Rosemary F. G. Wyse

Deciphering the Last Major Invasion of the Milky Way

We present first results from a spectroscopic survey of ~2000 F/G stars 0.5-5 kpc from the Galactic plane, obtained with the Two Degree Field facility on the Anglo-Australian Telescope. These data show the mean rotation velocity of the thick disk about the Galactic center a few kiloparsecs from the plane is very different than expected, being ~100 km s-1 rather than the predicted ~180 km s-1. We propose that our sample is dominated by stars from a disrupted satellite that merged with the disk of the Milky Way some 10-12 Gyr ago. We do not find evidence for the many substantial mergers expected in hierarchical clustering theories. We find yet more evidence that the stellar halo retains kinematic substructure, indicative of minor mergers.

Faint stars in the Ursa Minor dwarf spheroidal galaxy: implications for the low-mass stellar initial mass function at high redshift

The stellar initial mass function at high redshift is an important defining property of the first stellar systems to form and may also play a role in various dark matter problems. We here determine the faint stellar luminosity function in an apparently dark-matter-dominated external galaxy in which the stars formed at high redshift. The Ursa Minor dwarf spheroidal galaxy is a system with a particularly simple stellar population-all of the stars being old and metal-poor-similar to that of a classical halo globular cluster. A direct comparison of the faint luminosity functions of the UMi dSph and of similar metallicity, old globular clusters is equivalent to a comparison of the initial mass functions and is presented here, based on deep HST WFPC2 and STIS imaging data. We find that these luminosity functions are indistinguishable, down to a luminosity corresponding to similar to0.3 M-.. Our results show that the low-mass stellar IMF for stars that formed at very high redshift is apparently invariant across environments as diverse as those of an extremely low-surface-brightness, dark-matter-dominated dwarf galaxy and a dark-matter-free, high-density globular cluster within the Milky Way.

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 deg 2 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 (BMGC < 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 disk, an old, intermediatemetallicity 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.