The EGNoG Survey: Gas Excitation in Normal Galaxies at z~0.3
  As observations of molecular gas in galaxies are pushed to lower star
formation rate galaxies at higher redshifts, it is becoming increasingly
important to understand the conditions of the gas in these systems to properly
infer their molecular gas content. The rotational transitions of the carbon
monoxide (CO) molecule provide an excellent probe of the gas excitation
conditions in these galaxies. In this paper we present the results from the gas
excitation sample of the Evolution of molecular Gas in Normal Galaxies (EGNoG)
survey at the Combined Array for Research in Millimeter-wave Astronomy (CARMA).
This subset of the full EGNoG sample consists of four galaxies at z~0.3 with
star formation rates of 40-65 M_Sun yr^-1 and stellar masses of ~2x10^11 M_Sun.
Using the 3 mm and 1 mm bands at CARMA, we observe both the CO(1-0) and CO(3-2)
transitions in these four galaxies in order to probe the excitation of the
molecular gas. We report robust detections of both lines in three galaxies (and
an upper limit on the fourth), with an average line ratio, r_31 = L'_CO(3-2) /
L'_CO(1-0), of 0.46 \pm 0.07 (with systematic errors \lesssim 40%), which
implies sub-thermal excitation of the CO(3-2) line. We conclude that the
excitation of the gas in these massive, highly star-forming galaxies is
consistent with normal star-forming galaxies such as local spirals, not
starbursting systems like local ultra-luminous infrared galaxies. Since the
EGNoG gas excitation sample galaxies are selected from the main sequence of
star-forming galaxies, we suggest that this result is applicable to studies of
main sequence galaxies at intermediate and high redshifts, supporting the
assumptions made in studies that find molecular gas fractions in star forming
galaxies at z~1-2 to be an order of magnitude larger than what is observed
locally.
