The Galactic Inner Halo: Searching for White Dwarfs and Measuring the Fundamental Galactic Constant, Vo/Ro

Abstract

We establish an extragalactic, zero-motion frame of reference within the deepest optical image of a globular star cluster, an HST 123-orbit exposure of M4 (GO 8679, cycle 9). The line of sight beyond M4 (l,b (deg) = 351,16) intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance of 7.6 kpc (for Ro = 8 kpc). We isolate these spheroid stars from the cluster based on their proper motions over the 6-year baseline between these and previous epoch HST data (GO 5461, cycle 4). Distant background galaxies are also found on the same sight line using image-morphology techniques. This fixed reference frame allows us to independently determine the fundamental Galactic constant, Vo/Ro = 25.3 +/- 2.6 km/s/kpc, thus providing a velocity of the Local Standard of Rest, v = 202.7 +/- 24.7 km/s for Ro = 8.0 +/- 0.5 kpc. Secondly, the galaxies allow a direct measurement of M4's absolute proper motion, mu_total = 22.57 +/- 0.76 mas/yr, in excellent agreement with recent studies. The clear separation of galaxies from stars in these deep data also allow us to search for inner-halo white dwarfs. We model the conventional Galactic contributions of white dwarfs along our line of sight and predict 7.9 (thin disk), 6.3 (thick disk) and 2.2 (spheroid) objects to the limiting magnitude at which we can clearly delineate stars from galaxies (V = 29). An additional 2.5 objects are expected from a 20% white dwarf dark halo consisting of 0.5 Mo objects, 70% of which are of the DA type. After considering the kinematics and morphology of the objects in our data set, we find the number of white dwarfs to be consistent with the predictions for each of the conventional populations. However, we do not find any evidence for dark halo white dwarfs.

31 pages, including 6 diagrams and 2 tables. Accepted for publication in ApJ

References in corpus (9)

• The White Dwarf Cooling Sequence of the Globular Cluster Messier 4
• Direct Detection of Galactic Halo Dark Matter
• High-velocity white dwarfs: thick disk, not dark matter
• The Lower Main Sequence and Mass Function of the Globular Cluster Messier 4
• An Upper Limit to the Age of the Galactic Bar
• HST Astrometry of M4 and the Galactic Constant V_0/R_0
• Halo White Dwarfs, Thick Disks and a Sanity Check
• A Proper Motion Survey for White Dwarfs with the Wide Field Planetary Camera 2
• Illuminating the darkness: More evidence from faint-star proper-motions for a cool and warm component to the local dark matter in the Galaxy

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