Radio Galaxy Zoo Talk

Galaxies merging in SDSS image?

  • andymarrison by andymarrison

    Not a big radio signal, but the SDSS image suggests it's from the two galaxies at centre of SDSS frame (both have same redshift). Looks like they are merging or colliding. If not, they look nice, anyway!!

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  • ivywong by ivywong scientist, admin

    Excellent find @andymarrison! #merger

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  • WizardHowl by WizardHowl

    QUENCHED SPECTRUM. Sorry for caps but this needs to be shouted! Increasing depth in the H absorption lines from H beta through to H delta in the right hand galaxy SDSS J130117.75+211436.1 at Z_sp=0.081. The radio emission seems like it is probably a mixture of core emission and star formation, though a small corejet might also be possible.

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

    Very nice find! 😃

    This merging system is not, repeat NOT, one of the 3002 quenched objects in the GZ Quench project! 😮 Cool!! 😄

    Contour overlay image coming right up ...

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

    Contour overlay image coming right up ...

    A mere 35 minutes' later ...

    enter image description here

    enter image description here

    And the very distinctive spectrum:
    enter image description here

    Yes, definitely quenched; however, also definitely broadline QSO. The radio emission is extended, not at all simply a point-source; jets?

    The galaxy to the E is SDSS J130118.55+211428.7, and it too has a spectrum (z_sp 0.080). The SDSS spectroscopic pipeline classifies it as STARBURST, and it's rather faint nuclear (point-source) radio emission is consistent with this.
    enter image description here

    The image in this post was created from sources, and using methods, described in this RGZ Talk post. The object at the center of the image is SDSS J130117.75+211436.1.

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  • JeanTate by JeanTate

    Just posted this in Quench Talk: Nice quenched object not in Quench

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  • WizardHowl by WizardHowl

    Quite a coincidence for the discussion about UGC 6081 ( http://radiotalk.galaxyzoo.org/#/boards/BRG0000003/discussions/DRG000097c ) to come up at about the same time and interesting to compare the three systems (these two and the one from ARG0000hga). All have emission from the host galaxy AGN, however two have more extended emission. In this case, the emission does not extend beyond the boundary of the optical host, which is in mid-merger (though small jets/corejet is still possible). Only one has clear radio lobes extending beyond the host galaxy, which in that case is not mid-merger though it's shape is disturbed and thus potentially post-merger/interaction.

    My guess would be that UGC 6081 is the most recently quenched of the three, with the weakest H absorption, implying that the quenching happened very quickly during the early stages of the merger. Then this case here, which has a clearly quenched spectrum but is still mid-merger with both interacting galaxies still close and some star formation still going on in oneof the two galaxies and perhaps amongst the tidal debris but only small jets, if any, from the quenched galaxy. ARG0000hga would then seem to show the older quenched system, with the other galaxy it interacted with either absorbed or having moved away and with radio lobes extending beyond the optical boundary of the quenched galaxy.

    If this speculation is broadly how things are happening, then it's interesting to see star formation stopping so soon during mergers. I have recently read something about how mergers can shut down star formation by heating up the cold gas reservoirs that are needed for it to take place (NGC 3226 was the galaxy, it featured in a recent NASA press release) so perhaps something like that is going on here. I suppose the activity of the AGN might then be a factor in whether or not the gas can cool enough after the merger for stars to form once again, or whether it's enough to turn the galaxy into a red and dead elliptical. Fascinating objects!

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  • ivywong by ivywong scientist, admin

    @WizardHowl: Glad you brought this up. When galaxies experience strong tidal interactions (which may or may not lead to mergers), the gas in the galaxies get compressed and a strong starburst is usually triggered. In terms of star formation history, this will lead to a spike in star formation which uses up a fair bit of gas but then the gas that does not get compressed get stripped off in the tidal process. So the cumulative effect is a sudden loss of gas then plunges the star formation history. On top of everything, if there is an AGN in any of these galaxies, the remaining disturbed and/or diffused gas will get further ionised and blown out. Hence resulting in what appears to be a sudden shutdown. A few of us has a submitted paper recently about the sudden truncation of galaxies so once it's accepted, I'll post a blog about it and link it to the paper in the Journal Club. It's definitely an exciting area of astrophysics 😃

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  • andymarrison by andymarrison

    Sorry for my Ignorance, guys, but what particular feature of the spectrum tells us that the galaxy is post quench (by which I take to mean that its star formation has been turned off prematurely compared with more typical spirals)? More hydrogen than otherwise and not as many of the heavier elements? Had a quick look at the link to the Quench thread, thanks #jeantate, but couldn't find a ready answer and am not familiar with this other very interesting GZ project.

