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 [C ii] 158 μm line from a sample of intermediate redshift (0.2 < z < 0.8) (ultra)-luminous infrared galaxies, (U)LIRGs (LIR > 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) × 10 of the far-infrared continuum luminosity of our sources, and appears to arise from photodissociation regions on the surface of molecular clouds. TheL[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.