Astronomy and Astrophysics – Astrophysics – Galaxy Astrophysics
Scientific paper
2011-10-20
Astronomy and Astrophysics
Astrophysics
Galaxy Astrophysics
To appear in PASJ 2012, Vol 64, No. 2
Scientific paper
A grand rotation curve of the Milky Way Galaxy is constructed, which covers a wide range of radius from the Galactic Center to $\sim 1$ 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 $\Mb=(1.652 \pm0.083)\times 10^{10}\Msun$, $\ab=0.522 \pm 0.037$ kpc, $\Md=(3.41 \pm 0.41)\times 10^{10}\Msun$ and $\ad=3.19 \pm 0.35$ 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 $h=12.5 \pm 0.9$ kpc and $\rho_0=(1.06 \pm 0.14)\times 10^{-2} \Msun {\rm pc}^{-3}$. The local dark matter density near the Sun at $R_0=8$ kpc is estimated to be $\rho_0^\odot=(6.12 \pm 0.80)\times 10^{-3} \Msun {\rm pc}^{-3} = 0.235 \pm 0.030 {\rm GeV cm}^{-3}$. The total mass inside the gravitational boundary of the Galaxy at $R\sim 385$ kpc, a half distance to M31, is estimated to be $\Mh \sim (7.03\pm 1.01)\times 10^{11} \Msun$. This leads to the stellar baryon fraction of $(\Mb+\Md)/(\Mb+\Md+\Mh)=0.072\pm 0.018$. 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 $\sim 10^6$K and emission measure $\sim 10^{-5} \pccm $. Such hot halo gas may share a small fraction of the observed X-ray background emission.
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