The Ages and Chemistry of the Oldest Stellar Halo Populations

NGST is capable of reaching the main sequence turnoff of the oldest stellar populations (MV = +4.5) out to a distance ≈ 2 Mpc. There is now evidence that the oldest, most metal poor star clusters are coeval to within 1 Gyr over a 200 kpc radius centered on the Milky Way. NGST can be used to make an equally definitive test of this in the halo and globular cluster system of M31, NGC 147, 185, and perhaps also M81. We also propose to image field halo populations in galaxies spanning a full range of luminosity and Hubble type. The metallicity distribution can be measured using giants with MV < −1, permitting study of galaxies as distant as the Virgo cluster ≈ 16 Mpc. This study will determine whether halo star properties correlate with Hubble type or galaxy luminosity, or whether halos are built from disrupted dwarf galaxies, globular clusters, and other low luminosity stellar systems (the so-called Searle-Zinn hypothesis). An excess of extreme blue horizontal branch stars at large radii in stellar halos could be the fossil remnant of a population III component. Deep imaging in the outer disks of M31 and M33 will reveal the star formation histories of disk populations, which should agree with any inferred peak redshift of disk formation. ASWG DRM Proposal The Ages and Chemistry of the Oldest Stellar Halo Populations Observing Summary: Target RA Dec KAB Configuration/mode Days M31-CLUST 00 40 +41 00 V=30.5 OPT/CAM R10 1 M31-DISK 00 40 +41 00 V=30.5 OPT/CAM R10 1 M31-DWARFS 00 40 +41 00 V=30.5 OPT/CAM R10 1 M33 HALO, DISK 01 31 +30 23 V=30.5 OPT/CAM R10 1 VIRGO CLUST SURVEY 12 40 +20 00 V=32 OPT/CAM R10 20 SOUTH POLAR GROUP 00 30 -30 00 V=32 OPT/CAM R10 6 M81 GROUP 00 40 +41 00 V=32 OPT/CAM R10 4 NGC 5128, 5102 13 20 -36 00 V=30.5 OPT/CAM R10 2