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

We present a systematic observational study of the relationship between barsand boxy/peanut-shaped (B/PS) bulges. We first review and discuss proposedmechanisms for their formation, focussing on accretion and bar-bucklingscenarios. Using new methods relying on the kinematics of edge-on disks, wethen look for bars in a large sample of edge-on spiral galaxies with a B/PSbulge and in a smaller control sample of edge-on spirals with more spheroidalbulges. We present position-velocity diagrams of the ionised gas obtained fromoptical long-slit spectroscopy. We show that almost all B/PS bulges are due toa thick bar viewed edge-on, while only a few extreme cases may be due to theaccretion of external material. This strongly supports the bar-bucklingmechanism for the formation of B/PS bulges. None of the galaxies in the controlsample shows evidence for a bar, which suggests conversely that bars aregenerally B/PS. We consider the effects of dust in the disk of the galaxies, but concludethat it does not significantly affect our results. Unusual emission line ratioscorrelating with kinematical structures are observed in many objects, and weargue that this is consistent with the presence of strong bars in the disk ofthe galaxies. As expected from N-body simulations, the boxy--peanut transitionappears to be related to the viewing angle, but more work is required to derivethe precise orientation of the bars in the bulges. The reliable identificationof bars in edge-on spiral galaxies opens up for the first time the possibilityof studying observationally their vertical structure.