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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.

Lehmer, B.D. and Alexander, D.M. and Bauer, F.E. and Brandt, W.N. and Goulding, A.D. and Jenkins, L.P. and Ptak, A. and 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). pp. 559-571.


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).

Item Type:Article
Keywords:Cosmology, Observations, Galaxies, Starburst, Infrared, X-rays, Binaries.
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Date accepted:No date available
Date deposited:03 January 2014
Date of first online publication:November 2010
Date first made open access:No date available

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