A twisted ring of cold and dense dust revealed by the Hi-GAL Survey around the Galactic Center: a past AGN phase for the Milky Way ?
J. Bally, L. Testi, A. Noriega-Crespo and the Hi-GAL Team
Thermal images of cold dust in the Central Molecular Zone of the Milky Way, obtained with the far-infrared cameras on-board the Herschel satellite, reveal a 3×107 solar masses ring of dense and cold clouds orbiting the Galactic Center. Using a simple toy-model, an elliptical shape having semi-major axes of 100 and 60 parsecs is deduced. The major axis of this 100-pc ring is inclined by about 40 degrees with respect to the plane-of-the-sky and is oriented perpendicular to the major axes of the Galactic Bar.
The 100-pc ring appears to trace the system of stable x2 orbits predicted for the barred Galactic potential.
SgrA* is displaced with respect to the geometrical center of symmetry of the ring. The ring is twisted and its morphology suggests a flattening-ratio of 2 for the Galactic potential, which is in good agreement with the bulge flattening ratio derived from the 2MASS data.
The ring may be a fossil of what may have been a large dusty torus in possible past AGN phase of the Milky Way, as the recent FERMI-LAT observations also seem to suggest.