John Dickey (University of Tasmania, Hobart)

How does the Milky Way disk fade away? - John Dickey Colloquium

ATNF Marsfield Lecture Theatre

Abstract

The outer disk of the Milky Way is an interesting laboratory for the thermal phases of the atomic hydrogen in the interstellar medium. Data from the recent Galactic Plane Surveys at 21-cm wavelength show that the absorption and emission in the HI line decrease together with Galactic radius, but their ratio, the excitation or spin temperature of the line, stays roughly constant. This means that on average the mixture of warm and cool phase HI gas does not change much with radius in the outer Galaxy, which is a surprise since the average gas pressure itself must change dramatically. What seems to be happening is that the filling factor of the neutral atomic medium is decreasing, and the cool neutral medium (Tspin < 100 K) is confined to large complexes with diameters 100 to 300 pc and masses of 10**5 M_sun or more. These have very low molecular gas fraction, but are otherwise similar to giant molecular clouds in the inner Galaxy. It is not clear what holds these cloud-complexes together.