2007A&A...463..481P

Young galactic X-ray point sources (XPs) closely trace the ongoing star formation in galaxies. From measured XP number counts we extract the collective 2-10keV luminosity of young XPs, L_x^yXP, which we use to gauge the current star formation rate (SFR) in galaxies. We find that, for a sample of local star-forming galaxies (i.e., normal spirals and mild starbursts), L_x^yXP correlates linearly with the SFR over three decades in luminosity. A separate, high-SFR sample of starburst ULIRGs can be used to check the calibration of the relation. Using their (presumably SF-related) total 2-10keV luminosities we find that these sources satisfy the SFR-L_x^yXP relation, as defined by the weaker sample, and extend it to span ∼5 decades in luminosity. The SFR-L_x^yXP relation is also likely to hold for distant (z∼1) Hubble Deep Field North galaxies, especially so if these high-SFR objects are similar to the (more nearby) ULIRGs. It is argued that the SFR-L_x^yXP relation provides the most adequate X-ray estimator of instantaneous SFR by the phenomena characterizing massive stars from their birth (FIR emission from placental dust clouds) through their death as compact remnants (emitting X-rays by accreting from a close donor). For local, low/intermediate-SFR galaxies, the simultaneous existence of a correlation of the instantaneous SFR with the total 2-10keV luminosity, L_x, which traces the SFR integrated over the last ∼10⁹yr, suggests that during such epoch the SF in these galaxies has been proceeding at a relatively constant rate.