2003A&A...398..959H

We present ¹²CO (J=2-1) and CO (J=3-2) observations of quiescent Virgo Cluster spiral galaxies with the KOSMA 3m submm telescope. The beam sizes of 80" at 345GHz and 120" at 230GHz are well suited for the investigation of global properties of Virgo Cluster galaxies. The observed sample was selected based on previous ¹²CO (J=1-0) detections by Stark et al. (1986ApJ...310..660S), performed with the AT&T Bell Laboratory 7 m telescope (beam size ∼100"). We were able to detect 18 spiral galaxies in ¹²CO (2-1) and 16 in ¹²CO (3-2). Beam matched observations of the lowest three ¹²CO transitions allow us to compare our results with previous high spatial resolution studies of (moderate) starburst galaxies and galactic core regions. We discuss the global excitation conditions of the ISM in these quiescent spiral galaxies. The resulting CO (3-2)/(1-0) integrated line ratios vary over a relatively narrow range of values from 0.35 to 0.14 (on a K.km/s-scale) with increasing CO (2-1)/(1-0) ratio (from 0.5 to 1.1). The line ratios between the three lowest rotational transitions of CO cannot be fitted by any radiative transfer model with a single source component. A two-component model, assuming a warm, dense nuclear and a cold, less dense disc component allows us to fit the observed line ratios for most of the galaxies individually by selecting suitable parameters. The two-component model, however, fails to explain the observed correlation of the line ratios. This is due to a variation of the relative filling factor of the warm gas alone, assuming a typical set of parameters for the two components common for all galaxies.