X-Ray Sources in the Star-Forming Galaxies NGC 4194 and NGC 7541

Abstract

We examine the X-ray point-source population and 2-10 keV luminosity for two galaxies with high star formation rates (SFRs), NGC 4194 and NGC 7541. The X-ray point-source luminosity function (XLF) for these two galaxies is consistent with the XLF found by Grimm et al. for a sample of star-forming galaxies. Combining our results with a sample of galaxies with SFRs above 1 M_⊙ yr^-1, we find that the number of X-ray point sources above a luminosity of 2 × 10³⁸ ergs s^-1 is N = (1.8 +/- 0.4) SFR/(M_⊙ yr ^-1) . This number is lower than previously inferred by Grimm et al. based on a sample of galaxies with lower SFRs. We find that the ratio of X-ray luminosity in the 2-10 keV band to SFR is L_X/(10⁴⁰ ergs s^-1) = (0.37 +/- 0.08) SFR/(M_⊙ yr^-1). This value may serve as a calibration in attempts to use X-ray luminosity to measure the SFR of galaxies at cosmological distances. The ratio of mass accreted onto compact objects versus mass used to form stars is near 10^-6. This ratio may be useful in constraining population synthesis models of X-ray binary formation in actively star-forming systems.