Warm H₂ in the Galactic center region

Abstract

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 ~ 500 pc of the Galaxy. We also present C¹⁸ O and {¹³ CO 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 10^3.5-4.0 cm^-3 and H₂ column densities of a few 10²² cm^-2. We have corrected the H₂data 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 ~ 150 K while the S(4) and S(5) lines indicate temperatures of ~ 600 K. The warm H₂ 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 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 NH_3 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. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA.}