Warm H in the Galactic center region ^*

¹ Observatorio Astronómico Nacional, IGN, Apartado 1143, 28800 Alcalá de Henares, Spain
² Max-Planck-Institut für Radioastronomie, Postfach 2024, 53010 Bonn, Germany
³ Sub-mm Telescope Observatory, Steward Observatory, The University of Arizona, Tucson, AZ 85728, USA
⁴ Radioastronomisches Institute der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Corresponding author: N. J. Rodríguez-Fernández, nemesio@oan.es

Received: 10 August 2000
Accepted: 6 October 2000

Abstract

We present ISO observations of several 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 and and observations of these sources made with the IRAM-30 m telescope. With the CO data we derive densities of 10^3.5-4.0 cm^-3 and column densities of a few 10²² cm^-2. We have corrected the data for ∼30 magnitudes of visual extinction using a self-consistent method. In every source, we find that the 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 column density is typically ∼1-2 10²² cm^-2, and is predominantly gas with K. This is the first direct estimate of the total column density of the warm molecular gas in the Galactic center region. These warm column densities represent a fraction of ∼30% of the gas traced by the CO isotopes emission. The cooling by in the warm component is comparable to that by CO. Comparing our 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 lines. Alternatives for the heating mechanisms are discussed.