We present the Spitzer /IRS mid-infrared spectrum of the infrared-luminous ( L _ { IR } = 4 \times 10 ^ { 11 } L _ { \sun } ) brightest cluster galaxy ( BCG ) in the X-ray-luminous cluster Z3146 ( z = 0.29 ) .
The spectrum shows strong aromatic emission features , indicating that the dominant source of the infrared luminosity is star formation .
The most striking feature of the spectrum , however , is the exceptionally strong molecular hydrogen ( H _ { 2 } ) emission lines , which seem to be shock-excited .
The line luminosities and inferred warm H _ { 2 } gas mass ( \sim 10 ^ { 10 } M _ { \sun } ) are 6 times larger than those of NGC 6240 , the most H _ { 2 } -luminous galaxy at z \lesssim 0.1 .
Together with the large amount of cold H _ { 2 } detected previously ( \sim 10 ^ { 11 } M _ { \sun } ) , this indicates that the Z3146 BCG contains disproportionately large amounts of both warm and cold H _ { 2 } gas for its infrared luminosity , which may be related to the intracluster gas cooling process in the cluster core .