Radio Galaxy Zoo Talk

ARG0000hga - the "missing link" disk galaxy! Magic word: "quenched"

  • JeanTate by JeanTate

    SDSS J123847.46+520302.1 is DR8 ObjId 588013382745391278, and by a quirk of cosmic fate it's also AGS00000q9. Its z_sp is 0.221, and its spectrum instantly recognizable to a small cadre of zooites (as well as most, I would guess, extra-galactic optical astronomers):

    enter image description here

    enter image description here

    Yep, that's clearly not a boring elliptical, much like a post-merger or late-merger system, at least one of the formerly independent galaxies being a classic disk, rich in star-forming regions. The spectrum clinches it; barely tens of millions of years' ago (not counting the time it took the light to get to us), this galaxy (or galaxies) was ablaze with star-formation; now it's all but stopped, quenched in fact.

    The spectrum also shows an active nucleus, and its FIRST triple nature confirms radio activity:

    enter image description here

    And the 'quirk of cosmic fate'? Just yesterday I wrote*:

    if not-boring-ellipticals with doublelobes seem to include lots of late/post-mergers (Cen A anyone?), maybe post-quenched galaxies - many of which are clearly spirals - will also host doublelobes?

    I have a new research project; ivywong (among others), would you care to join in?

    Boilerplate: SDSS image per http://skyservice.pha.jhu.edu/DR10/ImgCutout/getjpeg.aspx, FIRST (red) contours derived from the FITS file produced using SkyView with Python code described in this RGZ Talk thread. Image center is the galaxy SDSS J123847.46+520302.1, near the center of the ARG image (ARG0000hga; J2000.0). "z_sp" is an SDSS spectroscopic redshift of the galaxy in the center.

    *source is page 9 of the Hourglass sources associated with spiral galaxies thread ("Posted August 5 2014 5:13 PM")

    Posted

  • ivywong by ivywong scientist, admin

    Hi Jean,

    Good one! A post-starburst. Now you have my attention (well, RGZ always did.. but yesterday, I was down with the flu and it was one of few days in the year when I was not regularly checking RGZTalk)

    For the moment, it'd be good to put them in with the spiral sample. I'd say an interesting question to now pose is "how many of the radio sources we spot with spiral hosts have similar post-starburst properties?"

    The only thing is that given the redshift of the galaxy, it's more difficult to infer post-merger properties without deeper observations searching for merger signatures such as tidal features.

    Posted

  • WizardHowl by WizardHowl

    I checked my collection of disturbed-looking galaxies and found three others with lobed emission and spectra resembling that of SDSS J123847.46+520302.1 to some degree. Are these also quenched galaxies or just dusty? They are listed below, the closest match to the spectrum 1st:

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0001coj -- triple, host is very disturbed galaxy as if in merger SDSS J085356.80+362455.9 Z_sp=.260 spectrum classed AGN broadline

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0002yox -- triple, badly disrupted host SDSS J095740.37+111620.4 Z_sp=0.276

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0002coz -- overedge triple, host has disturbed features suggestive of an interaction, SDSS J112558.74+200554.3 Z_sp=0.133

    I also found one with compact emission only (if this is still useful?):

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0003764 -- compact, SDSS J152412.58+083241.2 Z_sp=.037 faint hint of radio emission on SE rim matches bluer area in SDSS - Voorwerp?

    Posted

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

    Very cool! 😃

    I'll comment on each of them later, but for now just some notes on "post-starburst", "E+A", "K+A", "quenched", etc galaxies. Big caveat: what follows is my own understanding of the relevant astronomy and astrophysics; I hope someone like ivywong will jump in and correct the mistakes (of which there are surely some).

    These galaxies have very distinctive spectra, looking much like a combination of A (spectral) class star and K, or like a typical elliptical (E) and A. This SDSS webpage has some nice template spectra, what various classes of object look like when their spectra are taken by the original (not BOSS) spectrograph. For A stars, the hydrogen Balmer series is really strong, in absorption, with 'line depth' increasing with wavelength (i.e. H-beta is stronger than H-alpha; H-gamma stronger than H-beta; etc).

    Why post-starburst? If a galaxy has a strong starburst - huge numbers of stars formed in a very short period of time, followed by close-to-zero ongoing star formation - once the most massive and brightest stars have died (become supernovae, etc), the Main Sequence A stars will dominate the light, along with some red giants and the old stellar population that was there before the starburst (these collectively have a spectrum resembling that of a K class star); hence A+K.

    Of course, few galaxies are such simple systems, and have such simple histories. There may be some on-going star formation - hence some H-alpha, [OIII], and [OII] (etc) emission lines - and many will have AGN (hence the same emission lines, plus, perhaps, [NeIII]; the Balmer emission lines may be broadened too).

    Here's the spectrum of the Quench project object AGS000000x, a very clean example:

    enter image description here

    This galaxy is also 'white' in SDSS images; a distinctive and common feature of these sorts of galaxies*:

    enter image description here

    "Common" is not the same as "universal", of course; in particular, dust can redden the white, to nice shades of salmon and orange .... (examples? just ask! 😃).

    *I wrote about this in two GZF OOTDs, White, and A Paler Shade of White

    Posted

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

    Thanks Ivy!

