A Hot Helium Plasma in the Galactic Center Region

Recent X-ray observations by the space mission Chandra confirmed theastonishing evidence for a diffuse, hot, thermal plasma at a temperature of 9.$10^7$ K (8 keV) found by previous surveys to extend over a few hundred parsecsin the Galactic Centre region. This plasma coexists with the usual componentsof the interstellar medium such as cold molecular clouds and a soft (~0.8 keV)component produced by supernova remnants, and its origin remains uncertain.First, simple calculations using a mean sound speed for a hydrogen-dominatedplasma have suggested that it should not be gravitationally bound, and thusrequires a huge energy source to heat it in less than the escape time. Second,an astrophysical mechanism must be found to generate such a high temperature.No known source has been identified to fulfill both requirements. Here weaddress the energetics problem and show that the hot component could actuallybe a gravitationally confined helium plasma. We illustrate the new prospectsthis opens by discussing the origin of this gas, and by suggesting possibleheating mechanisms.Comment: 9 pages, accepted for publication in APJ