4.7. "Bulges" Made of Population I Material
A suggestive clue to the origin of bulgelike disks is provided by their stellar content. Many of them contain or even are dominated by Population I material. For example, SB galaxies frequently contain nuclear hot spots of young stars and gas. Well known examples include NGC 1097 (Hummel, van der Hulst, and Keel 1987), NGC 4314 (Hubble Atlas), and NGC 4321 (Arsenault et al. 1988); general references are Sérsic and Pastoriza (1965), Alloin and Kunth (1979), and Buta and Crocker (1991). The same is true of "bulges" in some oval galaxies, including the prototype NGC 4736. Bulges are most likely to contain substantial Population I material at low luminosities and late Hubble types (Bica and Alloin 1987; Frogel 1992).
Even S0 bulges can contain molecular gas and star formation (e.g., at this meeting: NGC 4710, Wrobel and Kenney 1992, 1993; see many papers in Combes and Casoli 1991, but especially Sofue 1991). Also, a modest fraction contain older starbursts that give them A-type integrated spectra (e.g., Gallagher, Faber, and Balick 1975; Burstein 1979; Sparke, Kormendy, and Spinrad 1980; Sil'chenko 1993), although some of the young stars may result from accretion events. And central dust disks are very common in early-type galaxies, both S0s ("S03" objects in the Hubble Atlas) and ellipticals (see Kormendy and Djorgovski 1989 for a review).
Young stars are not always present: in early-type galaxies, r1/4-law disks are usually made of old stars. However, the above observations show that when gas is present, it knows how to find the center and it likes to make stars there.