The Galactic Inner Halo: Searching for White Dwarfs and Measuring the Fundamental Galactic Constant, Θ0/R0*

摘要

We establish an extragalactic, zero-motion frame of reference within the deepest optical image of a globular star cluster, a Hubble Space Telescope (HST) 123 orbit exposure of M4 (GO 8679, Cycle 9). The line of sight beyond M4 (l,b = 351°,16°) intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance of 7.6 kpc (for R0 = 8 kpc). The main sequence of this population can be clearly seen on the color-magnitude diagram (CMD) below the M4 main sequence. We isolate these spheroid stars from the cluster on the basis of their proper motions over the 6 yr baseline between these observations and others made at a previous epoch with HST (GO 5461, Cycle 4). Distant background galaxies are also found on the same sight line by using image-morphology techniques. This fixed-reference frame allows us to determine an independent measurement of the fundamental Galactic constant, Ω0 = Θ0/R0 = 25.3 ± 2.6 km s-1 kpc-1, thus providing a velocity of the local standard of rest vLSR = Θ0 = 202.7 ± 24.7 km s-1 for R0 = 8.0 ± 0.5 kpc. Second, the galaxies allow a direct measurement of M4's absolute proper motion, μα = -12.26 ± 0.54 mas yr-1, μδ = -18.95 ± 0.54 mas yr-1, 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 M☉ 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.