The gas deficiency of the galactic halo.
Abstract
The γ-ray diffuse emission has been recently observed with unprecedented accuracy by the Compton Gamma Ray Observatory (CGRO) at photon energies in the range between 100MeV and 10GeV. A residual isotropic diffuse radiation is measured for the first time at low galactic latitude, with little variation over different portions of the sky. That measurement translates into a tight constraint on the abundance of diffuse gas in the dark matter halo surrounding our galaxy. If that halo contained significant amounts of gas, cosmic-ray protons originating from the galactic disc would interact with it, yielding a γ-ray flux which CGRO would have observed. By using a diffusion model which correctly reproduces the radial distribution of cosmic-rays along the galactic plane, we infer an upper limit of ~2 to 4% on the fraction of gas in diffuse form or in clouds. The flatter the halo, the stronger the bound. That result only applies in the region where cosmic-rays are confined, i.e., for a galactocentric radius R<20kpc and a height |z|<4kpc. In particular, a thin disc is severely constrained by the γ-ray measurements.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- September 1996
- Bibcode:
- 1996A&A...313....1S
- Keywords:
-
- DARK MATTER;
- GALAXY: HALO;
- COSMIC RAYS: OBSERVATIONS;
- ISM: GENERAL