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Astron. Astrophys. 337, 9-16 (1998)

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1. Introduction

Most spiral galaxies that can be traced in neutral hydrogen exhibit a warped plane (e.g. Sancisi 1976; Bosma 1981; Briggs 1990, Bottema 1995). Our own Galaxy is strongly warped from the solar radius (e.g. Burton 1992, Diplas & Savage 1991).

While the wide majority of warps have been detected in HI, outside the optical disks, some optical warps exist (M31, Innanen et al 1982; van der Kruit 1979, van der Kruit & Searle 1981, 82). In our own Galaxy, the stellar disk appears to follow the HI warp (Porcel & Battaner 1995, Reed 1996). The best evidence for warped optical disks occurs in tidally interacting galaxies (for instance UGC 3697, Burbidge et al 1967, or NGC 4656, Weliachew et al 1978). Sanchez-Saavedra et al (1990) have surveyed 86 edge-on galaxies in the northern sky, and claim that about half of the galaxies display some sort of warping of the plane. This high percentage would suggest that most galaxies are warped, since projection effects must mask at least some fraction of the warps, those with line of nodes perpendicular to the line of sight (Sanchez-Saavedra et al estimate that the observed fraction of warps should be multiplied by 1.7 for this). Sanchez-Saavedra et al results were confirmed by Reshetnikov (1995) from the study of a complete sample of 120 northern edge-on spiral galaxies.

Extended warps represent a dynamical puzzle, since they should be rapidly washed out by differential precession, and end up in corrugated disks. Many models have been proposed for their persistence, from an intergalactic magnetic field (Battaner et al 1990), discrete bending modes (Sparke 1984, Sparke & Casertano 1988), misaligned dark halos (Dubinski & Kuijken 1995), or cosmic infall and outer gas accretion (e.g. Binney 1992). Another possibility is that warps are self-gravitating, which reduces differential precession (Pfenniger et al 1994). In any case, the bending of the plane above the equator defined by the inner galaxy allows to explore the 3D-shape of dark matter, and to probe its potential. It is therefore of prime importance to tackle the warp formation mechanisms, and to enlarge the statistical data, especially on optical warps (Sects. 2 and 3). Optical warps are weaker than HI warps and can be easily contaminated by dust or projected spiral arms, when the galaxy is not exactly edge-on; we thus undertake simulations of dusty spiral galaxies, with or without warps, to estimate quantitatively the biases (Sect. 4).

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998