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] 158um
line from a sample of intermediate redshift (0.2<z<0.8) (ultra)-luminous
infrared galaxies, (U)LIRGs (LIR>10^11.5 Lsun), using the SPIRE-Fourier
Transform Spectrometer (FTS) 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] 158um line emission from over 80% of the
sample. We find that the [CII] 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[CII]/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[CII]/LIR and
L[CII]/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 luminous infrared galaxies is likely to have
occurred in the last 5 billion years.
