Knapen, J. H.; Allen, R. J.; Heaton, H. I.; Kuno, N.; Nakai, N.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 455, Issue 3, September I 2006, pp.897-902
Fecha de publicación:
9
2006
Revista
Número de citas
7
Número de citas referidas
7
Descripción
Context: .At least a fraction of the atomic hydrogen in spiral galaxies
is suspected to be the result of molecular hydrogen which has been
dissociated by radiation from massive stars. Aims: .In this paper,
we extend our earlier set of data from a small region of the Western
spiral arm of M 81 with CO observations in order to study the interplay
between the radiation field and the molecular and atomic hydrogen.
Methods: .We report CO(1-0) observations with the Nobeyama 45 m dish and
the Owens Valley interferometer array of selected regions in the Western
spiral arm of M 81. Results: .From our Nobeyama data, we detect
CO(1-0) emission at several locations, coinciding spatially with HI
features near a far-UV source. The levels and widths of the detected CO
profiles are consistent with the CO(1-0) emission that can be expected
from several large photo-dissociation regions with typical sizes of some
50 × 150 pc located within our telescope beam. We do not detect
emission at other pointings, even though several of those are near
far-UV sources and accompanied by bright HI. This non-detection is
likely a consequence of the marginal area filling factor of
photo-dissociation regions in our observations. We detect no emission in
our Owens Valley data, consistent with the low intensity of the CO
emission detected in that field by the Nobeyama dish. Conclusions:
.We explain the lack of CO(1-0) emission at positions farther from
far-UV sources as a consequence of insufficient heating and excitation
of the molecular gas at these positions, rather than as an absence of
molecular hydrogen.