Galactic star formation rates gauged by stellar end-products

Free access article

Issue		A&A Volume 463, Number 2, February IV 2007


Page(s)		481 - 492
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20054146

A&A 463, 481-492 (2007)
DOI: 10.1051/0004-6361:20054146

Galactic star formation rates gauged by stellar end-products

M. Persic¹ and Y. Rephaeli^{2, 3}

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, $L_{\rm x}^{\rm yXP}$ , 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), $L_{\rm x}^{\rm yXP}$ 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 2-10 keV luminosities we find that these sources satisfy the SFR- $L_{\rm x}^{\rm yXP}$ relation, as defined by the weaker sample, and extend it to span ${\sim}5$ decades in luminosity. The SFR- $L_{\rm x}^{\rm yXP}$ relation is also likely to hold for distant ( $z \sim 1$ ) 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- $L_{\rm x}^{\rm yXP}$ 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 2-10 keV luminosity, $L_{\rm x}$ , which traces the SFR integrated over the last ${\sim}10^9$ yr, suggests that during such epoch the SF in these galaxies has been proceeding at a relatively constant rate.

Key words: galaxies: starburst -- infrared: galaxies -- radio continuum: galaxies -- X-rays: binaries -- X-rays: galaxies

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