of them are galaxies? Fiber taking light to the spectrograph entered on the nucleus (or, if no obvious nucleus, the photocenter)? Etc?

    Posted

  • JeanTate by JeanTate

    Petro_R50 > 20"

    Unless there is a very good reason to keep these huge galaxies - if huge galaxies they be - it's hard to understand how the spectra from just a tiny part of the galaxy can tell us anything sensible.

    There are five QS objects this big, but only one QC one.

    All but one of these six are problematic; the one which does seem to have the fiber centered on (actually only vaguely near) nucleus of the galaxy zooites classified AND the Petro_R50 value seems about right is the QS object AGS00001b2 (588017702934872264):

    enter image description here Petro_R50 = 28.2"

    AGS00001b0 (588017564953280629), another QS object, is also an Eos (edge-on spiral), but this time the fiber is nowhere near the nucleus:

    enter image description here Petro_R50 = 22.7"

    The only QC object is an overlap AGS00002qp (587726014001578079); the fiber is centered on the nucleus of a distant, background galaxy (zsp=0.160), but the Petro_R50 refers to the huge foreground one (zsp=0.004):

    enter image description here Petro_R50 = 24.6"

    The remaining three QS objects are indeed galaxies ... but the Petro_R50 values are wrong, hopelessly so in one case, AGS00000s1 (587732469848015025; the photometric pipeline was fooled by the glare of the nearly bright star1):

    enter image description here Petro_R50 = 22.5"

    The last two - both QS galaxies - have Petro_R50 values which are far too high ... but not as ridiculously wild as in the previous two objects. First, AGS000021c (587742062663041028):

    enter image description here Petro_R50 = 25.6"

    And AGS000017a (588298664111112243; the fiber isn't centered on the nucleus either):

    enter image description here Petro_R50 = 20.4"

    1 I'm still looking into this, but so far I've found a good dozen other, similar, examples (of Petro_R50 being obviously, spectacularly, wrong, due to proximity to a bright star)

    Posted

  • JeanTate by JeanTate

    10" < Petro_R50 < 20"

    In terms of area, for a galaxy with a Petro_R50 of 10", the SDSS fiber which feeds the spectrographs covers only ~2% of the galaxy on-the-sky real estate from which 50% of its observed flux comes. Of course, there are many caveats; for example, some light from further out than 1.5" (the fiber radius) does get into the spectrograph (typical seeing - which is FWHM - in SDSS is 1.4"), and Petro_50 may be, um, misleading.

    But maybe there's a good QC counterpart to each - or at least some - QS galaxy with 10" > Petro_R50 > 20"; "good counterpart" in the sense that the redshift and Log_mass are ... close. Let's find out.

    The raw numbers: there are nine QS objects in this group, and eight QC ones. A good start.

    Eliminating the QS and two QC 'off-center' objects; AGS00000iz (587731187819937915; QS), AGS00002p7
    (587738411398070358), and AGS000034r (5877242401516421621) respectively:

    enter image description here enter image description here enter image description here

    Then there are the QS and the QC objects which - very likely - have wrong values of Petro_R50; AGS00001cm (758881522663490016; QS), and AGS000049f (587728666117603339; QC); "P50" is short for Petro_R50:

    enter image description here P50=11.3" enter image description here P50=10.3"

    Can the remaining seven QS and five QC galaxies be matched? How closely?

    Here's Laura, on the experimental design (source):

    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.

    All but one, maybe two, of the 12 remaining galaxies are "within a redshift of 0.02" of every other one; total stellar mass - as measured by log_mass - ranges from 7.8 to 10.9. So let's try matching on log_mass; QS galaxy first, and the numbers in brackets are (Petro_R50, log_mass, redshift).

    AGS00000sl (587729387683250194), AGS00004kq (587739114167009565):

    enter image description here (11.1, 8.6, 0.004) enter image description here (13.7, 8.3, 0.009)

    AGS00000dj (587728670415126752), AGS00004k3 (587742772950728790):

    enter image description here (10.9, 9.1, 0.013) enter image description here (12.1, 8.8, 0.006)

    AGS00001od (587739504470720606), AGS00002br (587741726576607381):

    enter image description here (10.8, 9.7, 0.023) enter image description here (11.5, 10.2, 0.015)

    AGS00001uz (587739828743962776), AGS00003p8 (588017604151541855):

    enter image description here (10.8, 10.7, 0.016) enter image description here (10.1, 10.9, 0.024)

    What of the remaining three QS and one QC galaxies? Curiously, two of the QS galaxies are also Eos ... is there something special about the spectra of large (physical size), low-redshift Eos galaxies which leads to their being selected by the "find me E+A galaxies!" selection algorithm?

    AGS00001az (588017703469187164), AGS00001n2 (587737827831382151):

    enter image description here (13.1, 9.2, 0.007) enter image description here (11.3, 9.6, 0.017)

    The QC galaxy is AGS00004ah (588017705079865511), a local dwarf:

    enter image description here (11.1, 7.8, 0.005)

    And the final QS galaxy is ... odd, AGS00001rq (587739406760870120); I zoomed in, and added the DR7 and DR10 images, so you can see what's odd:

    enter image description here (10.9, 9.3, 0.012)

    enter image description here enter image description here

    1 The QC catalog has another problem: if you click on the SkyServer link in the Quench/GZ Examine page, you end up with a totally different location!:

    enter image description here

    Posted

  • JeanTate by JeanTate

    From the last post, AGS00000sl (587729387683250194) illustrates a possible problem in using a spectrum with a tiny fiber covering fraction, to determine 'post-quench' status:

    enter image description here

    The galaxy clearly has many, very active star-forming regions ... so it cannot possibly be called 'post-quench'! However, I guess the spectrum (link) - which comes from a circle ~touching in the inner ends of the cross-hairs - has at least some post-quench features (why else would it be in the QS catalog?) ... but it has obvious HII region features:

    enter image description here

    Perhaps if more of the galaxy had been sampled by the spectrograph, the emission lines would be even more dominant, and the continuum bluer?

    Posted

  • JeanTate by JeanTate

    5" < Petro_R50 < 10"

    There are 89 QS objects with Petro_R50 between 5" and 10", and 95 QC ones. At 5", the fiber covering fraction is just a whisker under 10%.

    A dashboard I created, showing the size (Petro_R50) vs redshift distribution, for both QS and QC:
    http://tools.zooniverse.org/#/dashboards/galaxy_zoo_starburst/521849c2a6742469fc000136

    (I can't force the QS scatterplot's x-axis to go to 0.10, so the horizontal scales are not the same 😦)

    Aside from illustrating the tendency of the QS galaxies to be smaller than their QC counterparts (see also the non-Tools plots in this post), there are some QC outliers (numbers in brackets are (Petro_R50, log_mass, redshift)):

    AGS00002o8 (587736619320213712) - P50 is 11.18" in DR9 😮:

    enter image description here (9.1, 10.5, 0.081)

    AGS00003xb (587736813672792218) - P50 is 4.49" in DR9:

    enter image description here (9.8, 10.3, 0.068)

    AGS00002vf (587742551211769895) - a fascinating merger, and the main object (587742551211769894) has a P50 of 5.85" in DR9; AGS00002vf's itself is 0.60"

    enter image description here (6.6, 9.7, 0.087)

    Posted