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

    Hi andymarrison.

    what particular feature of the spectrum tells us that the galaxy is post quench

    It's the fact that the hydrogen Balmer series lines are 'in absorption', especially from Hβ on. This is a very distinctive feature, particularly noticable in the spectrum of "A" stars (i.e. stars whose spectroscopic class is "A"). Here are two examples from the SDSS templates (the second is an "F/A transition star"):

    enter image description here
    enter image description here

    Although they're not marked in the galaxy's spectrum (and many Balmer lines are 'off the chart' to the left in the star spectra), narrow and deep Balmer absorption lines can be seen beyond the last one marked (Hδ), at least to Hη (I think). Whether Hα is also 'in absorption' is hard to tell from the PNG file posted, but using the SDSS interactive tool you can see that it's actually an emission line.

    How come? Because in the nucleus is a supermassive black hole surrounded by an accretion disk; that disk is shining so brightly that it ionizes the surrounding interstellar medium, and by the magic of atomic physics and quantum mechanics that results in the atomic hydrogen emitting huge numbers of Balmer-alpha photons (and Lyman-alpha ones too, but they're too far into the ultraviolet to be detected by the SDSS spectrograph). And these Hα photons more than 'make up for' absorbed by the photospheres of the A stars.

    For more background, you may be interested in some GZ forum Object of the Day posts/threads I wrote, Saturday, 17th August, 2013: White, Saturday, 31st August, 2013: A Paler Shade of White, and Saturday, 19th February 2011: new galaxy type - "E+A progenitor".

    I do not know how familiar you are with spectra, the Balmer series, and so on, so if any of this is too much, please say so and I'd be happy to have a go at explaining it in more detail.

    To finish, here's something really neat: in the spectrum of the "F/A transition star", you can see some (~seven?) broad squiggles at the right, like a series of absorption lines getting fainter and closer together towards the left (blue). I cannot be 100% sure, but I think these are the 'tail end' of the next hydrogen series, Paschen. The lines seem to begin at 902nm, which would be Paschen-η. Can at least some of the Paschen series absorption lines be seen in the spectrum of SDSS J130117.75+211436.1? If so, what does this tell us about the star formation history (of at least that part of the galaxy whose light enters the SDSS spectrograph)?

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  • mlpeck by mlpeck

    Hi @andymarrison:

    what particular feature of the spectrum tells us that the galaxy is
    post quench

    Optical spectroscopic features that are taken to indicate a recently quenched starburst are strong absorption in higher order Balmer lines (Hγ, Hδ, Hε, ...) along with lines that are seen in cooler stars. Balmer lines are strongest in A stars that have lifetimes of order 0.1 - 1 Gyr. Lines like Ca II H&K (at 3933 and 3968Å), Magnesium around 5175Å, and Na D are characteristic of cooler stars. Redward of Hα there are also molecular bands from TiO and (I think) CN (maybe others as well) that are seen in K and M stars.

    Seeing a strong contribution from A stars is generally interpreted to mean that star formation was truncated ∼0.1-1 Gyr ago. One complicating factor is that young stars could still be hidden in their "birth clouds" of dust and not seen at visual wavelengths. When Goto published the first large catalog of post-starburst galaxies from SDSS he required weak emission from O II and Hα+N II along with strong Hδ absorption as a way of selecting against dusty star forming galaxies. @ivywong knows something about this subject by the way.

    I only recently noticed that Goto had compiled a catalog of post-starburst AGN that apparently remains unpublished. Those were selected to have strong Hδ absorption and emission line ratios characteristic of AGN. It's actually not too hard to construct one's own catalog with a CasJobs query. I've found about 800 in DR7 with rather restrictive selection criteria. Unfortunately the MPA group was the only one producing public measurements of absorption line indices and for some reason they are officially "deprecated" as of DR10; their spectroscopic processing pipeline has not been run on any BOSS targets.

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  • andymarrison by andymarrison

    @JeanTate and @mlpeck - thank you so much for taking the time to explain - I really do appreciate it. I'm familiar with emission and absorption lines etc, but where I'm sadly lacking is in knowing how the presence or absence of certain lines in the spectra of stars indicate at what stage that star, or population of stars, has reached in its evolution. My initial guess was that younger stars would show predominantly hydrogen and helium lines, with lines for heavier elements becoming gradually more present in older populations (as those stars exhaust their supplies of hydrogen and start to fuse helium, etc etc). Is that broadly right? I think your pits mentioned a "template" set of spectra for different classes of stars, @JeanTate?
    Thanks again guys - it's not an easy subject to get to grips with and I appreciate your help.

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

    You're very welcome, andymarrison.

    ... mentioned a "template" set of spectra for different classes of stars

    Yes. This SDSS page has 33 Spectral cross-correlation templates. They were used in DR5 to at least DR7, for automatically assigning a spectral class identification to the then SDSS spectra*; for extra-galactic objects (i.e. galaxies and QSOs) they were used to determine the redshift. What's displayed on this page is only the 'low res' gif spectrum; what was used by the pipeline was the 'full res' FITS one. They're quite educational, but also far from complete; for example, there's no 'post quenched' template 😉

    *today the spectra are taken with a new spectrograph, pipeline, etc, BOSS

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