|Annu. Rev. Astron. Astrophys. 2004. 42:
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8.3. Disky Distortions in Elliptical Galaxies
Some ellipticals contain central disky distortions (Bender, Döbereiner, & Möllenhoff 1988; see Bender et al. 1989; Kormendy & Djorgovski 1989; Bender 1990b; Kormendy & Bender 1996; Rest et al. 2001 for reviews). In the more extreme cases, the line-of-sight velocity distributions show asymmetries or even a two-component structure indicative of a cold, rotating nuclear disk embedded in a more slowly rotating elliptical host (e.g., Franx & Illingworth 1988; Bender 1990a; Bender, Saglia, & Gerhard 1994; Scorza & Bender 1995). Because these disks are not self-gravitating, the processes discussed in this paper cannot operate. Therefore there must be some embedded nuclear disks in the earliest-type galaxies that are not related to the themes of this paper.
How are they produced? Minor accretion events in which an elliptical swallows a gas-rich dwarf almost certainly produce some of them, along with the central dust disks commonly observed in ellipticals (see Kormendy & Djorgovski 1989 for a review and, Jaffe et al. 1994; van Dokkum & Franx 1995; Martini et al. 2003a for HST images). There is evidence that dust disks gradually turn into small stellar disks (Kormendy et al. 1994, 2004). Alternatively, gas shed by dying stars in the elliptical may, in some cases, cool and fall to the center. These processes are quite different from secular evolution driven by nonaxisymmetries in dominant, self-gravitating disks.
We do not know whether the tiny nuclear disks seen, for example, in NGC 3115 (Kormendy & Richstone 1992; Lauer et al. 1995; Scorza & Bender 1995; Kormendy et al. 1996b; Emsellem, Dejonghe, & Bacon 1999) and NGC 4594 (Burkhead 1986, 1991; Kormendy 1988b; Wagner, Dettmar, & Bender 1989; Crane et al. 1993; Emsellem et al. 1996; Seifert & Scorza 1996; Kormendy et al. 1996a) are more nearly related to pseudobulges or to the disky distortions discussed above. The dividing line between the above processes and those that make pseudobulges deserves further investigation. However, we emphasize that this uncertainty affects only a minority of disky components embedded in the largest, earliest-type classical bulges. It is not a problem for the identification of most pseudobulges in Sb and later-type galaxies.