The hot gas and the cold dust in the Galactic Centre clouds as seen by ISO

Details The hot gas and the cold dust in the Galactic Centre clouds as seen by ISO The hot gas and the cold dust in the Galactic Centre clouds as seen by ISO

Mar 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999esasp.427..711m&link_type=abstract

The Universe as Seen by ISO. Eds. P. Cox & M. F. Kessler. ESA-SP 427., p. 711

Physics

5

Galactic Center, Ism, Shock Waves

Scientific paper

We present the first results of the LWS and SWS ISO observations of 18 molecular clouds in the Galactic centre (GC) region. H2 emission from several rotational lines in the ground vibrational state is detected towards all molecular clouds. The rotational temperatures derived from these lines are between 150 and 500 K. Remarkably, the derived column densities of the hot H2, ~ 2 1022 cm-2, are similar to those obtained for the cold gas. >From the non-detection of the S(3) line at 9.7 μm, we derive that the hot gas must be located behind more than 30 mag of visual extinction of cold gas and dust. The LWS continuum spectra indicate that the cold dust has temperatures of 25-35 K and column densities similar to those required to explain the extinction of the H2 emission. The observations of the J=2-1 and the J=1-0 line of C18O seem to sample only the cold (J=2-1/J=1-0 line ratios of 0.7-1.4) gas located in front of the hot gas. This indicates that the low J transition of C18O does not trace the total molecular gas in the GC. The LWS continuum spectra impose stringent limits (<= 1 per cent) to the hot dust associated to the hot H2. Towards most of the clouds we also detected emission from the fine structure lines of ionized species like SIII and NeII, and in some cases also from NeIII and OIII. In contract with the previous idea that the heating of the GC clouds is dominated by shocks, our data suggest that photoelectric heating by UV radiation can be the dominant mechanism. The effective temperature of the ionization radiation derived from the NeIII/NeII ratio is 35.000 K (typical of an O7 star). These ISO data when combined with the upper limits to the intensities of the recombination lines measured at millimeter wavelengths indicate that the UV radiation is very diluted. Cavities with sizes larger than 2 pc surrounding the ionizing star(s) are required to explain the mentioned dilution. The origin of the ionized cavities and the implications of these findings on the heating of the molecular clouds in the GC are briefly discussed.

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