    Some time ago, I decided that to belong to 'the spiral sample' a candidate host needed to meet just one criterion*, "not a boring elliptical". Sure, this may be too broad - so the spiral sample collected using this criterion will contain objects we later decide do not belong - but I think it's better too broad than too narrow (i.e. risks excluding objects we'd really want to include); the old 'Type I vs Type II' conundrum.

    The inevitable redshift bias is interesting; on the one hand, 'spirals' are hard to robustly identify beyond z ~0.25 (though there are quantitative tools to help distinguish 'elliptical' from 'spiral', out to 0.4 and even beyond; an Eos, for example, cannot be mistaken for an elliptical, if big enough); on the other hand, at low-z we're very likely to miss the "J2345-0449"-like spirals because their lobes are so far from the host (and too diffuse to show up in FIRST)^.

    In the old days - before the discovery of red spirals and blue ellipticals - an object with a post-starburst spectrum would, by definition, be 'not a boring elliptical' (ellipticals were thought to be so devoid of cold gas and dust as to make starbursts an impossibility); today, post-starburst objects may not (no longer) have disks!

    To me it's rather strange; 'mergers' used to be 'peculiar' or 'irregular', yet we now know they comprise ~1% of all galaxies. And we also now know that a great many 'boring ellipticals' have quite distinct stellar disks, despite their morphology. Even the obvious presence of a dust lane ain't what it used to be ...

    *in terms of its features in the optical/NIR part of the spectrum; radio-wise a key criterion is that the radio emission appears to extend (well) beyond the optical boundary of the candidate host

    ^though Dolorous Edd may find them anyway

    Posted

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

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0001coj -- triple, host is very disturbed galaxy as if in merger SDSS J085356.80+362455.9 Z_sp=.260 spectrum classed AGN broadline

    SDSS J085356.80+362455.9, z_sp 0.260:

    enter image description here

    enter image description here

    The SDSS apparent color is similar to that of a post-starburst galaxy, and the fact that it's apparently involved in a major merger is consistent. The Balmer series switches to absorption from H-delta, perhaps H-gamma, and absorption lines in the series are apparent blue-ward of the H&K lines (this is usually a very messy part of most galaxy's SDSS spectrum), so it does have features in common with a post-starburst galaxy. However, the AGN component is very strong, so it's likely only a quantitative analysis could reveal just how close this is to being (or containing) a post-starburst.

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0002yox -- triple, badly disrupted host SDSS J095740.37+111620.4 Z_sp=0.276

    SDSS J095740.37+111620.4, z_sp 0.276:

    enter image description here

    enter image description here

    No apparent significant Balmer series absorption, though the obvious AGN features may be drowning them out (or not). Doesn't strike me as an 'E+A' spectrum, even allowing for the AGN component. Dust? Hard to say, without a quantitative analysis. Definitely 'NOT a boring elliptical'! 😃

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0002coz -- overedge triple, host has disturbed features suggestive of an interaction, SDSS J112558.74+200554.3 Z_sp=0.133

    SDSS J112558.74+200554.3, z_sp 0.133:

    enter image description here

    enter image description here

    Like 2yox, the AGN component is very strong; it even produces a rise in the continuum in the UV; also like 2yox, it is obviously NOT a boring elliptical! 😃 No post-starburst features in the spectrum, that I can see.

    I also found one with compact emission only (if this is still useful?):

    Yes.

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0003764 -- compact, SDSS J152412.58+083241.2 Z_sp=.037 faint hint of radio emission on SE rim matches bluer area in SDSS - Voorwerp?

    SDSS J152412.58+083241.2, z_sp 0.037:

    enter image description here

    enter image description here

    Again, the spectrum has strong AGN features. I don't think the Balmer series absorption is really like that of an A star - blue-ward of H-delta, for example, there seems to be little. In this case, however, the aperture effect is likely to be huge ... the spectrum is of light from the central ~3" (diameter) only; clearly, the galaxy beyond this is very different from the center. That means we can't really say anything about most of the galaxy, unlike in the other three objects. But its color is anything but white, so it's unlikely to be a post-starburst object, especially as the redshift is low and we're seeing it near face-on (so there are unlikely to be obscuring dust lanes).

    Well, that's my personal opinion; perhaps an astronomer could weigh in?

    Posted

  • WizardHowl by WizardHowl

    Another quenched radio galaxy has shown up, first commented on by @andymarrison and also noted by @ivywong as being in a merger. Only one of the two galaxies in the merger has a quenched spectrum, interestingly.

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0002a8x

    The right hand galaxy SDSS J130117.75+211436.1 at Z_sp=0.081 is the one with the quenched spectrum. The other one, which seems to be a disk galaxy, has only very weak radio emission and a spectrum classed as starburst. The quenched galaxy has a spectrum classified as AGN broadline. The radio emission from the quenched galaxy is extended and it seems to me to be likely a mixture of emission from the AGN and from star formation, though a small corejet might be a possibility.

    Posted

  • ivywong by ivywong scientist, admin

    I think quenching is interesting in general but the great thing about these objects is the timescale. They are all experiencing "sudden death" and very fast quenching. Understanding the extreme physical mechanisms that cause this sudden quenching is what drives much of my research interests too.

    Posted