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Durham University

Department of Physics

Staff profile

Publication details for Prof David Alexander

Lehmer, B.D., Alexander, D.M., Bauer, F.E., Brandt, W.N., Goulding, A.D., Jenkins, L.P., Ptak, A. & Roberts, T.P. (2010). 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. Astrophysical journal 724(1): 559-571.

Author(s) from Durham

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 × 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 sun. 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 sstarf) 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 sstarf, respectively, we constrain the relation L gal HX = αM sstarf + 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 sun and β = (1.62 ± 0.22) × 1039 erg s-1 (M sun 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 sstarf gsim5.9 × 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 sstarf >~ 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).