BOX / PEANUT structures in galaxies

Recent statistics based on the 734 disk galaxies from RC3 (de Vaucouleurs et al., 1991), shows the large fraction (~45 %) of the disk galaxies having box or peanut-shaped bulges and irregularities or/and asymmetries in the bulges (Luettike & Dettmar, 1998a, Luetticke et al., 2000). As dividing disk galaxies to “disk” and “nucleus”, these objects are quite important to study evolution of disk galaxies. Phisical processes in galaxies with unusuall nuclei – e.g. Box/Peanut bulges are not clear yet. According to Binney and Petrou, 1985 and Whitemore and Bell, 1988, these structures could be possible mergers remnant or merging satellites. Alternatively, disk instabillities after some resonances in the nuclei could have as a result Box/Peanut bulge (Kormedy, 1993). As a result of N-body simulations of stars movements in the nuclear potential, simulated nucleus resembled observational ones. Freeman, 1996 shows B/P nucleus can originate in quite stable disk with common instability or interactions of near galaxies. Such a nucleus can arise after accreting of nearby galaxy, so the two hypothesis for Box/Peanut forming could be combined (Mihos et al.,1995).

We accept a quite large mass concentration in the nucleus have a result its dissotiation – i.e. the evolution of galaxies is from SA to SB and back to SA. The nucleus is growing up in such a way the Box/Peanut structures arise or after accreting material or mergers. If this is realised, the evolution along the Hubble sequence could be only from Sd to S0/Sa (Pfenniger, 1993).

So to check this Box/Peanut bulges structure have to be studied in details as being a critical phase in galaxy evolution. A large number of Box/Peanut galaxies amongst disk edge-on galaxies were selected from R,G,I CCD_frames (Barteldrees & Dettmar, 1994). Practically no galaxy without satellite has Box/Peanut bulge or no galaxy with satellite demonstrate “normal” elliptical nucleus

Explanation of this forms probably lies in connection of stellar disk with the bar, soft-merging of companions to formso called tick boxy bulges or some mixed scenarios (Bureau & Freeman, 1997). Detailed surface photometry often showsevidences for additional inner component - thinner and

usually with the same scale-length (Dettmar & Barteldress,1990). Studies of the possible relations between the internalstructures and the environment of galaxies with B/P bulgesshow no connection on large scales, but the small scaleenvironment is important for the existence of B/P bulges (Luettike & Dettmar, 1998b). These structures usually are well seen at large inclinations of the galaxies in question - as close as possible to edge-on position.

Following Luetticke et al.,(2000) the basic characteristics of Box/Peanut galaxies could be marked as:

· B/P structures are seen in all morphological tipes

· Evidences for blue color distortion of B/P structures as compared with the color of the bulges

· No correlation between B/P structures and large scale environment (cluster members)

· Correlation between small scale environment (satellite galaxies) and existence of B/P structures

· Two type of B/P structures – “normal” and “thick boxy bulges”, the latter with different characterictics

· The B/P bulges are not strongly influenced by dust (from NIR frames)

· Strong correlation between bars and B/P bulges – i.e. bars viewed under different angles in edge-on galaxies are the main explanation of different shapes of B/P bulges

G.T.Petrov, 2004