Warm H$_\mathsf{2}$ in the Galactic center region

• Published in: Astronomy and astrophysics (Berlin) 2001-01, Vol.365 (2), p.174-185
• Main Authors: Rodríguez-Fernández, N. J., Martín-Pintado, J., Fuente, A., de Vicente, P., Wilson, T. L., Hüttemeister, S.
• Format: Article
• Language: English
• Subjects: Astrophysics; extinction - galaxy: center -infrared: ISM: continuum - infrared: ISM: lines and bands; Galactic Astrophysics; ISM: clouds -ISM: molecules -ISM: dust; Physics
• ISSN: 0004-6361; 1432-0746; 1432-0756
• DOI: 10.1051/0004-6361:20000020

We present ISO observations of several ${\mathrm H_2}$ pure-rotational lines (from S(0) to S(5)) towards a sample of 16 molecular clouds distributed along the central ∼500 pc of the Galaxy. We also present ${\mathrm C^{18} \mathrm O}$ and ${^{13}\mathrm{CO}}$ $J=1\rightarrow0$ and $J=2 \rightarrow 1$ observations of these sources made with the IRAM-30 m telescope. With the CO data we derive ${\mathrm H_2}$ densities of 103.5-4.0 cm-3 and ${\mathrm H_2}$ column densities of a few 1022 cm-2. We have corrected the ${\mathrm H_2}$ data for ∼30 magnitudes of visual extinction using a self-consistent method. In every source, we find that the ${\mathrm H_2}$ emission exhibits a large temperature gradient. The S(0) and S(1) lines trace temperatures (T) of ∼150 K while the S(4) and S(5) lines indicate temperatures of ∼600 K. The warm ${\mathrm H_2}$ column density is typically ∼1-2 1022 cm-2, and is predominantly gas with $T=150$ K. This is the first direct estimate of the total column density of the warm molecular gas in the Galactic center region. These warm ${\mathrm H_2}$ column densities represent a fraction of ∼30% of the gas traced by the CO isotopes emission. The cooling by ${\mathrm H_2}$ in the warm component is comparable to that by CO. Comparing our ${\mathrm H_2}$ and CO data with available ammonia (NH3) observations from literature one obtains relatively high NH3 abundances of a few 10-7 in both the warm and the cold gas. A single shock or Photo-Dissociation Region (PDR) cannot explain all the observed ${\mathrm H_2}$ lines. Alternatives for the heating mechanisms are discussed.