7.7. "Boxy" or "peanut" bulges and "x" galaxies

Boxy or peanut bulge galaxies have been an active subject of study the past 10 years. Major lists of the best cases have been compiled by Jarvis (1986) and Shaw (1987). The phenomenon has been variously explained in terms of cylindrical rotation (see Rowley, 1988, and references therein), edge-on or other preferred views of bars (Combes and Sanders, 1981, Combes et al., 1990), or merger effects (Whitmore and Bell, 1988). The boxy or peanut character is usually very subtle in photographs, although it can be extremely obvious in some cases, as in, for example, NGC 128 (Sandage, 1961; Jarvis, 1989) and IC 4767 (Whitmore and Bell, 1988). With suitable image processing, the latter object also shows an X-shaped structure crossing its inner regions, which has been interpreted by Whitmore and Bell as evidence for a recent merger. The highly inclined, but non-edge-on, S0 galaxy NGC 7020 shows a similar but possibly unrelated feature (Buta, 1990c).

7.8. Ring, polar ring, and "hoag-type" ring galaxies

These rare phenomena have been the subject of a fair amount of recent research. Ring and polar ring galaxies are believed to be produced by collisions or mergers. Recent studies of morphology and statistics of ring galaxies have been made by Few and Madore (1986), Appleton and Struck-Marcell (1987 and references therein), and Arp and Madore (1987). A catalogue of polar rings has been compiled by Whitmore et al. (1990), who also give many of the previous relevant references.

The prototype of "Hoag-type" ring galaxies is Hoag's Object (A1515 + 2146), which has been studied in great detail recently. This object is remarkable for having a clear ring surrounding a distinct spheroidal galaxy, with no trace whatsoever of a bar. It has been interpreted as a ringed galaxy whose bar dissolved after the ring formed (Brosch, 1985), and as a case where an E galaxy accreted a small companion into something like a polar ring (Schweizer et al., 1987). Wakamatsu (1990) studied another example, NGC 6028, but demonstrated the presence of a small bar in the central region and suggested that at least in this case, bar-driven gas dynamics could explain the presence of the ring. A possible related object is NGC 7187 (Buta, 1990b).