  Multilevel observations of formaldehyde (H_2CO) and methanol (CH_3OH) toward
the nearby spiral galaxies NGC 253, Maffei 2, IC 342, M 82 and NGC 6946 are
presented. H_2CO was detected in all galaxies (tentatively in NGC 6946). CH_3OH
was detected in all objects with the notable exception of M 82. H_2CO 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 H_2CO in NGC 253 is at a density of ~10^4 cm^-3 ,
while the H_2CO lines in M 82 and IC 342 trace two different, spatially
separated gas components with densities of <= 10^4 cm^-3 and ~10^6 cm^-3. The
south-western molecular hotspot in M 82 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 IC 342. The CH_3OH
abundance in NGC 253 and the non-starburst nuclei of IC 342 and Maffei 2 are
comparable. A map of the 3_k-2_k lines in NGC 253 shows that CH_3OH traces
clumpy structures better than other molecules requiring high gas density to be
excited. CH_3OH toward M 82 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.
