Galactic star formation rates gauged by stellar end-products

¹ INAF/Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy e-mail: persic@oats.inaf.it
² School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
³ CASS, University of California, San Diego, La Jolla, CA 92093, USA

Received: 5 September 2005
Accepted: 12 October 2006

Abstract

Young galactic X-ray point sources (XPs) closely trace the ongoing star formation in galaxies. From measured XP number counts we extract the collective 2-10 keV luminosity of young XPs, , 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), 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 2-10 keV luminosities we find that these sources satisfy the SFR– relation, as defined by the weaker sample, and extend it to span decades in luminosity. The SFR– relation is also likely to hold for distant () 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– 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 2-10 keV luminosity, L_x, which traces the SFR integrated over the last  yr, suggests that during such epoch the SF in these galaxies has been proceeding at a relatively constant rate.