Radio Galaxy Zoo Talk

SDSS vs IR

  • basst82 by basst82

    I'm still trying to nail down the relationship between the IR picture we're seeing and SDSS. Most of the time it makes perfect sense, but then there's images like this one. The central source of this #triple comes up as a #star, so that follows the pattern that would indicate this as a quasar. But, there are four strong IR signals all clustered together near the central source of this radio signal, yet SDSS shows nothing there. Even the "star" categorization in SDSS is barely a detectable smudge on the zoomed in window. So what are those other 3 IR signals if not galaxies or stars in SDSS?

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  • WizardHowl by WizardHowl

    I find the source of this triple to be SDSS J153915.16+141603.2 which is labelled as a galaxy (with photometric redshift estimates of ~0.68 and ~0.38). This is the pink smudge to the lower-right on the SDSS, a little way from the centre and in this case you can see the same pattern of objects below it that corresponds to that seen in the IR image from WISE.

    It does take a little practice to see which sources match up between the SDSS and IR. The best way to be sure you have found an optical counterpart is to click on the WISE link here in RGZ which will take you to the IR image with a larger field of view and as seen in the different bands (wavelengths) by the WISE satellite. There the lower-left image (WISE band 1, wavelength 3.4 microns) is the one used in RGZ: hover the cursor over the IR source and you will see the co-ordinates appear in a box at the top-right of the window - the Eq-J2000 co-ordinates are what you need to make a note of. Input those co-ordinates into the SDSS DR10 navigate tool (top-left) and it will centre the image on those co-ordinates. If there is a source in SDSS within 2 arcminutes it will highlight the closest source's information on the right for you to examine, otherwise it will report no sources. Check that the source it gives you information on is the one you want (important when several are in close proximity) and matches up with where the radio object is - the IR images have a lower resolution and the radio signal sometimes seems offset because it's coming from an object that is not properly resolved.

    Note that the SDSS survey will not show faint or distant sources - it takes a lot of time to make deep survey images on optical telescopes. The bright, elliptical galaxies that are typically the sources of the kind of radio emission seen here may only appear as fuzzy red blobs if they are very distant (they will not normally be seen beyond a redshift of about 0.7). The IR survey from WISE is 'deeper' in that it can observe more distant sources and some objects will simply not be visible in the optical. The radio images are deeper still, and sometimes there are objects that will not have any match in either IR or optical.

    Hope this helps!

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