The Millennium Galaxy Catalogue: Star counts and the structure of the Galactic stellar halo

We derive a star catalogue generated from the images taken as part of the ∼37.5 deg² Millennium Galaxy Catalogue. These data, alone and together with colours gained from the Sloan Digital Sky Survey Early Data Release, allow the analysis of faint star counts (B^MGC < 20) at high Galactic latitude (41° < b < 63°), as a function of Galactic longitude (239° < l < 353°). We focus here on the inner stellar halo, providing robust limits on the amplitude of substructure and on the large-scale flattening. In line with previous results, the thick disc, an old, intermediate-metallicity population, is clearly seen in the colour-magnitude diagram. We find that the Galactic stellar halo within ∼10 kpc (the bulk of the stellar mass) is significantly flattened, with an axial ratio of (c/a) = 0.56 ± 0.01, again consistent with previous results. Our analysis, using counts-in-cells, angular correlation functions and the Lee 2D statistic, confirms tidal debris from the Sagittarius dwarf but finds little evidence for other substructure in the inner halo, at heliocentric distances of ≲5 kpc. This new quantification of the smoothness in coordinate space limits the contribution of recent accretion/disruption to the build-up of the bulk of the stellar halo.