The Outskirts of Spiral Galaxies: Evidence for Multiple Stellar Populations

We present an analysis of the metallicity distribution functions of fieldsprojected along the minor axis for a sample of inclined spiral galaxies inorder to search for evidence of the presence of multiple stellar populations.In all cases, the stellar populations appear to have asymmetric metallicitydistributions with very high confidence levels. The mean metallicities of bothstellar subpopulations, determined from mixture modelling of the metallicitydistribution functions, correlate with parent galaxy luminosity. This suggeststhat the vast majority of field stars have probably formed in galacticfragments that were already embedded in the dark matter halo of the finalgalaxy. The steeper correlation between the mean stellar metallicity and parentgalaxy luminosity is driven by an increasing fraction of metal-rich stars withincreasing galaxy luminosity. Metal-poor components show larger dispersion inmetallicity than metal-rich components. These properties are in strikingsimilarity with those of globular cluster subpopulations around early-typegalaxies. The properties of field stars along the minor axis are consistentwith a formation scenario in which the metal-poor stars formed in all galaxies,possibly as a result of tidal disruption of dwarf-like objects. An additionalmetal-rich component might be related to the formation of the bulge and/or thedisk.Comment: 7 pages, 2 figures. ApJ, accepte