Although it contains a small fraction (1%) of the luminous mass of the Galaxy, the low density of the halo reduces the rate of tidal disruption and merging, allowing it to retain much of its merging history over long timescales in the form of fossil streams of stars (e.g., Tremaine 1993). Over tens of Gyr, merger remnants will spread out into tidal streamers that extend across the sky (fig. 9). The accretion of multiple satellites may result in tidal streamers covering a large fraction of the sky (>10%), with the observed covering fraction being diagnostic of the mass and total number of merged satellites. For example, if 100 objects of mass ~105Msun were accreted by the halo in the last 10 Gyr, the resulting tidal streamers would cover 10% of the sky (Johnston 1998).
For minor mergers, the low surface density of the resulting streamers (and high background contamination) suggests that star counts alone will be insufficient to detect the streamers. Instead, the spectroscopic identification of large-scale structures with coherent kinematic motions (proper motion, radial velocity) and metallicities is required (e.g., halo moving groups--Majewski et al. 1996; discovery of the Sgr dwarf--Ibata et al. 1994). The measured kinematics and current structure of the streamer can be used to reconstruct the encounter and recover information on the Galactic potential.
A Representative SWIFT Project: Halo Tidal Streamers
Simulations indicate that a survey of 4000 ^(10% of sky)
will reveal significant structure in the halo.
Since the kinematic and metallicity properties of halo streamers may not
differ drastically from the mean halo population (e.g., Majewski et al.
1996), dense sampling in each pointing (1.75sq.deg.)
is needed for
the robust identification of a coherent structure against statistical
fluctuations in the background halo population.
At a spectroscopic magnitude limit of V=22, we will be able to detect F and G halo dwarfs to 25 kpc where we expect a few 1000 dwarfs/sq.deg. (Reid & Majewski 1993). Assuming the pessimistic case that the halo streamer population represents only a small fraction (10%) of all halo stars, we would identify ~100 halo streamer stars per square degree. This would be sufficient to identify and quantify the mean kinematics and metallicity of the streamer population. SWIFT spectroscopy would be used to identify and eliminate interloping galaxies and to determine radial velocities and metallicities for all of the halo stars in order to search for coherent groups of stars in kinematics and metallicity.
Beyond quantifying the merger history of the halo, this project would also map out the kinematics and metallicities of 4 million halo dwarfs, possibly resulting in the detection of ultra metal-poor stars ([Fe/H] < -4) created in primordial star formation events. There are a host of other exciting discoveries possible in such a large dataset. Similar studies of the halos of nearby galaxies (e.g., LMC, M31, M33, M81) can be used to probe their formation histories for comparison with that of the Milky Way. Issues of interest include studies of galactic potentials, chemical enrichment histories, and existence of tidal streamers. SWIFT's field of view and high slit density are well-matched to the projected density of halo giants in these systems. Such studies may find convergent evolution occuring in galaxies with different progenitor populations. For example, the halo of M31 is thought to be much more metal rich than the Milky Way halo (e.g., Rich et al. 1996). Detailed abundances and kinematics for M31 halo giants can be used to explore the dynamical history of the halo and the origin of the high metal abundance.