Grand rotation curve and dark-matter halo in the Milky Way Galaxy

摘要

A grand rotation curve of the Milky Way Galaxy was constructed, which covers a wide range of radius from the Galactic Center to ～ 1 Mpc, and was deconvolved into bulge, disk, and halo components by least-squares fittings. We determined the masses and scale radii of the bulge and disk to be M _b = (1.652 ± 0.083) × 10 ~(10) M ⊙, a _b = 0.522 ± 0.037 kpc,M _d = (3.41 ± 0.41) × 10 ~(10) M ⊙, and a _d = 3.19 ± 0.35 kpc. The dark halo was fitted by the Navaro-Frenk-White (NFW) density profile, ρ = ρ _0/[(R/h)(1 + R/h) ~2], and the fit yielded h = 12.5 ± 0.9 kpc and ρ _0 = (1.06 ± 0.14) × 10 ~(-2)M ⊙ pc ~(-3). The local dark-matter density near the Sun at R _0 = 8 kpc was estimated to be ρ ⊙ _0 = (6.12 ± 0.80) × 10 ~(-3)M ⊙ pc ~(-3) = 0.235 ± 0.030GeVcm ~(-3). The total mass inside the gravitational boundary of the Galaxy at R ～ 385 kpc, a half distance to M31, was estimated to be M b+d+h = (7.03 ± 1.01) × 10 ~(11)M ⊙. This leads to a stellar baryon fraction of M _(b+d)/M _(b+d+h) = 0.072 ± 0.018. Considering the expected baryon fraction in the Local Group, we suggest that baryons in the form of hot gas are filling the dark halo with a temperature of ～ 10 ~6 K and an emission measure of ～ 10 ~(-5) pc cm ~2. Such hot halo gas may share a small fraction of the observed X-ray background emission.