Stellar population in M31
Ultradeep ACS imaging of a halo field in M31 reaching V=30.7, well below the old main sequence turnoff, has lead to the discovery that the halo of this galaxy includes a major high-metallicity, intermediate age component that appears to be absent in the Milky Way halo. In various proportions this population is also present in the M31 stream and disk, also observed to comparable depth with ACS. Kinematic, radial velocities of individual stars via Keck spectroscopy have allowed us to further characterize the composite stellar populations of this galaxy. HST refurbishment is awaited to continue exploring other fields, in particular including several M31 globular clusters. Other main results so far achieved include the determination of the periods of RR Lyraes in the M31 halo, again enlightening an appreciable difference with their Galactic counterpart, and the age determination of an M31 globular cluster via its main sequence turnoff, indicating that is 1-2 Gyr younger than Milky Way globulars of the same metallicity. Most recently, HST/ACS observations have been carried out along the minor axis up to a projected distance of 35 kpc, extending the characterization of the stellar populations in the halo of M31. Slowly, a picture is emerging indicating that star formation, metal enrichment, and mass assembly of M31 has proceeded in measurably different ways compared to our own Galaxy.
People: A. Renzini
Collaboration: T. Brown (STScI), M. Rich (UCLA), A. Sweigart (Goddard), H. Ferguson (STScI), et al.
Publications: Brown et al. (2008), ApJ 685,121; Brown et al. (2007), ApJ 658,L95