Lopsidedness in WHISP galaxies: I. Rotation curves and kinematic
  lopsidedness
  The frequently observed lopsidedness of the distribution of stars and gas in
disc galaxies is still considered as a major problem in galaxy dynamics. It is
even discussed as an imprint of the formation history of discs and the
evolution of baryons in dark matter haloes. Here, we analyse a selected sample
of 70 galaxies from the Westerbork HI Survey of Spiral and Irregular Galaxies.
The HI data allow us to follow the morphology and the kinematics out to very
large radii. In the present paper, we present the rotation curves and study the
kinematic asymmetry. We extract the rotation curves of receding and approaching
sides separately and show that the kinematic behaviour of disc galaxies can be
classified by five different types: symmetric velocity fields where the
rotation curves of receding and approaching sides are almost identical; global
distortions where the rotation velocities of receding and approaching side have
an offset which is constant with radius; local distortions which lead to large
deviations in the inner and negligible deviations in the outer parts (and vice
versa); and distortions which split the galaxies into two kinematic systems,
visible in the different behaviour of the rotation curves of receding and
approaching sides, which leads to a crossing and a change in side. The
kinematic lopsidedness is measured from the maximum rotation velocities,
averaged over the plateau of the rotation curves. This gives a good estimate of
global lopsidedness in the outer parts of the sample galaxies. We find that the
mean value of the perturbation parameter denoting the lopsided potential as
obtained from the kinematic data is 0.056. 36% of all sample galaxies are
globally lopsided, which can be interpreted as the disc responding to a halo
that was distorted by a tidal encounter. In Paper II, we study the
morphological lopsidedness for the same sample of galaxies.
