find non-galaxies in the SDSS

The SDSS has issues with bright stars and galaxies: The pipelines were made for 17th-magnitude objects and therefore (a) the point-spread function doesn't include features like diffraction spikes or large-angle wings and (b) de-blending of much brighter galaxies is overly agressive. These are known problems with the SDSS but they haven't been analyzed (in the literature) in complete detail (meaning: many SDSS investigators know a lot about these things but they don't appear in papers in one coherent place).

In experiments conducted by Dusting Lang (CMU) and myself, we found that you can find rings of blue galaxies around red galaxies (really just blue spiral arms around red bulges) and you can find very elongated galaxies pointing at bright stars (really just unmodeled diffraction spikes in the PSF being modeled as galaxies). Both of these kinds of anomalies in the survey data are generally flagged: The SDSS pipelines are very clever about figuring out where they are going wrong or out-of-spec; most SDSS data analyses have used agressive flag cuts to remove possibly problematic galaxies. The most interesting objectives from my perspective are (1) building a full catalog or list or annotation of all (all in some category, like diffraction-spike) anomalous (that is, wrong) galaxies in the SDSS, (2) identifying anomalous galaxies that wouldn't have been caught by one of the standard, aggressive flag cuts, and (3) figuring out what fraction of agressively cut-out galaxies are in fact real and trustworthy.

I have made this post about the SDSS but of course it applies to any large imaging survey with an automatically produced catalog. Indeed, we did very similar things for USNO-B and there are more to do there. One amusing thing about the project is that the bright stars in the SDSS (the ones that show diffraction spikes) are themselves often classified as galaxies, because they become extended when they saturate. But that's a detail!


  1. Along those lines, there are many* real 2MASS stars that appears only in the reject catalog because they happened to be near predicted positions of artifacts, etc. WISE is so well matched to 2MASS that these can easily be found. One could in principle fix SDSS, 2MASS, WISE etc. with the information from each other.

    *many in number, but a tiny fraction, well within survey specifications.

  2. Excellent point! That could even be a second "Idea": Use catalog matching to put up quality or reliability flags on existing imaging surveys.