ISO observations of the Galactic center interstellar medium , Neutral gas and dust

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 ((O) 63 and 146 µm, (C) 158 µm, (Si  )3 5µm, (S  )2 5µ ma nd (Fe )2 6µm), 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 µm. 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 (C) 158 µ ma nd (Si) 35 µm 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 H2 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. The central 600 pc of the Galaxy (hereafter the Galactic center, GC) contain up to 10% of the neutral gas of the Milky Way. In spite of the high gas surface density in the GC (several or- ders of magnitude higher than in the disk of the Galaxy) the star formation rate is 10 times lower than that in the disk and the star formation efficiency is only similar to that in the disk. This is probably due to the particular physical conditions of the � 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. �� Figures 1-4 are only available in electronic form at