The molecular gas in Luminous Infrared Galaxies I: CO lines, extreme
  physical conditions, and their drivers
  We report results from a large molecular line survey of Luminous Infrared
Galaxies (L_{IR} >= 10^{11} L_sol) in the local Universe (z<=0.1), conducted
during the last decade with the James Clerk Maxwell Telescope (JCMT) and the
IRAM 30-m telescope. This work presents the CO and {13}CO line data for 36
galaxies, further augmented by multi-J total CO luminosities available for
other IR-bright galaxies from the literature. This yields a sample of N=70
galaxies with the star-formation (SF) powered fraction of their IR luminosities
spanning L_{IR} (10^{10}-2x10^{12}) L_sol and a wide range of morphologies.
Simple comparisons of their available CO Spectral Line Energy Distributions
(SLEDs) with local ones, as well as radiative transfer models discern a
surprisingly wide range of average ISM conditions, with most of the surprises
found in the high-excitation regime. These take the form of global CO SLEDs
dominated by a very warm (T_{kin}>=100 K) and dense (n>=10^4 cm^{-3}) gas
phase, involving galaxy-sized (~(few)x10^9 M_sol) gas mass reservoirs under
conditions that would otherwise amount only ~1% of mass per typical SF
molecular cloud in the Galaxy. Some of the highest excitation CO SLEDs are
found in the so-called Ultra Luminous Infrared Galaxies and seem irreducible to
ensembles of ordinary SF-powered regions. Highly supersonic turbulence and high
cosmic ray (CR) energy densities rather than far-UV/optical photons or
SNR-induced shocks from individual SF sites can globally warm the large amounts
of dense gas found in these merger-driven starbursts and easily power their
extraordinary CO line excitation.....
