* Astronomy

Title: The Sagittarius Dwarf Galaxy: a Model for Evolution in a Triaxial Milky Way Halo
Authors: David R. Law, Steven R. Majewski

We present a new N-body model for the tidal disruption of the Sagittarius (Sgr) dwarf that is capable of simultaneously satisfying the majority of angular position, distance, and radial velocity constraints imposed by current wide-field surveys of its dynamically young (< 3 Gyr) tidal debris streams. In particular, this model resolves the conflicting angular position and radial velocity constraints on the Sgr leading tidal stream that have been highlighted in recent years. While the model does not reproduce the apparent bifurcation observed in the leading debris stream, recent observational data suggest that this bifurcation may represent a constraint on the internal properties of the Sgr dwarf rather than the details of its orbit. The key element in the success of this model is the introduction of a non-axisymmetric component to the Galactic gravitational potential which can be described in terms of a triaxial dark matter halo whose minor/major axis ratio (c/a)_Phi = 0.72 and intermediate/major axis ratio (b/a)_Phi = 0.99 at radii 20 < r < 60 kpc. The minor/intermediate/major axes of this halo lie along the directions (l, b) = (7, 0), (0, 90), and (97, 0) respectively, corresponding to a nearly-oblate ellipsoid whose minor axis is contained within the Galactic disk plane. We demonstrate that with simple assumptions about the star formation history of Sgr, tidal stripping models naturally give rise to gradients in the metallicity distribution function (MDF) along the stellar debris streams similar to those observed in recent studies.

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Title: Exploring the Sagittarius Stream with SEKBO Survey RR Lyrae Stars
Authors: Sayuri L Prior, G S Da Costa, Stefan C Keller

A sample of RR Lyrae (RRL) variables from the Southern Edgeworth-Kuiper Belt Object survey in regions overlapping the expected position of debris from the interaction of the Sagittarius (Sgr) dwarf galaxy with the Milky Way (RA ~ 20 and 21.5 h; distance 16-21 kpc) has been followed up spectroscopically and photometrically. The 21 photometrically confirmed type ab RRLs have mean [Fe/H] = -1.79 ±0.08, consistent with the abundances found for RRLs in a different portion of the Sgr tidal debris stream. The distribution of velocities in the Galactic standard of rest frame (V_GSR) of the 26 RRLs in the region is not consistent with a smooth halo population. Upon comparison with the Sgr disruption models of Law et al (2005), a prominent group of five stars having highly negative radial velocities (V_GSR ~ -175 kms-1) is consistent with predictions for old trailing debris when the Galactic halo is modelled as oblate. The observations also require that the recent trailing debris stream has a broader spread perpendicular to the Sgr plane than predicted by the models. We have also investigated the possible association of the Virgo Stellar Stream (VSS) with Sgr debris by comparing radial velocities for RRLs in the region with the same models, finding similarities in the velocity-position trends. As suggested by our earlier work, the stars in the VSS region with large negative V_GSR values are likely to be old leading Sgr debris, but we find that while old trailing Sgr debris may well make a contribution at positive V_GSR values, it is unlikely to fully account for the VSS feature. Overall we find that further modelling is needed, as trailing arm data generally favours oblate models while leading arm data favours prolate models, with no single potential fitting all the observed data.

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Title: On the nature of the Milky Way satellites
Authors: Yang-Shyang Li (1), Gabriella De Lucia (2), Amina Helmi (1) ((1) Kapteyn Astronomical Institute, Groningen, (2) INAF, Trieste)

We combine a series of high-resolution simulations with semi-analytic galaxy formation models to follow the evolution of a system resembling the Milky Way and its satellites. The semi-analytic model is based on that developed for the Millennium Simulation, and successfully reproduces the properties of galaxies on large scales, as well as those of the Milky Way. In this model, we are able to reproduce the luminosity function of the satellites around the Milky Way by preventing cooling in haloes with Vvir < 16.7 km/s (i.e. the atomic hydrogen cooling limit) and including the impact of the reionisation of the Universe. The physical properties of our model satellites (e.g. mean metallicities, ages, half-light radii and mass-to-light ratios) are in good agreement with the latest observational measurements. We do not find a strong dependence upon the particular implementation of supernova feedback, but a scheme which is more efficient in galaxies embedded in smaller haloes, i.e. shallower potential wells, gives better agreement with the properties of the ultra-faint satellites. Our model predicts that the brightest satellites are associated with the most massive subhaloes, are accreted later (z \lta 1), and have extended star formation histories, with only 1 per cent of their stars made by the end of the reionisation. On the other hand, the faintest satellites were accreted early, are dominated by stars with age > 10 Gyr, and a few of them formed most of their stars before the reionisation was complete. Objects with luminosities comparable to those of the classical MW satellites are associated with dark matter subhaloes with a peak circular velocity \gta 10 km/s, in agreement with the latest constraints.

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Mystery of the missing mini-galaxies
Like moths about a flame, thousands of tiny satellite galaxies flutter about our Milky Way. For astronomers this is a dream scenario, fitting perfectly with the established models of how our galaxy's cosmic neighbourhood should be. Unfortunately, it's a dream in more ways than one and the reality could hardly be more different.
As far as we can tell, barely 25 straggly satellites loiter forlornly around the outskirts of the Milky Way.

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