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

TL;DR

This study analyzes X-ray point sources in two star-forming galaxies, finding a correlation between X-ray luminosity and star formation rate, and providing a potential calibration for measuring galaxy SFRs at cosmological distances.

Contribution

It presents the X-ray luminosity function for NGC 4194 and NGC 7541 and refines the relationship between X-ray sources and star formation rates in galaxies.

Findings

XLF consistent with previous star-forming galaxy samples

Number of X-ray sources scales with SFR as N ≈ 1.8 × SFR

X-ray luminosity to SFR ratio useful for cosmological measurements

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. (2003) for a sample of star-forming galaxies. Combining our results with a sample of galaxies with SFRs above 1 solar mass/year, we find that the number of X-ray point sources above a luminosity of 2E38 erg/s is N = (1.8 +/- 0.4) SFR/(solar mass/year). 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/(10E40 erg/s) = (0.37 +/- 0.08) SFR/(solar mass/year). 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 1E-6. This ratio may be useful in constraining population synthesis models of X-ray binary formation in actively star forming systems.