HISTORICAL AND SCIENTIFIC BACKGROUND
Recent statistics based on 734 disk galaxies selected from RC3 catalogue shows that 45.0% +- 4.5% of S0-Sd galaxies have box/peanut-shaped bulges (hereafter b/p) (Lutticke et al. 2000). Therefore, the research on these bulges is important to clarify the evolution of disk galaxies. However, the physical processes leading to b/p bulges are not clear yet.
Several theories of bulge formation are currently discussed: primordial scenarios (monolithic collapse: Eggen et al. 1962 (ELS); clumpy collapse: Kauffmann et al. 1993, Baugh et al. 1996; inside-out formation: van den Bosch 1998, Kepner 1999), secular evolution scenarios (merger: Wyse et al. 1997 and references therein, dynamical evolution due to gravitational instabilities, such as bars: Combes et al. 1990), and combinations of both (Combes 2000). The proposed dynamical processes of bulge formation should be tested statistically. Statistics of b/p bulges is of particular interest to demonstrate the importance of these structures for disk galaxies and to point out the relevant evolution scenarios of bulges in general. From the frequency of b/p bulges the likeliness of their formation process can be estimated. External cylindrically symmetric torques (May et al. 1985) or mergers of two disk galaxies (Binney & Petrou 1985; Rowley 1988) require very special conditions (Bureau 1998) to result in b/p bulges. Therefore, such evolutionary scenarios can explain only a very low frequency of b/p bulges. Accretion of satellite galaxies is a formation process (Binney & Petrou 1985; Whitmore & Bell 1988), which could produce a higher frequency of b/p bulges. However, an oblique impact angle of the satellite is needed for the formation of a b/p bulge and a massive accretion event would disrupt the stellar disk (Barnes 1992; Hernquist 1993).
According to the most popular scenario b/p bulges are explained by dynamical processes in bar potential (Combes & Sanders 1981, Combes et al.1990, Raha et al. 1991, Pfenniger & Friedli 1991, Lutticke et al. 2000). This scenario is supported mainly by N-body simulations (e.g. Combes & Sanders 1981). Additional support comes from spectroscopic observations (characteristic bar signatures in the velocity fields are found, Kuijken & Merrifield 1995) and from statistical studies (the overall frequency of barred galaxies, about 55%, is able to explain the high fraction of b/p bulges, Lutticke et al. 2000, Knapen et al. 2000). On the other hand, the connection between b/p bulges and the presence of a bar is difficult to be proved by surface photometry - b/p bulges are observable only in almost edge-on galaxies (inclination less then about 75 degree for an edge-on bar, Shaw et al. 1990, Combes et al. 1990, Lutticke et al. 2000). However, there is photometric evidence in a few edge-on galaxies from cuts parallel to the major axis (de Carvalho & da Costa 1987, Dettmar & Ferrara 1996) and in two intermediately inclined galaxies (NGC 4442, Bettoni & Galletta 1994 and NGC 7582, Quillen et al. 1997) pointing to bars. In particular, the observations of intermediately inclined galaxies is important in clarifying the nature of b/p bulges because in such cases both the bar and the b/p bulge can be observed simultaneously.
The estimated frequency of galaxies with a b/p bulge has steadily increased from 1.2% (Jarvis 1986), over 13% (de Souza & dos Anjos 1987) and 20% (Shaw 1987), up to 45% (Dettmar & Barteldrees 1988) and more. This rise results from differences in sample selection, sample size, detection method, and criteria to identify b/p bulges. The subsequent statistical studies have shown that b/p bulges are not really as peculiar as they were supposed to be in the past. Therefore very common processes are required to explain their high frequency. No complete statistics and lists of galaxies with b/p bulges, based on observations, have been published in the last ten years. Galactic evolution would greatly benefit by a reliable value for the frequency of b/p bulges, by the determination of the most likely scenario for the formation of b/p bulges and by the subsequent conclusions on the evolution of bulges in general. A complete list of b/p bulges is a base for a detailed analysis of these structures. The first search for b/p bulges was conducted by Jarvis (1986). The final sample (Luettike & Dettmar, 2000) contains 1343 galaxies from RC3.
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