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The Large Deficit of HMXB Emission from Luminous Infrared Galaxies: the Case of the Circumnuclear Starburst Ring in NGC 7552

Published onJun 01, 2020
The Large Deficit of HMXB Emission from Luminous Infrared Galaxies: the Case of the Circumnuclear Starburst Ring in NGC 7552

Luminous infrared galaxies (LIRGs), the most extreme star-forming galaxies in the nearby (D < 30 Mpc) Universe, show a notable X-ray emission deficiency (up to a factor of ~10) compared with predictions from scalings of the high mass X-ray binary (HMXB) luminosity function with star-formation rate. In the nearby (≈20 Mpc) LIRG NGC 7552, the majority of the IR emission originates in a circumnuclear starburst ring, which has been resolved into several discrete knots of star formation. We present results from recent Chandra observations of NGC 7552, which reveal a major deficit in the 2-7 keV X-ray emission from two of the three most powerful star-forming knots. The luminous HMXB populations in these knots are thought to be either (1) missing due to relatively high metallicity and/or young ages in these knots or (2) obscured by a very large column density. We aim to distinguish between these possibilities with data from recent NuSTAR observations, whose sensitivity above 10 keV is expected to uncover any obscured HMXB population, since emission at these energies is more immune to absorption effects.

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