The progenitors of the Milky Way stellar halo: big bricks favoured over little bricks

We present a census of blue horizontal branch (BHB) and blue straggler (BS) stars belonging to dwarf galaxies and globular clusters, and compare these counts to that of the Milky Way stellar halo. We find, in agreement with earlier studies, that the ratio of BS-to-BHB stars in these satellite populations is dependent on stellar mass. Dwarf galaxies show an increasing BS-to-BHB ratio with luminosity. In contrast, globular clusters display the reverse trend, with N-BS/N-BHB (less than or similar to 1) decreasing with luminosity. The faintest (L < 10(5) L circle dot) dwarfs have similar numbers of BS and BHB stars (N-BS/N-BHB similar to 1), whereas more-massive dwarfs tend to be dominated by BS stars (N-BS/N-BHB similar to 2-40). We find that the BS-to-BHB ratio in the stellar halo is relatively high (N-BS/N-BHB similar to 5-6), and thus inconsistent with the low ratios found in both ultra-faint dwarfs and globular clusters. Our results favour more-massive dwarfs as the dominant 'building blocks' of the stellar halo, in good agreement with current predictions from Lambda cold dark matter models.