2001A&A...365..174R

We present ISO observations of several H₂ pure-rotational lines (from S(0) to S(5)) towards a sample of 16 molecular clouds distributed along the central ∼500pc of the Galaxy. We also present C¹⁸ O and ¹³CO J=1->0 and J=2 ->1 observations of these sources made with the IRAM-30 m telescope. With the CO data we derive H₂ densities of 10^3.5–4.0cm^–3 and H₂ column densities of a few 10²²cm^–2. We have corrected the H₂ data for ∼30 magnitudes of visual extinction using a self-consistent method. In every source, we find that the H₂ emission exhibits a large temperature gradient. The S(0) and S(1) lines trace temperatures (T) of ∼150K while the S(4) and S(5) lines indicate temperatures of ∼600K. The warm H₂ column density is typically ∼1-2x10²²cm^–2, and is predominantly gas with T=150K. This is the first direct estimate of the total column density of the warm molecular gas in the Galactic center region. These warm H₂ column densities represent a fraction of ∼30% of the gas traced by the CO isotopes emission. The cooling by H₂ in the warm component is comparable to that by CO. Comparing our H₂ 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 H₂ lines. Alternatives for the heating mechanisms are discussed.