Near-infrared and optical observations of galactic warps: A common,
  unexplained feature of most discs
  Context: Warps occurring in galactic discs have been studied extensively in
HI and in the optical, but rarely in the near-infrared (NIR) bands that trace
the older stellar populations.
  Aims: We provide NIR data of nearby edge-on galaxies, combined with optical
observations, for direct comparison of the properties of galactic warps as a
function of wavelength, and calculate warp curves for each galaxy and obtain
the characteristic warp parameters. We discuss these properties as possible
constraints to the different mechanisms that have been proposed for the
development and persistence of galactic warps.
  Methods: We observed 20 galaxies that were selected from a statistically
complete diameter-limited subsample of edge-on disc galaxies. We used the Cerro
Tololo Infrared Imager (CIRIM) at the CTIO 1.5m Ritchey-Chretien telescope to
acquire the NIR data. We used the 1.54m Danish and 0.92m Dutch telescopes at
the European Southern Observatory's La Silla site for our optical observations.
  Results: Our results show that 13 of our 20 sample galaxies are warped, with
the warp more pronounced in the optical than at NIR wavelengths. In the
remaining seven galaxies, no warp is apparent within the limitations of our
automated detection method. The transition between the unperturbed inner disc
and the outer, warped region is rather abrupt. S0 galaxies exhibit very small
or no warps. The magnetic model remains one of a number of interesting
formation scenarios.
