Annu. Rev. Astron. Astrophys. 1997. 35: 637-675 Copyright © 1997 by Annual Reviews. All rights reserved

3.3. Bulges in Formation at z < 0.1?

A few local exceptional systems are candidates for young bulges. Gravitational torques during interactions can act to drive gas to the central regions (e.g. Mihos & Hernquist 1994), where it may form stars, and which may, depending on the duration of star formation and of the interaction, be heated into a bulge. Schweizer (1990) discusses local disk galaxies with blue bulges, presenting them as evidence for recent bulge-building in this manner. These galaxies include (the dwarf) NGC 5102, an S0 galaxy with a bluer bulge than disk and strong Balmer absorption lines in its central regions. Classic merger remnants such as NGC 7252 are forming disks in their central parts, which may imply that these galaxies perhaps have evolved into S0s, or early type spirals (e.g. Whitmore et al 1993).

A more dramatic example of gas-rich mergers is Arp 230, which shows classical shells in the bulge component and a young disk rich in gas, as displayed in Figure 6 (D Schiminovich & J van Gorkom, private communication and in preparation).

Figure 6. An optical image of Arp 230, with overlaid HI contours. This galaxy shows evidence for shells in its outer bulge, which indicates a recent substantial accretion event, and also has a young gas-rich disk (D Schiminovich & J van Gorkom, private communication).