Old and Young Bulges in Late‐Type Disk Galaxies

ABRIDGED: We use HSTACS and NICMOS imaging to study the structure and colorsof a sample of nine late-type spirals. We find: (1) A correlation between bulgeand disks scale-lengths, and a correlation between the colors of the bulges andthose of the inner disks. Our data show a trend for bulges to be moremetal-enriched than their surrounding disks, but otherwise no simpleage-metallicity connection between these systems; (2) A large range in bulgestellar population properties, and, in particular, in stellar ages.Specifically, in about a half of the late-type bulges in our sample the bulk ofthe stellar mass was produced recently. Thus, in a substantial fraction of thez=0 disk-dominated bulged galaxies, bulge formation occurs after theformation/accretion of the disk; (3) In about a half of the late-type bulges inour sample, however, the bulk of the stellar mass was produced at early epochs;(4) Even these "old" late-type bulges host a significant fraction of stellarmass in a young(er) c component; (5) A correlation for bulges between stellarage and stellar mass, in the sense that more massive late-type bulges are olderthan less massive late-type bulges. Since the overall galaxy luminosity (mass)also correlates with the bulge luminosity (mass), it appears that the galaxymass regulates not only what fraction of itself ends up in the bulge component,but also "when" bulge formation takes place. We show that dynamical friction ofmassive clumps in gas-rich disks is a plausible disk-driven mode for theformation of "old" late-type bulges. If disk evolutionary processes areresponsible for the formation of the entire family of late-type bulges, CDMsimulations need to produce a similar number of initially bulgeless disks inaddition to the disk galaxies that are observed to be bulgeless at z=0.