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Complementing the data gathered by looking back to high redshift is information gleaned by ``archeological'' studies of objects at z ~ 0. The oldest components of the Milky Way provide evidence that mergers of small galaxies played an important role [40]:

  1. A ``second parameter'' - which may not be age [41] - is required to account for variations in the stellar content of globular clusters.

  2. This second parameter is correlated with orbital direction; clusters with retrograde orbits have Oosterhoff class I variables [42].

  3. Halo stars with [Fe/H] ~ -1 have a large range of [alpha/Fe] values [43, 44].

  4. The outer halo exhibits retrograde rotation with respect to the rest of the galaxy [45].

  5. The halo is not completely well-mixed, as indicated by observations of star streams and moving groups [45, 46, 47].

Items 1-3 indicate that different parts of the halo have different enrichment histories, items 2 & 4 imply that some part of the halo fell in on a retrograde orbit, and item 5 is direct evidence for the gradual dissolution of fragments after merging.

Halo accretion is clearly an ongoing process, as shown by the discovery of the Sgr I dwarf galaxy [48] and by observations of high-latitude A stars [49]. But two different arguments suggest that the bulk of the halo fell into place long ago.

First, halo stars are old. The halo as a whole shows a well-defined turn-off at B - V ~ 0.4, corresponding to ages gtapprox 10 Gyr; only ~ 10% of the stars appear younger [50]. To be sure, this does not rule out recent accretions of objects containing only old stars, but most dwarf galaxies in the local group contain intermediate-age stars as well. Thus, unless the accreted galaxies were unlike those we observe today, most fell in more than 10 Gyr ago.

Second, galactic disks are dynamically fragile; accretion of satellite galaxies can easily ruin a stellar disk. Analytic estimates limit the mass accreted by the Milky Way to less than 4% in the past 5 Gyr [51]. N-body experiments show less disk heating than the analytic work predicts; dark halos absorb much of the damage, and disks may tilt as well as thicken [52, 53, 54]. Still, accretion events of any size increase the disk's vertical dispersion, sigmaz. Significant structure is seen in the sigmaz-age relation; most striking is the jump from sigmaz appeq 20 to 40 km sec-1 which marks the transition to the ~ 10 Gyr-old thick disk [55].

In sum, the Milky Way last suffered a significant merger at least 10 Gyr ago; relics of this event include the outer stellar halo and possibly the thick disk. Presumably, the Milky Way's dark halo was largely in place at this time, since a major merger would have disrupted even the thick disk.

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