Rare CO isotope observations of M 51a (NGC 5194)

Abstract

We present observations of the central regions of M 51a (NGC5194) in the J = 1-0 rotational transitions of 13CO, C18O, and C 17O. The last two are the first detections reported for this object. We have combined these data with published HCN and CO(J = 1-0) observations with the same telescope to carry out some LVG modeling (single-cloud and two-cloud mixtures) of the line ratios. The results are compatible with the presence of a region emitting most of the HCN and another component at lower temperatures and densities emitting most of the rest of the molecular emission. The observed high C18O/C17O ratio, together with our models, suggest that this is caused by an underabundance of [C17O] (compared with Galactic values). This can be explained by different evolutionary histories. The C 18O emission line has a clearly asymmetric profile, implying the presence of significant excitation differences within the observed region. Finally, there is an indication that the CO(1-0) is overestimating the molecular gas mass in the central kpc of this galaxy. This is similar to what is found in the Milky Way at similar spatial scales, and possibly associated with a change in the conversion factor due to the average cloud properties in the central regions of this object. © 2008. Astronomical Society of Japan.