Dense gas in nearby galaxies XI . H CO and CH OH : Molecular abundances and physical conditions

Multilevel observations of formaldehyde (H2CO) and methanol (CH3OH) toward the nearby spiral galaxies NGC 253, Maffei 2, IC342, M82 and NGC6946 are presented. H2CO was detected in all galaxies (tentatively in NGC6946). CH3OH was detected in all objects with the notable exception of M82. H2CO line intensity ratios point out differences in gas density both between galaxies and within the central regions of individual objects. Model calculations show that the bulk of the gas emitting H2CO in NGC253 is at a density of ∼ 10 cm, while the H2CO lines in M82 and IC 342 trace two different, spatially separated gas components with densities of ≤ 10 cm and ∼ 10 cm. The south-western molecular hotspot in M82 and the center of IC 342 are the regions with the highest density. Methanol is subthermally excited in all galaxies, with the lowest excitation temperatures found in IC342. The CH3OH abundance in NGC253 and the non-starburst nuclei of IC 342 and Maffei 2 are comparable. A map of the 3k−2k lines in NGC253 shows that CH3OH traces clumpy structures better than other molecules requiring high gas density to be excited. CH3OH toward M82 is at least an order of magnitude less abundant than in otherwise comparable galaxies. This confirms the existence of global chemical differences, and thus very large scale variations in the state of the molecular gas phase, even between galaxies commonly classified as starburst nuclei.