Free access article

A&A 427, 217-229 (2004)
DOI: 10.1051/0004-6361:20041370

ISO observations of the Galactic center interstellar medium

Neutral gas and dust

N. J. Rodríguez-Fernández¹, J. Martín-Pintado², A. Fuente³ and T. L. Wilson<sup>4

1</sup>  LERMA (UMR 8112), Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
    e-mail: nemesio.rodriguez@obspm.fr
²  DAMIR, Instituto de Estructura de la Materia, Consejo Superior de Investigaciones Científicas (CSIC), Serrano 121, 28006 Madrid, Spain
³  Observatorio Astronómico Nacional, Instituto Geográfico Nacional, Apdo. 1143, 28800 Alcalá de Henares, Spain
⁴  Max-Planck-Institut für Radio Astronomie, Auf dem Hügel 69, 53121 Bonn, Germany

(Received 28 May 2004 / Accepted 9 July 2004)

Abstract
The 500 central pc of the Galaxy (hereafter GC) exhibit a widespread gas component with a kinetic temperature of 100-200 K. The bulk of this gas is not associated to the well-known thermal radio continuum or far infrared sources like Sgr A or Sgr B. How this gas is heated has been a longstanding problem. With the aim of studying the thermal balance of the neutral gas and dust in the GC, we have observed 18 molecular clouds located at projected distances far from thermal continuum sources with the Infrared Space Observatory (ISO). In this paper we present observations of several fine structure lines ([OI] 63 and 146  m, [CII] 158  m, [Si II] 35  m, [S I] 25  m and [Fe II] 26  m), which are the main coolants of the gas with kinetic temperatures of several hundred K. We also present the full continuum spectra of the dust between 40 and 190  m. All the clouds exhibit a cold dust component with a temperature of ~15 K. A warmer dust component is also required to fit the spectra. The temperature of this dust component changes between 27 and 42 K from source to source. We have compared the gas and the dust emission with the predictions from J-type and C-type shocks and photodissociation region (PDRs) models. We conclude that the dust and the fine structure lines observations are best explained by a PDR with a density of 10 ³ cm ^-3 and an incident far-ultraviolet field 10 ³ times higher than the local interstellar radiation field. The fine structure line emission arises in PDRs in the interface between a diffuse ionized gas component and the dense molecular clouds. The [CII] 158  m and [Si II] 35  m lines also have an important contribution from the ionized gas component. PDRs can naturally explain the discrepancy between the gas and the dust temperatures. However, these PDRs can only account for 10-30% of the total H ₂ column density with a temperature of ~150 K. We discuss other possible heating mechanisms for the rest the warm molecular gas, such as non-stationary PDRs, X-ray Dominated Regions (XDRs) or the dissipation of supersonic turbulence.

Key words: ISM: lines and bands -- infrared: galaxies -- galaxies: ISM -- Galaxy: center -- ISM: dust, extinction

SIMBAD Objects

© ESO 2004

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

• If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
• You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
• You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.