SYNMAG Photometry: A Fast Tool for Catalog-Level Matched Colors of
  Extended Sources
  Obtaining reliable, matched photometry for galaxies imaged by different
observatories represents a key challenge in the era of wide-field surveys
spanning more than several hundred square degrees. Methods such as flux
fitting, profile fitting, and PSF homogenization followed by matched-aperture
photometry are all computationally expensive. We present an alternative
solution called "synthetic aperture photometry" that exploits galaxy profile
fits in one band to efficiently model the observed, PSF-convolved light profile
in other bands and predict the flux in arbitrarily sized apertures. Because
aperture magnitudes are the most widely tabulated flux measurements in survey
catalogs, producing synthetic aperture magnitudes (SYNMAGs) enables very fast
matched photometry at the catalog level, without reprocessing imaging data. We
make our code public and apply it to obtain matched photometry between SDSS
ugriz and UKIDSS YJHK imaging, recovering red-sequence colors and photometric
redshifts with a scatter and accuracy as good as if not better than
FWHM-homogenized photometry from the GAMA Survey. Finally, we list some
specific measurements that upcoming surveys could make available to facilitate
and ease the use of SYNMAGs.
