Population Studies. X. Constraints on the Mass and Extent of the Galaxy's Dark Corona
Abstract
We report the results of a search for blue horizontal branch (BHB) stars at galactocentric distances of ~40-65 kpc. Heliocentric line-of- sight velocities are presented for 10 horizontal branch stars (nine BHB and one RR Lyrae object) in this distance interval and are used to place constraints on the extent and mass of the Galaxy's dark corona. The dispersion of the radial component of galactocentric velocity of this group of objects is 111 +/- 25 km s^-1^. While interpretation of these data is restricted by a lack of knowledge concerning the nature of the orbital properties of material at such large distances they, together with results for satellites in the range 50-240 kpc and Population II material having distances 10-40 kpc, suggest that the mass of the dark corona continues to grow out to at least 40 kpc. The data are discussed within the framework of Little & Tremaine. If the stellar orbits are isotropic the data show little evidence for a departure from a flat rotation curve having V_c_~200 km s^-1^, and the mass within 50 kpc is ~6 x 10^11^M_sun_ . If the orbits are radial the data for the horizontal branch stars may be interpreted in terms of a corona which extends only out to ~40 kpc, and in this case the mass of the Galaxy is 5(^+2^_-1_) x 10^11^ M_sun_ . We argue, however, that the assumptions of such a finite corona and radial orbits are inconsistent with the observed distribution of data for the 26 objects currently known to have distances greater than 40 kpc. Should the orbits be principally tangential, as has been advocated by some workers, the mass of the Galaxy will be higher than the above estimates: the present data argue against the model of this type presented by Sommer-Larsen. These results may be compared with recent mass estimates based on the distant satellites of the Galaxy. They are not inconsistent with the timing argument presented by Zaritsky et al. for Leo I, that the corona has a lower mass limit of 13 x 10^11^ M_sun_. They are also consistent with the result of Carney, Latham, & Laird, who argue from studies of local high-velocity stars that the Galaxy has lower limits to its mass and radius of 5 x 10^11^ M_sun_ and 45 kpc, respectively, although in such a comparison the caveats of Leonard & Tremaine must be borne in mind. We emphasize that horizontal branch stars with B = 19.7-20.8 (heliocentric distances of 60-100 kpc) have unlimited potential for the examination of the growth of the corona, as opposed to the situation for the restricted sample of distant satellites for which the data set is now essentially complete.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 1991
- DOI:
- 10.1086/170566
- Bibcode:
- 1991ApJ...380..104N
- Keywords:
-
- Galactic Mass;
- Galactic Structure;
- Horizontal Branch Stars;
- Milky Way Galaxy;
- Blue Stars;
- Line Of Sight;
- Astrophysics;
- GALAXIES: STRUCTURE;
- GALAXIES: THE GALAXY;
- STARS: HORIZONTAL-BRANCH;
- STARS: STELLAR DYNAMICS