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 (SFR) 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 SFRs of 40-65 M_⊙ yr^-1 and stellar masses of ≈2 × 10¹¹ M_odot;. Using the 3 mm and 1 mm bands at CARMA, we observe both the CO(J = 1 → 0) and CO(J = 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 ± 0.07 (with systematic errors ≲ 40%), which implies sub-thermal excitation of the CO(J = 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 ultraluminous infrared galaxies. Since the EGNoG gas excitation sample galaxies are selected from the main sequence (MS) of star-forming galaxies, we suggest that this result is applicable to studies of MS 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.