An Extragalactic 12CO J=3-2 survey with the Heinrich-Hertz-Telescope
  We present results of a ^{12}CO 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'', ^{12}CO 3-2 emission was
detected in 114 targets. Based on 61 galaxies observed with equal beam sizes
the ^{12}CO 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 60mum/100mum 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_{25}/kpc)^2)] >
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 ^{12}CO J =
3-2 and the 1-0 lines. The slope of the log-log plots rises with compactness of
the respective galaxy subsample, indicating a higher average density and a
larger fraction of thermalized gas in distant luminous galaxies. While linear
or sublinear correlations for the ^{12}CO J = 3-2 line can be explained, if the
bulk of the observed J = 3-2 emission originates from molecular gas with
densities below the critical one, the case of the ^{12}CO J = 1-0 line with its
small critical density remains a puzzle.
