X-ray properties of K-selected galaxies at 0.5<z<2.0: Investigating
  trends with stellar mass, redshift and spectral type
  We examine how the total X-ray luminosity correlates with stellar mass,
stellar population, and redshift for a K-band limited sample of ~3500 galaxies
at 0.5<z<2.0 from the NEWFIRM Medium Band Survey in the COSMOS field. The
galaxy sample is divided into 32 different galaxy types, based on similarities
between the spectral energy distributions. For each galaxy type, we further
divide the sample into bins of redshift and stellar mass, and perform an X-ray
stacking analysis using the Chandra COSMOS (C-COSMOS) data. We find that full
band X-ray luminosity is primarily increasing with stellar mass, and at similar
mass and spectral type is higher at larger redshifts. When comparing at the
same stellar mass, we find that the X-ray luminosity is slightly higher for
younger galaxies (i.e., weaker 4000\AA breaks), but the scatter in this
relation is large. We compare the observed X-ray luminosities to those expected
from low and high mass X-ray binaries (XRBs). For blue galaxies, XRBs can
almost fully account for the observed emission, while for older galaxies with
larger 4000\AA breaks, active galactic nuclei (AGN) or hot gas dominate the
measured X-ray flux. After correcting for XRBs, the X-ray luminosity is still
slightly higher in younger galaxies, although this correlation is not
significant. AGN appear to be a larger component of galaxy X-ray luminosity at
earlier times, as the hardness ratio increases with redshift. Together with the
slight increase in X-ray luminosity this may indicate more obscured AGNs or
higher accretion rates at earlier times.
