Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...274..707t&link_type=abstract
Astronomy and Astrophysics, Vol. 274, NO. 3/JUL(III), P. 707, 1993
Astronomy and Astrophysics
Astrophysics
60
Scientific paper
We present high-resolution observations of the atomic hydrogen (HI) and Hβ emission lines in the southern grand-design spiral galaxy M 83. Contrary to the case of M 51, we have not detected streaming motions of a magnitude of 60-90 km s^-1^ (in the plane of the disk) across the spiral arms of M 83 either in H I or in Hβ. The Hβ observations hint at streaming motions at a level of 35 km s^-1^, comparable to the motions seen in M81. Significant deviations from circular rotation are observed in the regions where the bar connects with the spiral arms, both in H I and Hβ . The gas is moving tangentially faster in these regions compared to a simple model for the rotation in the disk of M 83. These motions are similar to the streaming motions observed in CO (Kenney & Lord 1991). The H I observations have revealed a large-scale displacement (700 pc) of the atomic gas from the dust lane along a 7 kpc stretch of the inner eastern spiral arm of M 83. This separation can not be explained by beam smoothing. The atomic gas in that part of the arm is coincident with the ridge of HII complexes along the outer edge of the spiral arm. The observations, as well as recent CO observations of the eastern arm of M 83, are discussed in the context of two scenarios that attempt to explain the observed morphology of the interstellar medium (ISM) in the eastern spiral arm. The dissociation scenario assumes the ISM to be predominantly molecular at the position of the dust lanes and to (partly) dissociate into the observed HI ridge downstream. The diffuse-shock scenario assumes that the massive clouds shock at the observed location of the CO, HI, and HII regions, whereas a shock in a more diffuse medium results in the dust lane upstream. We argue in this paper that the effect of heating of the CO in the spiral arm may have been underestimated in the interpretation of the CO observations. Heating may have increased the CO brightness downstream from the shock substantially in which case the observed CO distribution across the spiral arm does not accurately reflect the underlying distribution of the molecular gas. The dissociation scenario appears to apply in M 51, and the diffuse-shock scenario in M 81; these are two galaxies which may be at opposite extremes with regard to the strength of the density-wave shock and the fraction of ISM in molecular hydrogen. M 83 may be an intermediate case lacking the strong density-wave shock, a suggestion which is observationally supported by our failure to detect streaming motions like those in M51.
Allen Rosalind J.
Tilanus Remo P. J.
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