The Once and Future Andromeda Stream
Masao Mori (Senshu University) and R. Michael Rich (UCLA)
Accepted for publication in ApJ Letters (arXiv:0711.4589)
The interaction between an accreting satellite and the Andromeda galaxy (M31) has been
studied analytically and numerically, using a high-resolution N-body simulation with
4 x 107 particles. For the first time, we show the self-gravitating response
of the disk, the bulge, and the dark matter halo of M31 to an accreting satellite.
We reproduce the Stream and the shells at the East and West side of M31, by following
the evolution of the collision 4 Gyr into the future, and show that recently discovered
diffuse arcs on the south side of the minor axis are the remnants of a similar collision
that occurred 3-4 Gyr earlier than the Stream event. The present day integrity of the
M31 disk constrains the satellite mass to be M < 5 x 109 Msun.
The stars that were originally in the center of the satellite are now in the east shell.
Therefore, observations in this region might reveal additional clues about the nature of
satellite, such as the central core and any metallicity gradient.
Kinematic and Chemical constraints on the formation of M31's inner and outer halo
A. Koch, R.M. Rich, D. Reitzel, N.F. Martin, R.A. Ibata, S.C. Chapman, S.R. Majewski, M. Mori, Y.-S. Loh, J.C. Ostheimer
ApJ submitted (arXiv:0711.4588)
The halo of M31 shows a wealth of substructures that are consistent with satellite accretion. Here we report on kinematic and abundance results from Keck/DEIMOS spectroscopy in the calcium triplet region of over 3500 red giant star candidates along the minor axis and in off-axis spheroid fields of M31. Our data reach out to large radial distances of 160 kpc. The derived velocity distributions show a kinematically cold substructure at 17 kpc that has been reported before. We devise an improved method to measure accurate metallicities from the calcium triplet in low signal-to-noise spectra using a coaddition of the individual lines. The resulting distribution leads us to note an even stronger gradient in the abundance distribution along M31's minor axis than previously detected. The mean metallicity in the outer halo reaches below -2 dex, with individual values as low as -2.6 dex. In the inner spheroid, at 17-19 kpc, we find a sharp decline of ~0.5 dex in metallicity, which roughly coincides with the edge of an extended disk, previously detected from star count maps. A comparison of our velocities with those predicted by new N-body simulations argues that the event responsible for the giant Stream is most likely not responsible for the full population of the inner halo; we show further that the abundance distribution of the Stream is different from that of the inner halo, from which it becomes evident that the merger event that formed the outer halo cannot have contributed any significant material to the inner spheroid. All this evidence of severe structure changes in the halo suggests a high degree of infall and stochastic abundance accretion governing the build-up of M31's halo. A large fraction of red giants in the most distant fields are likely members of M33's overlapping halo (Abridged).