Radio Galaxy Zoo Talk

optical and IR-bright galactic nucleus but no radio - why?

  • WizardHowl by WizardHowl

    Whilst checking for IR sources for the compact radio emission here, it is noticeable that the bright galaxy in the West of this field of view, SDSS J160407.96+472058.8 Z_sp=0.042, is very much brighter in the longer wavelengths in WISE (dust emission?) but despite an extremely bright core and AGN-like spectrum it has zero radio emission. No radio emission means the super-massive black hole is not producing jets, yet if it is not active then why is the nucleus so bright and the spectrum (classed as starforming) showing such strong emission lines? A nuclear starburst would still produce radio emission...

    This galaxy might be a face-on disk, given the colours and redshift, which might also explain some features of the spectrum. To be so bright at longer wavelengths I think implies a lot of dust (or maybe cold gas?) which might also fit with being face-on but there are no obvious spiral features. It is brighter than all the surrounding galaxies in WISE band 4 and brighter in band 4 than band 3, whereas everything else gets fainter between bands 3 and 4.

    Could it be that this galaxy is in a state where there is a large disk around the SMBH but it is not accreting at a sufficient rate to produce an observable radio flux? i.e. Primed and ready for action but not yet active?


  • 42jkb by 42jkb scientist, admin

    My understanding is that if the object is brighter in the longer wavelengths of WISE, then chances are that there is a lot of dust and /or star formation.

    There are two options here:

    1. The SMBH could still be producing jets but they are only visible in the X-ray.
    2. The radio emission is too faint and not detected in the radio survey.


  • JeanTate by JeanTate

    Although it's about high-z AGN, Hickox+ 2007 may be relevant here. Except that I could not find any papers on trying to match identified AGN in "the 9 square degree Bootes" field with sources found in any radio surveys; do you know of any, 42jkb?

    SDSS J160407.96+472058.8 looks pretty cool, definitely like it has a bright, point-like nucleus:

    enter image description here


  • 42jkb by 42jkb scientist, admin

    There is a paper on Westerbork observations of the Bootes field ( Is this what you are looking for?


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

    Thanks 42jkb! 😃

    This is very helpful ... look at Table 1, for example (I'm not going to try to reproduce it; Talk will turn it into gibberish): there are 3172 WSRT-Bootes "detections" (at 1400 MHz), and just 749 FIRST ones (also at 1400 MHz), the difference being the "flux density limit" (a.k.a. sensitivity), 0.14 mJy vs 1.0 mJy*. So the 'WHAGN' ('WizardHowl AGN'; a catchier name than SDSS J160407.96+472058.8 😉) may be detected, at 1400 MHz, if you stared at it for longer (so to speak).

    Table 5 is also very cool, "Sources with known redshifts". Of the ten, nine have 0.01 < z < 0.05, and one is 1.237 (I guess this was before SDSS DR7). There's no mapping (cross-correlation?) in the paper, so we don't know what sort of radio source these ten are; I guess none of the five late-type galaxies (i.e. Sa to Irr) have double lobes (or similar), otherwise there'd have been papers published about them already, right?

    BTW, how did you find that paper? I tried to find just such a paper - I figured there had to be at least one - but failed.

    *there's also a difference in resolution, 13" x 27" vs 5"


  • 42jkb by 42jkb scientist, admin

    In radio things are slightly different then in optical. Rule of thumb is that brighter radio objects are usually further away and objects fainter in radio are closer - sort of confusing isn't it? The galaxies closer usually don't have an active black hole. For example our galaxy doesn't have an active black hole and the majority of radio emission comes from star formation regions and supernova remnants. A good paper describing the 1.4 GHz radio emission of our own galaxy is this one (I may be biased here as it is the Canadian survey and has polarisation information) and the images show the regions I was talking about. The radio emission that is not a result of an active black hole is hard to detect so the telescopes do need to observe these objects for a longer time then the bright radio sources of active black holes. This source just may not have an active black hole that is emitting in the radio, so observing longer will tell you this or show you the star formation regions. Either way, it is still good science.

    The ATLAS data that will be supplied to RGZ will have more of these star forming galaxies than the current images in RGZ because ATLAS is the deepest radio survey at 1.4 GHz. ATLAS is the pilot to the EMU project so we will get a taste of what ASKAP will observe. We will be really "opening" up the radio sky.

    As for searching for the paper, I think I just googled Bootes radio observations.