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R.-J. Dettmar, R. Lütticke, M. Pohlen (Astronomisches Institut der Ruhr-Universität Bochum, Germany)
The large fraction of 45% box- and peanut-shaped (b/p) bulges in edge-on galaxies (S0-Sd) derived from a visuell inspection of a complete sample of ~1224 RC3 galaxies (Lütticke et al. 2000, A&AS 145, 405) must be considered an important constraint for models explaining the formation process of such ``pseudo"-bulges. The most promising scenarios describing the origin of b/p bulges are the accretion of satellite galaxies and bar resonances coupled with bending instabilities. The importance of bars can directly be demonstrated by NIR imaging. In a sample of 60 edge-on galaxies we have analysed the galactic structure almost free from dust absorption along the line-of-sight. This sample allows us to identify an additional thin, central component in almost all target galaxies with b/p bulges. This inner component is easily seen in cuts parallel to the major axis and can be identified with a bar.
The length of this structure scaled by the length of the bulge correlates with the morphologically classified shape of the bulge, i.e. as box or peanut (cf. Lütticke et al 2000). This correlation can therefore be interpreted as the projection of a bar at different aspect angles.
We conclude that galaxies with peanut bulges have a bar seen nearly edge-on. The ratio of the bar length to its thickness can be directly measured for the first time and is found to be 14±4.
However, a small group of ~5% of all b/p bulges shows very peculiar boxy structures with remarkable morphological features such as large asymmetries, substructures, and a box-shape which is more prominent in the outer part of these bulges. Therefore we have introduced the term ``thick boxy bulges'' (TBB) for this kind of bulges. The extent of these box shaped structures is too large to result from a bar resonance (Dettmar & Lütticke 1999, ASP Conf. Ser. Vol. 165, p. 95). Irregularities and even possible merger remnants in the TBBs show that these bulges are most likely not yet dynamically settled. Therefore we consider soft merging to be the most likely formation process for these bulges.