A Herschel Space Observatory Spectral Line Survey of Local Luminous
  Infrared Galaxies from 194 to 671 Microns
  We describe a Herschel Space Observatory 194-671 micron spectroscopic survey
of a sample of 121 local luminous infrared galaxies and report the fluxes of
the CO $J$ to $J$-1 rotational transitions for $4 \leqslant J \leqslant 13$,
the [NII] 205 um line, the [CI] lines at 609 and 370 um, as well as additional
and usually fainter lines. The CO spectral line energy distributions (SLEDs)
presented here are consistent with our earlier work, which was based on a
smaller sample, that calls for two distinct molecular gas components in
general: (i) a cold component, which emits CO lines primarily at $J \lesssim 4$
and likely represents the same gas phase traced by CO (1-0), and (ii) a warm
component, which dominates over the mid-$J$ regime ($4 < J < 10$) and is
intimately related to current star formation. We present evidence that the CO
line emission associated with an active galactic nucleus is significant only at
$J > 10$. The flux ratios of the two [CI] lines imply modest excitation
temperatures of 15 to 30 K; the [CI] 370 um line scales more linearly in flux
with CO (4-3) than with CO (7-6). These findings suggest that the [CI] emission
is predominately associated with the gas component defined in (i) above. Our
analysis of the stacked spectra in different far-infrared (FIR) color bins
reveals an evolution of the SLED of the rotational transitions of water vapor
as a function of the FIR color in a direction consistent with infrared photon
pumping.
