High-ionization Fe K emission from luminous infrared galaxies

TL;DR

This study analyzes the X-ray properties of luminous infrared galaxies, revealing strong high-ionization Fe K emission and exploring potential origins such as starburst-driven hot gas or obscured active galactic nuclei.

Contribution

It provides new insights into the origin of Fe K emission in infrared galaxies and discusses the possible roles of starburst activity and hidden supermassive black holes.

Findings

Luminous IR galaxies are underluminous in 2-10 keV X-rays compared to less luminous galaxies.

Strong high-ionization Fe K emission is detected, incompatible with X-ray binaries.

The Fe K emission may originate from hot gas in starbursts or obscured AGN activity.

Abstract

The Chandra component of the Great Observatories All-Sky LIRG Survey (GOALS) presently contains 44 luminous and ultraluminous infrared galaxies with log (Lir/Lsun) = 11.73-12.57. Omitting 15 obvious AGNs, the other galaxies are, on average, underluminous in the 2-10 keV band by 0.7 dex at a given far-infrared luminosity, compared to nearby star-forming galaxies with lower star formation rates. The integrated spectrum of these hard X-ray quiet galaxies shows strong high-ionization Fe K emission (Fe XXV at 6.7 keV), which is incompatible with X-ray binaries as its origin. The X-ray quietness and the Fe K feature could be explained by hot gas produced in a starburst, provided that the accompanying copious emission from high-mass X-ray binaries is somehow suppressed. Alternatively, these galaxies may contain deeply embedded supermassive black holes that power the bulk of their infrared luminosity and only faint photoionized gas is visible, as seen in some ULIRGs with Compton-thick AGN.