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Astron. Astrophys. 347, 634-639 (1999)

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1. Introduction

CO gas is a tracer of molecular hydrogen observable at mm and sub-mm wavelengths. This is valid for molecular clouds in the Galaxy but also for extragalactic objects. In observations of galaxies, when the beamwidth subtends many molecular clouds at a time, care is needed to interpret the results. The intensity of the various line transitions can give us information about the local density, temperature and optical depth (e.g. Scoville & Solomon 1974; de Jong et al. 1975; Rohlfs & Wilson 1996). These possibilities have been used extensively by observers, especially in the framework of the Large Velocity Gradient (LVG) approximation and Monte-Carlo simulations. Additional information, which helps to interpret the physical situation in the clouds, has come from the various isotopomers of CO which observers have studied in addition to the 12CO observations. One general fact of all these studies emerges: the higher line transitions are indicators of warmer and denser gas.

Extensive observations of the CO(1-0) line and the CO(2-1) line have been made in hundreds of galaxies (e.g. Braine et al. 1993; Young et al. 1995; Bajaja et al. 1995). Some nearby galaxies have also been mapped in the 13CO transition (e.g. Loiseau et al. 1988; Harrison et al. 1999). The data on the CO(3-2) line transition is rather sparse in view of the lack of suitable telescopes on good sites to map the extended galaxies.

The first detection of the CO(3-2) line in the nearby starburst galaxy IC342 was reported by Ho et al. (1987). A few years later maps of the central regions of IC342 (Steppe et al. 1990) and of the core of M82 (Tilanus et al. 1991) were published. A report on CO(3-2) line intensities at 22 positions in the inner part of M51 was presented by Bash et al. (1990). In the multi-line studies of M51 some CO(3-2) observations were reported by Garcia-Burillo et al. (1993). Since then a couple of - mainly starburst - galaxies were observed in the CO(3-2) line, but they cover basically only the central regions of these objects (e.g. Devereux et al. 1994; Wilson et al. 1997; Petitpas & Wilson 1998; Harrison et al. 1999). Also the central positions were studied recently in the CO(3-2) line in 30 nearby galaxies by Mauersberger et al. (1999), also using the HHT. A CO(3-2) map of the central region of NGC 4945 was published by Mauersberger et al. (1996), claiming that the higher line transitions are concentrated to the nucleus. In this paper we present a discussion on multi-line studies of the three galaxies M51, NGC 278 and NGC 4631, suggesting that the warm and dense CO gas is rather extended in these objects.

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© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999