4.1. Embedded Disks: Spiral Structure, Star Formation
Renzini (1999) states the definition of a bulge: "It appears legitimate to look at bulges as ellipticals that happen to have a prominent disk around them [and] ellipticals as bulges that for some reason have missed the opportunity to acquire or maintain a prominent disk." Our paradigm of galaxy formation is that bulges and ellipticals both formed via galaxy mergers (e. g., Toomre 1977a; Steinmetz & Navarro 2002, 2003), a picture that is well supported by observations (see Schweizer 1990 for a review). But as observations improve, we discover more and more features that make it difficult to interpret every example of what we used to call a "bulge" as an elliptical living in the middle of a disk. Carollo and collaborators find many such galaxies in their HST snapshot survey of 75, S0 - Sc galaxies observed with WFPC2 in V band (Carollo et al. 1997, 1998; Carollo & Stiavelli 1998; Carollo 1999) and a complementary survey of 78 galaxies observed with NICMOS in H band (Carollo et al. 2001, 2002; Seigar et al. 2002). Figure 5 shows examples. These are Sa - Sbc galaxies, so they should contain bulges. Instead, their centers look like star-forming spiral galaxies. It is difficult to believe that, based on such images, anyone would define bulges as ellipticals living in the middle of a disk. Spiral structure happens only in a disk. Therefore these are examples of pseudobulges.
Figure 5. Sa - Sbc galaxies whose "bulges" have disk-like morphology. Each panel shows an 18" × 18" region centered on the galaxy nucleus and extracted from HST WFPC2 F606W images taken and kindly provided by Carollo et al. (1998). North is up and east is at left. Displayed intensity is proportional to the logarithm of the galaxy surface brightness.