Near-infrared and optical observations of galactic warps: a common, unexplained feature of most discs

¹ Dpto. Física Teórica y del Cosmos, Universidad de Granada, Granada, Spain e-mail: aguijarr@ugr.es
² Centro Astronómico Hispano Alemán, Almería, Spain
³ Kapteyn Astronomical Institute, Groningen, The Netherlands
⁴ Kavli Institute for Astronomy and Astrophysics and Department of Astronomy, Peking University, Beijing, PR China
⁵ Department of Physics & Astronomy, The University of Sheffield, UK

Received: 26 March 2010
Accepted: 10 June 2010

Abstract

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 1.5 m Ritchey-Chretien telescope to acquire the NIR data. We used the 1.54 m Danish and 0.92 m 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.