  The age distribution, and chemical elemental abundances, of stars in the halo
of the Milky Way provide constraints on theories of galaxy formation. As one
specific example, the accretion of satellite galaxies similar to the present
retinue of dwarf spheroidals (dSphs) would provide an observable metal-poor,
intermediate-age population. This paper presents a quantitative assessment of
any contribution made by such stars to the stellar halo. The bulk of the
stellar populations in the halo show a well-defined turn-off, at B-V ~ 0.4,
implying that the vast majority of the stars are old. The fraction of stars
which lie blueward of this well-defined turn-off, with metallicities similar to
that of the present dSphs, is used in this paper to place limits on the
importance of the recent accretion of such systems. Very few (< ~ 10%) stars
are found to be bluer (and by implication, younger) than this limit, with the
highest value found for the more metal-rich halo ([Fe/H] >~ -1.5 dex). Direct
comparison of this statistic with the colour distribution of the turnoff stars
in the Carina dwarf allows us to derive an upper limit on the number of mergers
of such satellite galaxies into the halo of the Milky Way. This upper limit is
~ 40 Carina-like galaxies. The higher metallicity data constrain satellite
galaxies like the Fornax dwarf; only <~ 5 of these could have been accreted
within the last < ~ 10 Gyr. We note that the low star-formation rates inferred
for dSphs predict distinctive elemental abundance signatures; future data for
field halo stars, including candidate younger stars, will provide a further
robust test of accretion models.
