Low lying rotational transitions of CO at mm-wavelengths are widely used as tracers of molecular hydrogen. This is valid for clouds in the Galactic disk, the Galactic center and also for extragalactic objects (e.g. Strong et al. 1988, Bitran et al. 1997, Mauersberger et al. 1996b). To date, however, the widespread use of CO and 2-1 spectroscopy (e.g. Young et al. 1995, Braine et al. 1993, Sage 1993, Chini et al. 1996 and Elfhag et al.1996) is not complemented by a systematic survey for higher rotational CO transitions (for a few exceptions, see Steppe et al. 1990, Tilanus et al. 1991, Güsten et al. 1993, White et al. 1994, Israel et al. 1995, Mauersberger et al. 1996a), so that important aspects of molecular excitation emphasizing warmer and denser molecular regions remain virtually unexplored.
The and 2 levels of CO are 5.5 and 17 K above the ground level. The state, however, is at 33 K, and hence traces a warmer gas component that may not be pervasive in galactic disks, but which is certainly important toward the central regions of galaxies, particularly toward the more active ones. In addition, the "critical density" of the CO transition (at which collisional excitation matches spontaneous decay in the optically thin case) is , vs. and for the two lower transitions.
We have used the Heinrich-Hertz Telescope on Mt. Graham to observe the CO 3-2 line in a number of galaxies which are known to be strong emitters in the and 2-1 transitions. The main purpose is to present a sample of sources with integrated line intensities of three CO transitions and almost identical beam widths (), and to provide a starting point for extended CO 3-2 surveys, either covering larger samples of sources or involving detailed maps and isotopic studies for individual sources.
© European Southern Observatory (ESO) 1999
Online publication: November 26, 1998