making a catalog is not easy

Today David Mykytyn (NYU undergrad) and I specified Mykytyn's project to be the construction of a "Large Galaxy Catalog" from the SDSS imaging, using the Tractor to do the galaxy measurements (sizes, surface brightnesses, colors, and magnitudes). Today we discussed many of the complicating issues, which include (but are not limited to) the facts that: (1) angularly large galaxies often overlap multiple SDSS fields, usually taken on different nights, (2) they often overlap very bright stars, which can dominate the photon count in the face of the galaxy, (3) the SDSS software (optimized for much more angularly small galaxies) shreds them often into many pieces, (4) lumps and bumps in the intensity field can be features of the galaxy or confusing foreground objects (stars) or background objects (distant galaxies). We have hacks for all of these issues (not yet implemented), but we would like some principled approaches. It's hard, because, as I have lamented before, despite a hundred years of expensive and painstaking work by thousands of very bright people, astronomers do not have a generative model for galaxies!

1 comment:

  1. If we did, we would have solved (1) galaxy formation (including stellar evolution) and (2) radiative transfer in very complex star-dust geometries. And if we made it that far, who cares about "correct" catalogs!