Excitation of the inner pc of the Galactic Center: Results from near-infrared spectroscopy

Abstract

We present near-infrared spectroscopy and line imaging of the central 0.5 pc of the Galaxy. The “mini-cavity” region south-west of SgrA∗ is a source of bright 2.217 μm emission now identified as due to [Fe III]. We infer that iron is released from grains in shocks triggered by fast wind blowing into the Galactic Center gas streamers. After cooling the shocked streamer gas is maintained at about 7000 K by the central UV field. Morphology and dynamics can be explained either by models invoking HeI emission line stars as a local wind source or by models in which a jet from SgrA∗ acts as an external wind source. Velocity-resolved spectroscopy of the brightest HeI emission line star in the Galactic Center has been used to constrain atmospheric models. The star is found to be similar to evolved massive stars of type Ofpe/WN9, undergoing heavy mass loss. Helium is overabundant in the atmosphere. Extrapolating from this star it is likely that the Galactic Center HeI stars contribute a large fraction of the total luminosity and a significant fraction of the Lyman continuum luminosity of the central parsec. Hotter stars are needed to provide the HeI ionizing flux.