Weighing the Galactic dark matter halo: a lower mass limit from the fastest halo star known

Abstract

The mass of the Galactic dark matter halo is under vivid discussion. A recent study by Xue et al. (2008, ApJ, 684, 1143) revised the Galactic halo mass downward by a factor of ~2 relative to previous work, based on the line-of-sight velocity distribution of ~2400 blue horizontal-branch (BHB) halo stars. The observations were interpreted in a statistical approach using cosmological galaxy formation simulations, as only four of the 6D phase-space coordinates were determined. Here we concentrate on a close investigation of the stars with highest negative radial velocity from that sample. For one star, SDSSJ153935.67+023909.8 (J1539+0239 for short), we succeed in measuring a significant proper motion, i.e. full phase-space information is obtained. We confirm the star to be a Population II BHB star from an independent quantitative analysis of the SDSS spectrum - providing the first NLTE study of any halo BHB star - and reconstruct its 3D trajectory in the Galactic potential. J1539+0239 turns out as the fastest halo star known to date, with a Galactic rest-frame velocity of 694${}_{-221}^{+300}$ km/s (full uncertainty range from Monte Carlo error propagation) at its current position. The extreme kinematics of the star allows a significant lower limit to be put on the halo mass in order to keep it bound, of M_halo$\ge 1.7_{-1.1}^{+2.3}\times 10^{12}$ Msun. We conclude that the Xue et al. results tend to underestimate the true halo mass as their most likely mass value is consistent with our analysis only at a level of 4%. However, our result confirms other studies that make use of the full phase-space information.

5 pages, 5 figures. ApJ, accepted

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• MMT Hypervelocity Star Survey. II. Five New Unbound Stars
• Bailing Out the Milky Way: Variation in the Properties of Massive Dwarfs Among Galaxy-Sized Systems
• The Hyper-MUCHFUSS project: probing the Galactic halo with sdB stars
• CD-62\degree1346: an extreme halo or hypervelocity CH~star?
• The Nature of Hypervelocity Stars and the Time Between Their Formation and Ejection
• Systematic uncertainties from halo asphericity in dark matter searches
• An extremely fast halo hot subdwarf star in a wide binary system
• Kinematics in galactic tidal tails - A source for Hypervelocity stars?
• Constraining the Milky Way Dark Matter Density Profile with Gamma-Rays with Fermi-LAT
• NLTE spectroscopic analysis of the ${}^3$He anomaly in subluminous B-type stars