Based on recent work on spectral decomposition of the emission of
star-forming galaxies, we assess whether the integrated 2-10 keV emission from
high-mass X-ray binaries (HMXBs), L_{2-10}^{HMXB}, can be used as a reliable
estimator of ongoing star formation rate (SFR). Using a sample of 46 local (z <
0.1) star forming galaxies, and spectral modeling of ASCA, BeppoSAX, and
XMM-Newton data, we demonstrate the existence of a linear SFR-L_{2-10}^{HMXB}
relation which holds over ~5 decades in X-ray luminosity and SFR. The total
2-10 keV luminosity is not a precise SFR indicator because at low SFR (i.e., in
normal and moderately-starbursting galaxies) it is substantially affected by
the emission of low-mass X-ray binaries, which do not trace the current SFR due
to their long evolution lifetimes, while at very high SFR (i.e., for very
luminous FIR-selected galaxies) it is frequently affected by the presence of
strongly obscured AGNs. The availability of purely SB-powered galaxies - whose
2-10 keV emission is mainly due to HMXBs - allows us to properly calibrate the
SFR-L_{2-10}^{HMXB} relation. The SFR-L_{2-10}^{HMXB} relation holds also for
distant (z ~ 1) galaxies in the Hubble Deep Field North sample, for which we
lack spectral information, but whose SFR can be estimated from deep radio data.
If confirmed by more detailed observations, it may be possible to use the
deduced relation to identify distant galaxies that are X-ray overluminous for
their (independently estimated) SFR, and are therefore likely to hide strongly
absorbed AGNs.Comment: Astronomy & Astrophysics, in press (15 pages, 7 figures, 4 tables
