A Grand Rotation Curve and Dark Matter Halo in the Milky Way Galaxy

Abstract

A grand rotation curve of the Milky Way Galaxy is constructed, which covers a wide range of radius from the Galactic Center to ~1 Mpc, and is deconvolved into bulge, disk and halo components by least-squares fitting. We determined the scale radii and masses of the bulge and disk to be M_b=(1.652 +/-0.083)x10^10 Msun, a_b=(0.522 +/- 0.037) kpc, M_d=(3.41 +/- 0.41)x10^10 Msun and a_d=(3.19 +/- 0.3) kpc. The dark halo was fitted by the Navaro-Frenk-White (NFW) density profile, rho=rho_0/[(R/h) (1+R/h)^2], and the fit yielded h=12.5 +/- 0.9 kpc and rho_0=(1.06 +/- 0.14)x10^-2 Msun pc^-3. The local dark matter density near the Sun at R_0=8 kpc is estimated to be rho_0_sun=(6.12 +/-0.80)x10^-3 Msun pc^-3 = 0.235 +/- 0.030 GeV cm^-3. The total mass inside the gravitational boundary of the Galaxy at R~ 385 kpc, a half distance to M31, is estimated to be M_h ~ (7.03 +/- 1.01)x10^11 Msun. This leads to the stellar baryon fraction of (M_b+M_d)/(M_b+M_d+M_h)=0.072 +/-0.018. Considering expected baryon fraction in the Local Group, we suggest that baryons in the form of hot gas are filling the dark halo with temperature of ~ 10^6 K and emission measure ~ 10^-5 pc cm^-6. Such hot halo gas may share a small fraction of the observed X-ray background emission.