Dense gas in nearby galaxies. V. Multilevel studies of CH3CCH and CH3CN.
Abstract
The first detections of methyl acetylene (CH_3_CCH) and methyl cyanide (CH_3_CN) emission from outside our Galaxy are reported. Five mm- wave lines of CH_3_CCH 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. CH_3_CCH 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. CH_3_CN 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 CH_3_CN 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 CH_3_CCH and CH_3_CN line intensity ratios. As a byproduct, mm-emission of the ring molecule C_3_H_2_ was detected toward M 82 and NGC 253. Toward the SW hotspot of M 82, the H41α recombination line was measured.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- July 1991
- Bibcode:
- 1991A&A...247..307M
- Keywords:
-
- Acetonitrile;
- Acetylene;
- Galactic Nuclei;
- Intergalactic Media;
- Methyl Compounds;
- Milky Way Galaxy;
- Astronomical Models;
- Gas Composition;
- Gas Density;
- Line Spectra;
- Millimeter Waves;
- Astrophysics