The Nature of Boxy/Peanut-Shaped Bulges in Spiral Galaxies

Abstract

We present a systematic observational study of the relationship between bars and boxy/peanut-shaped (B/PS) bulges. We first review and discuss proposed mechanisms for their formation, focussing on accretion and bar-buckling scenarios. Using new methods relying on the kinematics of edge-on disks, we then look for bars in a large sample of edge-on spiral galaxies with a B/PS bulge and in a smaller control sample of edge-on spirals with more spheroidal bulges. We present position-velocity diagrams of the ionised gas obtained from optical long-slit spectroscopy. We show that almost all B/PS bulges are due to a thick bar viewed edge-on, while only a few extreme cases may be due to the accretion of external material. This strongly supports the bar-buckling mechanism for the formation of B/PS bulges. None of the galaxies in the control sample shows evidence for a bar, which suggests conversely that bars are generally B/PS. We consider the effects of dust in the disk of the galaxies, but conclude that it does not significantly affect our results. Unusual emission line ratios correlating with kinematical structures are observed in many objects, and we argue that this is consistent with the presence of strong bars in the disk of the galaxies. As expected from N-body simulations, the boxy--peanut transition appears to be related to the viewing angle, but more work is required to derive the precise orientation of the bars in the bulges. The reliable identification of bars in edge-on spiral galaxies opens up for the first time the possibility of studying observationally their vertical structure.