LX-SFR relation in star-forming galaxies

We compare the results of Grimm, Gilfanov & Sunyaev and Ranalli, Comastri & Seti on the LX–SFR (X-ray luminosity–star formation rate) relation in normal galaxies. Based on the LX–stellar mass dependence for low-mass X-ray binaries (LMXBs), we show that low-SFR (≲1 M⊙ yr−1) galaxies in the Ranalli et al. sample are contaminated by the X-ray emission from LMXBs, unrelated to the current star formation activity. However, the most important conclusion from our comparison is that, after the data are corrected for the ‘LMXB contamination’, the two data sets become consistent with each other, despite differences in their content, variability effects, adopted source distances, X-ray fluxes and SFR determinations, and also in the cosmological parameters used in interpreting the Hubble Deep Field North (HDF-N) data. They also agree well, both in the low- and high-SFR regimes, with the predicted LX–SFR dependence derived from the parameters of the ‘universal’ high-mass X-ray binary (HMXB) luminosity function. This encouraging result emphasizes the potential of the X-ray luminosity as an independent SFR indicator for normal galaxies.