Old and Young Bulges in Late-Type Disk Galaxies

We use HST ACS and NICMOS imaging to study the inner B - I and I - H colors of nine late-type spiral galaxies, which we interpret on the basis of different star formation histories. The colors and scale lengths of the bulges of these late-type systems are correlated with those of the disks in which they are embedded. We find that in about half of the systems, the bulk of the bulge mass formed more recently than the disk. In the remainder, early bulge formation was supplemented by continuing "rejuvenating" star formation. More massive bulges are generally older. These results extend previous findings down to the smallest bulge mass scales, bordering on the masses of nuclear star clusters. The variety and extended star formation histories of late-type bulges could be naturally explained by the contribution of several processes at different epochs and operating on different timescales. On the other hand, the scaling relations between bulge stellar age and bulge/galaxy mass and between bulge and disk scale lengths hint at similar processes for all components and suggest that late-type bulges of all (masses and) stellar ages result from the internal evolution of the parent disks. We show that dynamical friction of massive clumps in gas-rich disks is also a plausible mode for the formation of late-type bulges, especially for those that are older than their surrounding disks. If disk evolutionary processes are indeed responsible for the formation of the entire family of late-type bulges, CDM simulations need to produce a similar number of initially bulgeless disks in addition to the disk galaxies that are observed to be bulgeless at z = 0.