X-rays across the galaxy population - I. Tracing the main sequence of star formation

We use deep Chandra imaging to measure the distribution of X-ray luminosities (L-X) for samples of star-forming galaxies as a function of stellar mass and redshift, using a Bayesian method to push below the nominal X-ray detection limits. Our luminosity distributions all show narrow peaks at L-X (less than or similar to) 10(42) erg s(-1) that we associate with star formation, as opposed to AGN that are traced by a broad tail to higher L-X. Tracking the luminosity of these peaks as a function of stellar mass reveals an ` X-ray main sequence' with a constant slope approximate to 0.63 +/- 0.03 over 8.5 less than or similar to logM(*)/ M-circle dot less than or similar to 11.5 and 0.1 less than or similar to z less than or similar to 4, with a normalization that increases with redshift as (1 + z) (3.79 +/- 0.12). We also compare the peak X-ray luminosities with UV-to-IR tracers of star formation rates (SFRs) to calibrate the scaling between L-X and SFR. We find that L-X alpha SFR0.83 X (1 + z)(1.3), where the redshift evolution and non-linearity likely reflect changes in high-mass X-ray binary populations of star-forming galaxies. Using galaxies with a broader range of SFR, we also constrain a stellar-mass-dependent contribution to LX, likely related to low-mass X-ray binaries. Using this calibration, we convert our X-raymain sequence to SFRs and measure a star-forming main sequence with a constant slope approximate to 0.76 +/- 0.06 and a normalization that evolves with redshift as (1 + z)(2.95 +/- 0.33). Based on the X-ray emission, there is no evidence for a break in the main sequence at high stellar masses, although we cannot rule out a turnover given the uncertainties in the scaling of L-X to SFR.