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 = 351O, 16O) intersects the inner halo (spheroid) of our Galaxy at a tangent-point distance of 7.6 kpc (for RO = 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-year 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. The clear separation of galaxies from stars in these deep data allows 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 solar mass 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.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 2003