Dark matter in the Milky Way

II. The HI gas distribution as a tracer of the gravitational potential

¹ Argelander-Institut für Astronomie, Universität Bonn (Founded by merging of the Sternwarte, Radioastronomisches Institut and Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn.) , Auf dem Hügel 71, 53121 Bonn, Germany e-mail: [pkalberla;ldedes;jkerp]@astro.uni-bonn.de
² Tartu Observatory, 61602 Toravere, Estonia e-mail: urmas@aai.ee

Received: 8 September 2006
Accepted: 20 March 2007

Abstract

Context.Gas within a galaxy is forced to establish pressure balance against gravitational forces. The shape of an unperturbed gaseous disk can be used to constrain dark matter models.

Aims.We derive the 3D  volume density distribution for the Milky Way out to a galactocentric radius of 40 kpc and a height of 20 kpc to constrain the Galactic mass distribution.

Methods.We used the Leiden/Argentine/Bonn all sky 21-cm line survey. The transformation from brightness temperatures to densities depends on the rotation curve. We explored several models, reflecting different dark matter distributions. Each of these models was set up to solve the combined Poisson-Boltzmann equation in a self-consistent way and optimized to reproduce the observed flaring.

Results.Besides a massive extended halo of   10¹² , we find a self-gravitating dark matter disk with to 3 10¹¹ , including a dark matter ring at  kpc with to 2.8  10¹⁰ . The existence of the ring was previously postulated from EGRET data and coincides with a giant stellar structure that surrounds the Galaxy. The resulting Milky Way rotation curve is flat up to  kpc and slowly decreases outwards. The  gas layer is strongly flaring. The HWHM scale height is 60 pc at  kpc and increases to ~2700 pc at  kpc. Spiral arms cause a noticeable imprint on the gravitational field, at least out to  kpc.

Conclusions.Our mass model supports previous proposals that the giant stellar ring structure is due to a merging dwarf galaxy. The fact that the majority of the dark matter in the Milky Way for  kpc can be successfully modeled by a self-gravitating isothermal disk raises the question of whether this massive disk may have been caused by similar merger events in the past. The substructure in the Galactic dark matter disk suggests a dissipative nature for the dark matter disk.