Luminous and Dark Matter in the Milky Way

Abstract

(Abridged) Axisymmetric models of the Milky Way exhibit strong interrelations between the Galactic constants (R_0 and T_0), the stellar columndensity (S_*) and the shape of the dark matter (DM) halo. Here we present analytical relations that can be used to investigate the effects of the uncertain gaseous velocity dispersion on the HI flaring constraints. The contribution of cosmic rays and magnetic fields to the pressure gradients is small. A significantly flattened dark matter halo is only possible if R_0 <~ 6.8 kpc. If R_0 is larger than ~7 kpc, or T_0 >~ 170 km/s, we can rule out two DM candidates that require a highly flattened DM halo: 1) decaying massive neutrinos; and 2) a disk of cold molecular hydrogen. It is only possible to construct self-consistent models of the Galaxy based on the IAU-recommended values for the Galactic constants in the unlikely case that the the stellar columndensity is smaller than ~18 M_sun/pc^2. If we assume that the halo is oblate and S_* = 35 +/- 5 M_sun/pc^2, R_0 <~ 8 kpc and T_0 <~ 200 km/s. Combining the best kinematical and star-count estimates of S_*, we conclude that: 25 <~ S_* <~ 45 M_sun/pc^2. Kuijken & Gilmore's (1991) determination of the columndensity of matter with |z|<=1.1 kpc is robust and valid over a wide range of Galactic constants. Our mass models show that the DM density in the Galactic centre is uncertain by a factor 1000. In the Solar neighbourhood we find: rho_DM ~0.42 GeV/c^2/cm^3 or (11 +/- 5) mM_sun/pc^3 -- roughly 15% of rho_tot.