An Extragalactic 12CO J = 3-2 Survey with the Heinrich Hertz Telescope

We present results of a ¹²CO J = 3-2 survey of 125 nearby galaxies obtained with the 10 m Heinrich Hertz Telescope, with the aim to characterize the properties of warm and dense molecular gas in a large variety of environments. With an angular resolution of 22'', ¹²CO 3-2 emission was detected in 114 targets. Based on 61 galaxies observed with equal beam sizes the ¹²CO 3-2/1-0 integrated line intensity ratio R ₃₁ is found to vary from 0.2 to 1.9, with an average value of 0.81. No correlations are found for R ₃₁ to Hubble-type and far-infrared luminosity. Possible indications for a correlation with inclination angle and the 60 μm/100 μm color temperature of the dust are not significant. Higher R ₃₁ ratios than in "normal" galaxies, hinting at enhanced molecular excitation, may be found in galaxies hosting active galactic nuclei. Even higher average values are determined for galaxies with bars or starbursts, the latter being identified by the ratio of infrared luminosity versus isophotal area, log [(L _FIR/L _sun)/(D ² ₂₅/kpc²)] > 7.25. (U)LIRGs are found to have the highest averaged R ₃₁ value. This may be a consequence of particularly vigorous star formation activity, triggered by galaxy interaction and merger events. The nuclear CO luminosities are slightly sublinearly correlated with the global FIR luminosity in both the ¹²CO J = 3-2 and the 1-0 lines. The slope of the log-log plots rises with compactness of the respective galaxy sub-sample, indicating a higher average density and a larger fraction of thermalized gas in distant luminous galaxies. While linear or sublinear correlations for the ¹²CO J = 3-2 line can be explained, if the bulk of the observed J = 3-2 emission originates from the molecular gas with densities below the critical one, the case of the ¹²CO J = 1-0 line with its small critical density remains a puzzle.