Radio Galaxy Zoo Talk

Recent science paper: Bent-Tailed Radio Sources In The Australia Telescope Large Area Survey Of The Chandra Deep Field-South

  • KWillett by KWillett scientist, admin, translator

    Thought I'd point out a paper that appeared on the arXiv (the physics/astronomy pre-print server) a few days ago. Led by researchers at VUW in New Zealand, as well as science team member Ray Norris ( @raynorris ) as a co-author. They're analyzing images of both ATLAS galaxies (which we hope to put on RGZ soon) and VLA images similar to the ones that you see on the site right now. They're examining an area of the sky in the Southern Hemisphere which we haven't looked at yet in RGZ, which is called the CDFS (Chandra Deep Field South) and looking for bent-tailed radio sources.

    Their smaller survey finds 45 bent-tailed radio sources and seven complex, multi-component sources (along with a few diffuse objects). Do these sources look similar to what you've seen in RGZ?

    Figure 2 from Dehghan et al. (2014)

    Figure 2 from Dehghan et al. (2014)

    Link to the original paper is here - please discuss if you have questions, or read something interesting!


  • 42jkb by 42jkb scientist, admin

    Great post Kyle. I had a chance to read over this paper before it was submitted to the journal and it is great work! I would like to point out that these particular radio galaxies will become available in the next data addition to RGZ. I think you should write a blog post on this.


  • JeanTate by JeanTate

    Cool! 😃

    Do these sources look similar to what you've seen in RGZ?

    I'm not 100% sure I understand the color coding in the contours, but yes, I think I've seen RGZ objects that look similar to many (but not all!) of these. However, there seem to be more 'apparently unrelated' sources in these fields than in most RGZ fields; is this real? Might it lead to a situation - given some combo of resolution and sensitivity - where the sky is wall-to-wall radio sources (i.e. there's no radio sight-line to 'blank sky')?

    Related RGZ Talk thread: Has 'the new ATLAS data' been loaded into RGZ yet?


  • 42jkb by 42jkb scientist, admin

    The colour coding in these images has to do with the different resolutions of the radio data that they used in their analysis. ATLAS observes much fainter radio sources than the images we show on RGZ, as a result there may be more sources in the fields.

    Yes, if our observations are too deep and based on the resolution we can have a situation called "confusion" where the whole image is "wall-to-wall radio sources (i.e. there's no radio sight-line to 'blank sky')".


  • JeanTate by JeanTate in response to 42jkb's comment.

    Cool! 😃 Thanks.

    Re "confusion": thinking about this a bit, I wonder if there's a CRB (cosmic radio background), a spectrum counterpart to the CMB and CIB (but not necessarily physically related, of course)?

    Increased resolution will show the details of lobes, jets, relics, etc (and identify truly compact/point sources); increased sensitivity will show the extent - both physical and in sky real estate - of such extended objects. As the 'radio wall' is so much further away (higher z) than the optical one (once the Lyman limit is redshifted into the optical, the sky is dark), greater sensitivity and resolution will reveal not only more sources - at all redshifts - but also the extended sources will expand (current not-deep surveys merely detect the 'tip of the iceberg' of extended sources). And unlike in the optical - where galaxies do indeed 'get bigger' as sensitivity goes up, but not by much - the fainter regions of lobes, relics (etc) take up huge amounts of sky real estate.

    Do radio astronomers expect there to be a CRB? Or is there something about the nature and distribution of extended radio sources that means a CRB is quite unlikely, at any radio frequency (barring unknown, new, classes of radio object)?


  • enno.middelberg by enno.middelberg scientist, translator in response to JeanTate's comment.

    Keep in mind that the CMB is only called "microwave" background because it peaks in the mm regime. Since the CMB is a black body it has a tail which extends all the way to the radio, but it is more than three orders of magnitude fainter than at mm wavelengths. Check out for a spectrum.

    Of course, as one makes more and more sensitive observations radio sources will eventually fill the entire sky, which is then called "confusion", because we don't know where one source begins and the other one ends. Here's what it will look like: (paper:


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


    Ah yes, the Rayleigh–Jeans law and the red-tail of the black-body spectrum; if our eyes could see 'radio' and if all foregrounds could be removed, the CMB would look like a uniform, white haze, with a slightly bluish tinge; it'd be 'white hot' (except for it being nearly close to absolute zero).

    The paper provides a nice amount of detail to expand on what 42jkb wrote (here), about the two primary source of extra-galactic radio
    emission (AGNs and star-forming regions, SFRs).

    The radio emission from SFRs is compact, compared with that from AGNs: "[...] provides independent support for the claim that most faint radio sources are in star-forming galaxies. Such radio sources are usually somewhat smaller than their optical host galaxies, [...]" From my own RGZ classifications - and subsequent attempts to find hosts - this feels right ... if a radio source is extended, look for the host elsewhere, possibly overedge, if it's not obvious in the SDSS image; if it's compact (and especially if it's bright), it could be in a local spiral, or a very distant AGN (but it likely won't be a detached lobe).

    One part I don't quite understand: why either AGN or SFR? Why not both? And even if it's predominantly either/or locally (out to z~1, say), why should it be either/or at high z? Isn't the conclusion that "the two types of galaxies evolved at the same rate in the early Universe" only valid if you assume an either/or? Wild example: suppose that, at high z, a substantial percentage of spiral galaxies have both AGNs - with nice radio jets and lobes - and fiery SFRs; isn't the observed P(D) distribution also consistent with such a wild idea?

    One aspect of radio astronomy that's surely different from optical: in the radio, you can easily 'see through' - and so completely miss - extended sources! "Our new source count is about a factor of four lower than the Owen & Morrison (2008) count near [...],
    even though our fields overlap on the sky. What might cause this discrepancy and, for that matter, the surprisingly large scatter among all published faint-source counts (Condon 2007)?
    "* "Survey catalogs are complete to a fixed brightness [...] cutoff , so extended sources with lower brightnesses but higher integrated flux densities [...] will be missed."

    Best news: SKA (etc) won't be confusion limited! 😄

    *trying to reproduce the text in a paper in Talk is exceedingly fiddly and error-prone; I simply deleted the bits that are hard (and substituted [...])


  • ivywong by ivywong scientist, admin

    Hi Jean,

    Sorry for the slightly late response but better late than never...

    There are fewer massive galaxies at low redshift than at higher redshifts. AGNs usually found in higher mass galaxies so dwarfs do not host AGNs. Often, the emission is both AGN and SFR but typically the emission is dominated either by star formation or AGN. From our current understanding of galaxy evolution, AGNs are typically observed in galaxies that are passed their main stages of star formation. Very young galaxies have not had the time to have "grown" an AGN just yet.

    It's not so much radio astronomy but rather the interferometry technique where the balance between surface brightness sensitivity and angular resolution plays a big part in determining the source counts. 😃