A CHANDRA PERSPECTIVE ON GALAXY-WIDE X-RAY BINARY EMISSION AND ITS CORRELATION WITH STAR FORMATION RATE AND STELLAR MASS: NEW RESULTS FROM LUMINOUS INFRARED GALAXIES

Abstract

We present new Chandra observations that complete a sample of seventeen (17) luminous infrared galaxies (LIRGs) with D < 60 Mpc and low Galactic column densities of N H ≲ 5 × 10 20 cm −2 . The LIRGs in our sample have total infrared (8–1000 μm) luminosities in the range of L IR ≈ (1–8) × 10 11 L ☉ . The high-resolution imaging and X-ray spectral information from our Chandra observations allow us to measure separately X-ray contributions from active galactic nuclei and normal galaxy processes (e.g., X-ray binaries and hot gas). We utilized total infrared plus UV luminosities to estimate star formation rates (SFRs) and K -band luminosities and optical colors to estimate stellar masses ( M ⋆ ) for the sample. Under the assumption that the galaxy-wide 2–10 keV luminosity ( L gal HX ) traces the combined emission from high-mass X-ray binaries (HMXBs) and low-mass X-ray binaries, and that the power output from these components is linearly correlated with SFR and M ⋆ , respectively, we constrain the relation L gal HX = α M ⋆ + βSFR. To achieve this, we construct a Chandra -based data set composed of our new LIRG sample combined with additional samples of less actively star-forming normal galaxies and more powerful LIRGs and ultraluminous infrared galaxies (ULIRGs) from the literature. Using these data, we measure best-fit values of α = (9.05 ± 0.37) × 10 28 erg s −1 M −1 ☉ and β = (1.62 ± 0.22) × 10 39 erg s −1 ( M ☉ yr −1 ) −1 . This scaling provides a more physically meaningful estimate of L gal HX , with ≈0.1–0.2 dex less scatter, than a direct linear scaling with SFR. Our results suggest that HMXBs dominate the galaxy-wide X-ray emission for galaxies with SFR/ M ⋆ ≳5.9 × 10 −11 yr −1 , a factor of ≈2.9 times lower than previous estimates. We find that several of the most powerful LIRGs and ULIRGs, with SFR/ M ⋆ ≳ 10 −9 yr −1 , appear to be X-ray underluminous with respect to our best-fit relation. We argue that these galaxies are likely to contain X-ray binaries residing in compact star-forming regions that are buried under thick galactic columns large enough to attenuate emission in the 2–10 keV band ( N H ≳ 10 23 cm −2 ).