Disk Detective Talk

please help

  • onetimegolfer by onetimegolfer

    I know the SED disqualifies this one but it is a star and I need instruction on what to do to prove the SED correct as if it were a goodcandidate. I just do not know what to look at.


  • WizardHowl by WizardHowl

    I agree this looks like a star and colours at first seem wrong for a QSO, so here's what I do to look up what's known:

    1. Find a spectrum, if possible.

    I check SDSS DR10 first ( http://skyserver.sdss3.org/public/en/tools/toolshome.aspx then select Navigate and type in the co-ordinates in the box top-left), then NED. If these don't have anything, there's a slim chance a spectrum can be found amongst the references, if any, but unless the title of the paper gives this away I don't know where to look otherwise. For this star there is no spectrum and no references.

    Checking the SDSS DR10 image, there is a red object just 5" below the star we are interested in. From my experience looking up some objects like this over in Radio Galaxy Zoo, it looks like it might be a typical elliptical galaxy with a redshift of ~0.6 +/-0.1, purely from visual inspection (although SDSS classes it as a star). This might be affecting the SED as it could be brighter than the star in the IR. The centroid of the WISE images is well-centred and does not appear to change, however.

    1. Find a distance estimate, or measurement that constrains it (often proper motion).

    SDSS DR10 and NED again are good places to look and may have photometric redshift estimates for many QSO/AGN candidates. In this case the object is classified as a star in both and no estimate has been done.

    Next I check catalogues to look for proper motion: NOMAD and RAVE are the two I know about. In Vizier, just type NOMAD RAVE into the box for catalogues and select 'optical from the menu to the right', then press Find Catalogues. The latest release of RAVE (DR4) is the most recent and what you want here, so tick the box to the left of that and NOMAD (NOMAD is an all-sky catalogue but RAVE is not, as the illustrations on the right of this screen show, but RAVE has more precise measurements). Press 'Query selected tables' and then you are on the search page: here it's best to set preferences on the left before typing the co-ordinates in: I tick the ALL columns button so I'm not missing anything and set the max (above this) to 1000 (only in very dense starfields will this not cover everything within 2 arcmin, although you can also change that to 1 arcmin if you prefer).

    This star is not in RAVE but is in NOMAD, which also shows no other stars within 0.2arcmin (12") which means nothing too close to the red circle here in DD. Sadly it does not have a proper motion measurement, either.

    1. Find the magnitude measurements in different bands.

    The SED relies on these, using those listed in the image: DSS2 Blue, red and IR, 2MASS J,H and K and WISE bands 1,2,3 and 4. NOMAD also has B,V and R, so I'll list those additional values here: B=17.950mag, V=17.390mag and R=17.650mag. SDSS DR10 also has measurements in u,g,r,i and z bands that are in the visual and near-IR: u=19.10mag, g=18.29mag, r=17.84mag, i=17.67mag and z=17.66mag. The SDSS DR10 magnitudes are all listed as having errors of 0.02mag or less, so they should be OK.

    1. Put it all together.

    For this star we know only that it is fainter in the blue, about equally bright across the red but then continually gets brighter in near-IR and through the WISE bands. There is a small possibility of contamination in WISE but the image hasn't moved off the crosshairs which suggests it's seeing the right object. Therefore, this is probably a QSO/AGN. It can also be worth looking for other images of the object, for which I use http://skyview.gsfc.nasa.gov/current/cgi/query.pl and the radio surveys FIRST and NVSS sometimes give away a galaxy/QSO/AGN (no emission here, though).


  • onetimegolfer by onetimegolfer

    A confidence builder; Thank-you Wizardhowl. I will practice this on each #goodcandidate target; as I get them.


  • Shigeru by Shigeru moderator

    Amazing, great info WizardHowl!


  • jdebes by jdebes scientist, admin

    Another thing to check is whether this object has appreciable proper motion. If it doesn't that would be another clue that it might be extragalactic (or at least pretty far away in our own galaxy).