HERSCHEL OBSERVATIONS OF FAR-INFRARED COOLING LINES IN INTERMEDIATE REDSHIFT (ULTRA)-LUMINOUS INFRARED GALAXIES

We report the first results from a spectroscopic survey of the [CII] 158 mu m line from a sample of intermediate redshift (0.2 < z < 0.8) (ultra)-luminous infrared galaxies, (U)LIRGs (L sub(IR) > 10 super(11.5) L sub([middot in circle])), using the Spectral and Photometric Imaging REceiver-Fourier Transform Spectrometer on board the Herschel Space Observatory. This is the first survey of [CII] emission, an important tracer of star formation, at a redshift range where the star formation rate density of the universe increases rapidly. We detect strong [CII] 158 mu m line emission from over 80% of the sample. We find that the [CII] line is luminous, in the range (0.8-4) x 10 super(-3) of the far-infrared continuum luminosity of our sources, and appears to arise from photodissociation regions on the surface of molecular clouds. The L sub([CII])/L sub(IR) ratio in our intermediate redshift (U)LIRGs is on average ~ 10 times larger than that of local ULIRGs. Furthermore, we find that the L sub([CII])/L sub(IR) and L sub([CII])/L sub(CO(1-0)) ratios in our sample are similar to those of local normal galaxies and high-z star-forming galaxies. ULIRGs at z ~ 0.5 show many similarities to the properties of local normal and high-z star-forming galaxies. Our findings strongly suggest that rapid evolution in the properties of the star-forming regions of (U)LIRGs is likely to have occurred in the last 5 billion years.