A Far-Infrared Spectroscopic Survey of Intermediate Redshift (Ultra)
  Luminous Infrared Galaxies
  We present Herschel far-IR photometry and spectroscopy as well as ground
based CO observations of an intermediate redshift (0.21 < z < 0.88) sample of
Herschel-selected (ultra)-luminous infrared galaxies (L_IR > 10^11.5L_sun).
With these measurements we trace the dust continuum, far-IR atomic line
emission, in particular [CII]\,157.7microns, as well as the molecular gas of
z~0.3 (U)LIRGs and perform a detailed investigation of the interstellar medium
of the population. We find that the majority of Herschel-selected intermediate
redshift (U)LIRGs have L_CII/L_FIR ratios that are a factor of about 10 higher
than that of local ULIRGs and comparable to that of local normal and high-$z$
star forming galaxies. Using our sample to bridge local and high-z [CII]
observations, we find that the majority of galaxies at all redshifts and all
luminosities follow a L_CII-L_FIR relation with a slope of unity, from which
local ULIRGs and high-z AGN dominated sources are clear outliers. We also
confirm that the strong anti-correlation between the L_CII/L_FIR ratio and the
far-IR color L_60/L_100 observed in the local Universe holds over a broad range
of redshifts and luminosities, in the sense that warmer sources exhibit lower
L_CII/L_FIR at any epoch. Intermediate redshift ULIRGs are also characterised
by large molecular gas reservoirs and by lower star formation efficiencies
compared to that of local ULIRGs. The high L_CII/L_FIR ratios, the moderate
star formation efficiencies (L_LIR/L_CO or L_IR/M_gas) and the relatively low
dust temperatures of our sample (which are also common characteristics of
high-z star forming galaxies with ULIRG-like luminosities) indicate that the
evolution of the physical properties of (U)LIRGs between the present day and z
> 1 is already significant by z ~ 0.3.
