Hunting galaxies to (and for) extinction

In studies of star-forming regions, near-infrared excess (NIRX) sources - objects with intrinsic colors redder than normal stars - constitute both signal (young stars) and noise (e. g., background galaxies). We hunt down (identify) galaxies using near-infrared observations in the Perseus star-forming region by combining structural information, colors, and number density estimates. Galaxies at moderate redshifts (z = 0.1-0.5) have colors similar to young stellar objects (YSOs) at both near- and mid-infrared (e. g., Spitzer) wavelengths, which limits our ability to identify YSOs from colors alone. Structural information from high-quality near-infrared observations allows us to better separate YSOs from galaxies, rejecting two out of five of the YSO candidates identified from Spitzer observations of our regions and potentially extending the YSO luminosity function below K of 15 mag where galaxy contamination dominates. Once they are identified we use galaxies as valuable extra signals for making extinction maps of molecular clouds. Our new iterative procedure, the galaxies near-infrared color excess method revisited (GNICER), uses the mean colors of galaxies as a function of magnitude to include them in extinction maps in an unbiased way. GNICER increases the number of background sources used to probe the structure of a cloud, decreasing the noise and increasing the resolution of extinction maps made far from the galactic plane.