Constraints on galaxy formation from stars in the far outer disk of M31

Numerical simulations of galaxy formation within the cold dark matter (CDM) hierarchical clustering framework are unable to produce large disk galaxie s without invoking some form of feedback to suppress gas cooling and collap se until a redshift of unity or below. An important observational consequen ce of delaying the epoch of disk formation until relatively recent times is that the stellar populations in the extended disk should be of predominant ly young-to-intermediate age. We use a deep Hubble Space Telescope/Wide Fie ld Planetary Camera 2 archival pointing to investigate the mean age and met allicity of the stellar population in a disk-dominated field at 30 kpc alon g the major axis of M31. Our analysis of the color-magnitude diagram reveal s the dominant population to have a significant mean age (greater than or s imilar to8 Gyr) and a moderately high mean metallicity ([Fe/H] similar to - 0.7); tentative evidence is also presented for a trace population of ancien t (greater than or equal to 10 Gyr) metal-poor stars. These characteristics are unexpected in CDM models, and we discuss the possible implications of this result as well as alternative interpretations.