$\textit{Chandra}$ 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 $L_{IR}$ > 10$^{13}$ L$\odot$. They are thought to be closer counterparts of the more distant sub-millimeter galaxies, and should therefore be optimal targets to study the most massive systems in formation. We present deep $\textit{Chandra}$ observations of IRAS F15307+3252 (100 ks), a classical HyLIRG located at $z$=0.93 and hosting a radio-loud AGN ($L{1.4 GHz}$ ~3.5 × 10$^{25}$ W Hz$^{−1}$). The $\textit{Chandra}$ 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 (~2 keV) and bolometric X-ray luminosity (~3 × 10$^{43}$ erg s$^{−1}$) of the gas follow the expected $L_{X-ray}$–$T$ correlation for groups and clusters, and that the gas has a remarkably short cooling time of 1.2 Gyr. 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, $\textit{Hubble}$ images reveal an overdensity 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.