Probing the Nature of the G1 Clump Stellar Overdensity in the Outskirts of M31

We present deep HST/ACS observations of the G1 clump, a distinct stellaroverdensity lying at ~30 kpc along the south-western major axis of M31 close tothe G1 globular cluster (Ferguson et al. 2002). Our well-populatedcolour-magnitude diagram reaches ~7 magnitudes below the red giant branch tipwith 90% completeness, and allows the detection of various morphologicalfeatures which can be used to derive detailed constraints on the age andmetallicity of the constituent stellar population. We find that thecolour-magnitude diagram is best described by a population with a large agerange (>= 10 Gyr) and a relatively high mean metallicity [M/H]= -0.4. Thespread in metallicity is constrained to be <=0.5 dex. The star formation ratein this region has declined over time, with the bulk of the stellar mass havingformed >6 Gyr ago. Nonetheless, a non-negligible mass fraction (approximately10%) of the population has formed in the last 2 Gyr. We discuss the nature ofthe G1 Clump in light of these new stellar population constraints and arguethat the combination of stellar content and physical size make it unlikely thatthe structure is the remnant of an accreted dwarf galaxy. Instead, the strongsimilarity between the stellar content of the G1 Clump and that of the M31outer disk suggests the substructure is a fragment of the outer disk, perhapstorn off from the main body during a past accretion/merger event; thisinterpretation is consistent with extant kinematical data. If thisinterpretation is correct, our analysis of the stellar content provides furtherevidence that the outskirts of large disk galaxies have been in place for asignificant time.