2014ApJ...781L..15R

We report the first results from a spectroscopic survey of the [C II] 158 µm line from a sample of intermediate redshift (0.2 <z < 0.8) (ultra)-luminous infrared galaxies, (U)LIRGs (L_IR> 10^11.5 L_☉), using the Spectral and Photometric Imaging REceiver-Fourier Transform Spectrometer on board the Herschel Space Observatory. This is the first survey of [C II] 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 [C II] 158 µm line emission from over 80% of the sample. We find that the [C II] line is luminous, in the range (0.8-4)x10^–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_[C II]_/LIR_ ratio in our intermediate redshift (U)LIRGs is on average ∼10 times larger than that of local ULIRGs. Furthermore, we find that the L_[C II]_/LIR_and L_[C II]_/LCO(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.