X‐Ray Luminosity Functions of Normal Galaxies in the Great Observatories Origins Deep Survey

Abstract

We present soft (0.5-2 keV) X-ray luminosity functions (XLFs) in the Great Observatories Origins Deep Survey (GOODS) fields for galaxies at z ~ 0.25 and 0.75. SED fitting was used to estimate photometric redshifts and separate galaxy types, resulting in a sample of 40 early-type galaxies and 46 late-type galaxies. We estimate k-corrections for both the X-ray/optical and X-ray/NIR flux ratios, which facilitates the separation of AGNs from the normal/starburst galaxies. We fit the XLFs with a power-law model using both traditional and Markov-Chain Monte Carlo (MCMC) procedures. A key advantage of the MCMC approach is that it explicitly takes into account upper limits and allows errors on derived quantities, such as luminosity densities, to be computed directly (i.e., without potentially questionable assumptions concerning the propagation of errors). The slopes of the early-type galaxy XLFs tend to be slightly flatter than the late-type galaxy XLFs, although the effect is significant at only the 90% and 97% levels for z ~ 0.25 and 0.75. The XLFs differ between z 0.5 at >99% significance levels for early-type, late-type, and all (early- and late-type) galaxies. We also fit Schechter and lognormal models to the XLFs, fitting the low- and high-redshift XLFs for a given sample simultaneously assuming only pure luminosity evolution. In the case of lognormal fits, the results of MCMC fitting of the local FIR luminosity function were used as priors for the faint- and bright-end slopes (similar to fixing these parameters at the FIR values, except here the FIR uncertainty is included). The best-fit values of the change in log L* with redshift were Δ log L* = 0.23 ± 0.16 dex (for early-type galaxies) and 0.34 ± 0.12 dex (for late-type galaxies), corresponding to (1 + z)1.6 and (1 + z)2.3. These results were insensitive to whether the Schechter or lognormal function was adopted.