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 galaxieswithout invoking some form of feedback to suppress gas cooling and collapse toa redshift of unity or below. An important observational consequence ofdelaying the epoch of disk formation until relatively recent times is that thestellar populations in the extended disk should of be predominantlyyoung-to-intermediate age. We use a deep HST/WFPC2 archival pointing toinvestigate the mean age and metallicity of the stellar population in adisk-dominated field at 30 kpc along the major axis of M31. Our analysis of thecolor-magnitude-diagram reveals the dominant population to have significantmean age (>~8 Gyr) and a moderately-high mean metallicity ([Fe/H]~-0.7);tentative evidence is also presented for a trace population of ancient (>10Gyr) metal-poor stars. These characteristics are unexpected in CDM models andwe discuss the possible implications of this result, as well as alternativeinterpretations.Comment: Accepted to ApJ Letters, 4 pages, uses emulateapj5.sty. Minor revisions/additions to previously posted versio