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