towards and within the Galactic centre

Abstract

High-resolution spectroscopy of bright infrared sources in the centre of the Galaxy has resulted in the detection of in a remarkable array of dense and diffuse clouds along the 8000 parsec long line of sight, at a wide range of distances from the centre. Most prominent among these is a previously undetected, but very large amount of warm ( T ∼250 K) and diffuse ( n ∼100 cm 2 ) gas within a few hundred parsecs of the centre. The key to understanding the environment of the in this region is an absorption line at 3.53 μm from the metastable (3,3) rotational level, which has not been detected in dense or diffuse clouds outside of the Galactic centre (GC). We have used spectroscopy of this line along with other lines of and CO to characterize all of the clouds along the line of sight to the GC. The high abundance of in the central few hundred parsecs implies an ionization rate there that is several times larger than estimated for diffuse clouds outside the GC, and nearly two orders of magnitude greater than originally predicted for diffuse clouds.