Abstract
The first detections of methyl acetylene (CH3CCH) and methyl cyanide (CH3CN) emission from outside our Galaxy are reported. Five mm-wave lines of CH3CCH have been measured toward the molecular hotspot 15" SW of the center of M 82; four transitions have been observed toward the center of the starburst galaxy NGC 253. CH3CCH is the most complex molecule detected outside the Galaxy. Its abundances in M 82 and NGC 253 are similar to those measured in Galactic clouds. CH3CN has been detected in at least three mm-wave transitions toward the center of NGC 253, where its abundance is 'normal'. A sensitive search toward the center of M 82 shows that CH3CN is underabundant compared with Galactic clouds. These results confirm the different chemical compositions of the nuclear gas in M 82 and NGC 253. Aside of a likely difference in kinetic temperature, there must be additional effects causing the observed discrepancies; several alternatives are discussed.
For both galaxies studied, densities of a few times 10(4) cm-3 are required to match the observed CH3CCH and CH3CN line intensity ratios.
As a byproduct, mm-emission of the ring molecule C3H2 was detected toward M 82 and NGC 253. Toward the SW hotspot of M 82, the H41-alpha recombination line was measured.