The hot gas and the cold dust in the galactic center clouds as seen by ISO

J. Martin-Pintado ¹, N. Rodrigued-Fernandez ¹P. de Vicente ¹ A. Fuente ¹T.L. Wilson ²S. Huettermeister ³ & D. Kunze ^{4 1} Observatorio Astronomico Nacional, Apartado 1143, E-28800, Alcala de Henares, Spain

² Max Planck Institut für Radioastronomie, Postfach 2024, D-53010 Bonn, Germany

³ Astronomische Institute, Auf dem Huegel 71, D-53121 Bonn 1, Germany

⁴ MPE, Giesenbachstrasse 1, D-85748 Garching, Germany

The molecular clouds in the galactic center exhibit unusual characteristics with widespread high gas kinetic temperatures and relatively low dust temperatures.

Unlike the warm and hot cores in the molecular clouds of the galactic disk, where the UV photons from young stars heat the grains and the gas is heated by dust-gas collisions, the low dust temperatures observed in the galactic center require a different heating mechanism (cosmic rays, shocks, etc..) which acts directly on the gas.

We present the first results of our ISO observations toward 21 molecular clouds in the galactic center region. Towards all molecular clouds, we detect several

ground state rotational lines of H₂ and the derived rotational temperatures are typically between 150 and 500 K. The hot H₂ gas is located behind of more 30 mag of visual extinction, and the estimated H₂ column densities are comparable to those of the cold gas measured by C¹⁸O. This indicates that the low J transition of C¹⁸O do not trace the total molecular gas in the galactic center. Furthermore, the SWS and LWS continuum spectra can be explained by cold dust with temperatures of typically 25-35 K and column densities similar to those derived from C¹⁸O The LWS spectra imposes stringent limits (<2%) to the hot dust component associated to the hot H₂. This would imply that the hot gas has been heated by shocks. However, towards most of the clouds we also observed lines from ionized species like [SIII] and [NeII] and some in cases also from [NeIII] and [OIII] which favors the heating by UV radiation with an effective temperature of about 35.000 K (typical of an O7 star). These data when combined with the intensities of the recombination lines measured at millimeter wavelengths indicate that the UV radiation is very diluted filling rather large cavities surrounding the ionizing star(s). The origin of the hot bubbles and

the implications of these findings on the heating of other molecular clouds in the galactic center will be also discussed.