29 October 2007
Andreas Koch (UCLA)
"New results on the abundances and kinematics in the M31 halo from 10-150 kpc"
The halo of M31 shows a wealth of substructures, some of which are consistent with assembly from satellite accretion.
Here we report the kinematics and abundances of red giants in the halo of M31, based on spectroscopy in the calcium
triplet region from Keck/Deimos. We measure metallicities based on the infrared Ca triplet, using a new method that
significantly improves precision for low S/N spectra. Furthermore, from the resulting radial velocity measurements we report on
kinematic substructures such as cold, i.e., low velocity dispersion, streams. We report the following new results:
1. a sharp decline in metallicity at R~20 kpc that is coincident with the edge of the rotating disk in the Ibata et al. (2005) maps.
2. A strong gradient in metallicity, with confirmed individual abundances in the outer halo of M31 <-2 dex and reaching
as low as ~-2.6 dex, the first time such a metal poor halo has been detected in M31.
3. 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 population of the inner halo; we show further that the abundance distribution of the Stream is different
from that of the inner halo. All our results indicate that the inner and outer halo of M31 cannot entirely consist of debris
from one such collisional event. We also note the important finding that the overlapping halo of M33 contributes a
considerable fraction of stars to the outer halo of M31.