Broken degeneracies: the rotation curve and velocity anisotropy of the Milky Way halo

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 04/2012; 424(1). DOI: 10.1111/j.1745-3933.2012.01283.x
Source: arXiv


We use distant Blue Horizontal Branch stars with Galactocentric distances 16
< r/kpc < 48 as kinematic tracers of the Milky Way dark halo. We model the
tracer density as an oblate, power-law embedded within a spherical power-law
potential. Using a distribution function method, we estimate the overall
power-law potential and the velocity anisotropy of the halo tracers. We measure
the slope of the potential to be gamma ~ 0.4 and the overall mass within 50 kpc
is ~ 4 x 10^11 M_sol. The tracer velocity anisotropy is radially biased with
beta ~ 0.5, which is in good agreement with local solar neighbourhood studies.
Our results provide an accurate outer circular velocity profile for the Milky
Way and suggest a relatively high concentration dark matter halo (c_vir ~ 20).

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    Preview · Article · May 2012 · The Astrophysical Journal
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    ABSTRACT: We build a sample of distant (D > 80 kpc) stellar halo stars with measured radial velocities. Faint (20 < g < 22) candidate blue horizontal branch (BHB) stars were selected using the deep, but wide, multi-epoch Sloan Digital Sky Survey photometry. Follow-up spectroscopy for these A-type stars was performed using the Very Large Telescope (VLT) FOcal Reducer and low dispersion Spectrograph 2 (FORS2) instrument. We classify stars according to their Balmer line profiles, and find that seven are bona fide BHB stars and 31 are blue stragglers (BS). Owing to the magnitude range of our sample, even the intrinsically fainter BS stars can reach out to D ∼ 90 kpc. We complement this sample of A-type stars with intrinsically brighter, intermediate-age, asymptotic giant branch stars. A set of four distant cool carbon stars is compiled from the literature and we perform spectroscopic follow-up on a further four N-type carbon stars using the William Herschel Telescope (WHT) Intermediate dispersion Spectrograph and Imaging System (ISIS) instrument. Altogether, this provides us with the largest sample to date of individual star tracers out to r ∼ 150 kpc. We find that the radial velocity dispersion of these tracers falls rapidly at large distances and is surprisingly cold (σr ≈ 50–60 km s−1) between 100 and 150 kpc. Relating the measured radial velocities to the mass of the Milky Way requires knowledge of the (unknown) tracer density profile and anisotropy at these distances. Nonetheless, by assuming the stellar halo stars between 50 and 150 kpc have a moderate density fall-off (with power-law slope α < 5) and are on radial orbits (), we infer that the mass within 150 kpc is less than 1012 M⊙ and suggest it probably lies in the range (5–10) × 1011 M⊙. We discuss the implications of such a low mass for the Milky Way.
    Full-text · Article · May 2012 · Monthly Notices of the Royal Astronomical Society
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