Chandra X-ray observations of the hyper-luminous infrared galaxy IRAS F15307+3252

Abstract

Hyper-luminous infrared galaxies (HyLIRGs) lie at the extreme luminosity end of the IR galaxy population with $L_{\rm IR}>10^{13}$L$_\odot$. They are thought to be closer counterparts of the more distant sub-mm galaxies, and should therefore be optimal targets to study the most massive systems in formation. We present deep $Chandra$ observations of IRAS~F15307+3252 (100ks), a classical HyLIRG located at $z=$0.93 and hosting a radio-loud AGN ($L_{\rm 1.4 GHz}\sim3.5\times10^{25}$ W/Hz). The $Chandra$ images reveal the presence of extended ($r=160$ kpc), asymmetric X-ray emission in the soft 0.3-2.0 keV band that has no radio counterpart. We therefore argue that the emission is of thermal origin originating from a hot intragroup or intracluster medium virializing in the potential. We find that the temperature ($\sim2$ keV) and bolometric X-ray luminosity ($\sim3\times10^{43}$ erg s$^{-1}$) of the gas follow the expected $L_{\rm X-ray}-T$ correlation for groups and clusters, and that the gas has a remarkably short cooling time of $1.2$ Gyrs. In addition, VLA radio observations reveal that the galaxy hosts an unresolved compact steep-spectrum (CSS) source, most likely indicating the presence of a young radio source similar to 3C186. We also confirm that the nucleus is dominated by a redshifted 6.4 keV Fe K$\alpha$ line, strongly suggesting that the AGN is Compton-thick. Finally, Hubble images reveal an over-density of galaxies and sub-structure in the galaxy that correlates with soft X-ray emission. This could be a snapshot view of on-going groupings expected in a growing cluster environment. IRAS~F15307+3252 might therefore be a rare example of a group in the process of transforming into a cluster.