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

摘要

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 × 1020?cm–2. The LIRGs in our sample have total infrared (8-1000 μm) luminosities in the range of L IR≈ (1-8) × 1011 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 = αM + 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) × 1028?erg?s–1 M –1 ☉ and β = (1.62 ± 0.22) × 1039?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 1023?cm–2).