The [CII] 158 Micron Absorption Line Towards the Galactic Center: Connection With Bright IR Galaxies

High spectral resolution ISO-LWS data are presented for the centre of the Sgr B2 complex around the fine structure lines of [CII] at 157.7 microns and the [OI] lines at 63.2 and 145.5 microns. Both the [OI] 63.2 microns and the [CII] 157.7 micron lines are detected in absorption. Whereas the [OI] 63.2 micron line is seen in absorption over the entire wavelength range -200 to 100 km s^-1, the [CII] 157.7 micron line displays a more complex profile: absorption occurs at velocities <20 km s^-1 with an emission profile from the Sgr B2 complex at velocities greater than 20 km s^-1. Using CO isotopes and HI data, absorption components can be associated with many clouds along the Sagittarius B2 line of sight. From these data, we are able to disentangle three different layers, predicted by Photo-Dissociation Regions models, which contain atomic oxygen. The method used in that study leads to an estimate of the oxygen content in the intermediate layer where C⁰ is the dominant form of carbon. An O⁰/CO ratio of around 2.5 is computed in the internal cores of the clouds lying along the line of sight, which means that ∼70% of gaseous oxygen is in the atomic form and not locked into CO. The fact that the [CII] 157.7 micron line is detected in absorption could partially account for the deficiency in this line which has been recently found towards infrared bright galaxies.

To appear in ApJ, December 2002.