Signatures of the M31-M32 Galactic Collision

The unusual morphologies of the Andromeda spiral galaxy (M31) and its dwarf companion M32 have been characterized observationally in great detail. The two galaxies apparent proximity suggests that Andromedas prominent star-forming ring as well as M32s compact elliptical structure may result from a recent collision. Here we present the first self-consistent model of the M31-M32 interaction that simultaneously reproduces observed positions, velocities, and morphologies for both galaxies. Andromedas spiral structure is resolved in unprecedented detail, showing that a rare head-on orbit is not necessary to match Andromedas ring-like morphology. The passage of M32 through Andromedas disk perturbs the disk velocity structure. We find tidal stripping of M32s stars to be inefficient during the interaction, suggesting that some cEs are intrinsically compact. Additionally, the orbital solution implies that M32 is currently closer to the Milky Way than models have typically assumed, a prediction that may be testable with upcoming observations.

Observational Evidence from SDSS for a Merger Origin of the Milky Ways Thick Disk

We test competing models that aim at explaining the nature of stars in the Milky Way that are well away (|z|$gtrsim$ 1kpc) from the midplane, the so-called thick disk: the stars may have gotten there through orbital migration, through satellite mergers and accretion, or through heating of pre-existing thin disk stars. Sales et al. (2009) proposed the eccentricity distribution of thick disk stars as a diagnostic to differentiate between these mechanisms. Drawing on SDSS DR7, we have assembled a sample of 34,223 G-dwarfs with 6-D phase-space information and metallicities, and have derived orbital eccentricities for them. Comparing the resulting eccentricity distributions, p(e|z), with the models, we find that: a) the observed p(e|z) is inconsistent with that predicted by orbital migration only, as there are more observed stars of high and of very low eccentricity; b) scenarios where the thick disk is made predominantly through abrupt heating of a pre-existing thin disk are also inconsistent, as they predict more high-eccentricity stars than observed; c) the observed p(e|z) fits well with a gas-rich merger scenario, where most thick disk stars were born from unsettled gas in situ.