Introduction

Spirals constitute a large fraction of the giant galaxies and of the visible mass of the universe. Furthermore, a significant fraction of edge-on spiral galaxies, and therefore presumably of all spirals, show boxy or peanut-shaped isophotes in the bulge region. Because of the subjectivity involved in identifying boxy/peanut bulges, the fraction of bulges found to be boxy/peanut-shaped varies appreciably from study to study, but a typical number is 20-30% (Jarvis 1986: Shaw 1987; de Souza & dos Anjos 1987). Consequently, by their sheer number, boxy/peanut-bulge spiral galaxies constitute a significant class of object.

More importantly, boxy/peanut spirals display interesting characteristics. Firstly, the structure of boxy/peanut bulges represents a major departure from the usual density distribution of the more spheroidal bulges. Furthermore, it should be remembered that for a bulge to produce boxy/peanut isophotes when seen edge-on in projection, its 3D volume density distribution must be even more distorted. Secondly, what makes boxy/peanut bulges really interesting from a dynamical point of view is that many of them are cylindrical rotators; that is, their rotation velocity is independent of the distance from the plane of the galaxy (see Kormendy & Illingworth 1982; Rowley 1986). In the more spheroidal systems, the rotation velocity decreases rapidly with height. Other features, although less common, are also displayed by boxy/peanut-bulge galaxies. Some of them show isophotal twist, usually taken as a signature of triaxiality, and some even show directly the presence of a bar when the boxy/peanut shape is seen at moderate inclination (e.g. for NGC 4442, see Bettoni & Galletta 1994). It is of course not possible to identify a bar with certainty from the morphology alone in a galaxy seen very close to edge-on. The range of structural and dynamical features displayed by boxy/peanut spiral galaxies supports the view that they are an important and interesting class of objects. Understanding their present structure and dynamics is therefore of fundamental importance, as these are the primary clues to their formation mechanism(s).

It should be noted that the bulge of the Galaxy also displays boxy isophotes, even in the extinction-corrected near-infrared images from the Diffuse Infrared Background Experiment (DIRBE) (Weiland et al. 1994). In addition, there is now evidence for the presence of a bar in the Galaxy: asymmetries in the bulge light distribution (e.g. Blitz & Spergel 1991; Weiland et al. 1994), non-circular gas motions near the Galactic center (e.g. Binney et al. 1991), asymmetries in star counts (e.g. Nakada et al. 1991) and kinematics in the bulge (e.g. Beaulieu 1996), high gravitational microlensing optical depth toward the Galactic bulge (e.g. Paczynski et al. 1994), etc. This represents an additional motivation to study the structure and dynamics of boxy/peanut-bulge spiral galaxies as we might learn greatly about our own Milky Way by studying them.

In § 2, we introduce the accretion and bar-buckling scenarios for the formation of boxy/peanut bulges. In § 3, our observational programs to study those galaxies and their goals are described. These include optical long-slit spectroscopy, HI line-imaging, near-infrared imaging, and optical imaging. New spectroscopic and HI observations of a few of our sample galaxies are presented in § 4, and their implications are discussed in § 5. We conclude briefly in § 6.

© Copyright Astronomical Society of Australia 1997