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Astron. Astrophys. 326, 59-68 (1997)

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5. Conclusions

We have observed rotational transitions of the high gas density tracing molecules [FORMULA] and [FORMULA] toward a sample of nearby gas rich external galaxies, namely NGC 253, Maffei 2, IC 342, M 82 and NGC 6946. Our main results are:

  1. H2 CO was detected in all 5 galaxies searched (tentatively in NGC 6946).
  2. In the prominent starburst galaxy NGC 253, the bulk of the gas emitting [FORMULA] lines is at densities of [FORMULA] [FORMULA]. In both M 82 and IC 342, the [FORMULA] line ratios distinguish two different components of molecular gas. High densities dominate the SW molecular hotspot in M 82 and the central region of IC 342 ([FORMULA] [FORMULA]), while lower densities ([FORMULA] [FORMULA]) are prevalent toward the center of M 82 and the north of the nucleus of IC 342.
  3. We detected [FORMULA] lines in the same sample of galaxies, with the notable exception of M 82.
  4. In all galaxies detected, methanol is subthermally excited. In NGC 253, Maffei 2 and NGC 6946, moderate gas densities of [FORMULA] [FORMULA] are likely. Toward a position north of the center of IC 342, the H2 densities seem to be at [FORMULA] [FORMULA]. This might indicate that warmer gas is at lower densities in the non-starburst nucleus of IC 342, where [FORMULA] traces an entirely different gas component than [FORMULA].
  5. In M 82, no methanol could be detected down to an rms level of 10-15 mK, corresponding to an abundance at least an order of magnitude lower than in the other galaxies. This confirms large scale chemical differences in starburst centers. The likeliest cause of these are global differences in the temperature of the molecular material.
  6. A map of NGC 253 in the [FORMULA] line of methanol shows several peaks of emission and a greater amount of spatial structure than than single-dish maps of of other molecules, even high-density tracers. Thus, [FORMULA] emission is associated with smaller scale clumps. This indicates chemical fractionation not only between different galaxies but also within the bulge of NGC 253, possibly reflecting the temperature structure of the gas.
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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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