We derive new constraints on the mass of the Milky Way ’ s dark matter halo , based on a set of halo stars from SDSS as kinematic tracers .
Our sample comprises 2401 rigorously selected Blue Horizontal-Branch ( BHB ) halo stars at |z| \geq 4 kpc , and with distances from the Galactic center up to \sim 60 kpc , with photometry and spectra drawn from SDSS DR-6 .
With distances accurate to \sim 10 \% , this sample enables construction of the full line-of-sight velocity distribution at different Galactocentric radii .
To interpret these distributions , we compare them to matched mock observations drawn from two different cosmological galaxy formation simulations designed to resemble the Milky Way , which we presume to have an appropriate orbital distribution of halo stars .
Specifically , we select simulated halo stars in the same volume as the observations , and derive the distributions P ( V _ { los } / V _ { cir } ) of their line-of-sight velocities for different radii , normalized by the simulation ’ s local circular velocity .
We then determine which value of V _ { cir } ( r ) brings the observed distribution into agreement with the corresponding distributions from the simulations .
These values are then adopted as observational estimates for V _ { cir } ( r ) , after a small Jeans Equation correction is made to account for slight data/simulation differences in the radial density distribution .
This procedure results in an estimate of the Milky Way ’ s circular velocity curve to \sim 60 kpc , which is found to be slightly falling from the adopted value of 220 ~ { } km~ { } s ^ { -1 } at the Sun ’ s location , and implies M ( < 60 ~ { } kpc ) = 4.0 \pm 0.7 \times 10 ^ { 11 } M _ { \odot } .
The radial dependence of V _ { cir } ( r ) , derived in statistically independent bins , is found to be consistent with the expectations from an NFW dark matter halo with the established stellar mass components at its center .
If we assume an NFW halo profile of characteristic concentration holds , we can use the observations to estimate the virial mass of the Milky Way ’ s dark matter halo , M _ { vir } = 1.0 ^ { +0.3 } _ { -0.2 } \times 10 ^ { 12 } M _ { \odot } , which is lower than many previous estimates .
We have checked that the particulars of the cosmological simulations are unlikely to introduce systematics larger than the statistical uncertainties .
This estimate implies that nearly 40 % of the baryons within the virial radius of the Milky Way ’ s dark matter halo reside in the stellar components of our Galaxy .
A value for M _ { vir } of only \sim 1 \times 10 ^ { 12 } M _ { \odot } also ( re- ) opens the question of whether all of the Milky Way ’ s satellite galaxies are on bound orbits .