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-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 _ { 1.4 ~ { } GHz } \sim 3.5 \times 10 ^ { 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 ( \sim 2 keV ) and bolometric X-ray luminosity ( \sim 3 \times 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 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 .