4.3. NGC 7475

As a final example of a "bulge" that is really a disk, consider NGC 7457. This is a normal, unbarred S0 (Hubble Atlas) dominated by an exponential disk (Kormendy 1977). The "bulge" is faint, fractionally and in absolute luminosity (M_B - 18.5). Hubble Space Telescope observations by Lauer et al. (1991) show that it has a steep brightness profile, a very high central surface brightness (µ_0V 12.4 V mag arcsec^-2), and an unresolved core. The limits on the core parameters are extreme, but they are in the range expected for such a low luminosity (Fig. 7). This "bulge" is enormously different from a normal disk; these typically have µ_0V 21 V mag arcsec^-2 (Freeman 1970). The rotation curve has not been measured well enough to allow us to plot the galaxy in the V / - diagram. But = 65 km s^-1, making this the coldest "bulge" in Fig. 6.

Figure 7. Four of six fundamental plane correlations between core radius rc, central surface brightness µ0V, central velocity dispersion , and MB. Approximate seeing corrections are from Kormendy (1987). NGC 7457 (Lauer et al. 1991) is shown by plus signs.