Kinematics and physical conditions of HI in nearby radio sources. The
  last survey of the old Westerbork Synthesis Radio Telescope
  We present an analysis of the properties of neutral hydrogen (HI) in 248
nearby (0.02<z<0.25) radio galaxies with $S_{\rm 1.4\, GHz}>30$ mJy and for
which optical spectroscopy is available. The observations were carried out with
the Westerbork Synthesis Radio Telescope as the last large project before the
upgrade of the telescope with phased array feed receivers (Apertif). The sample
covers almost four orders of magnitude in radio power from $\log\,P_{\rm 1.4
\,GHz}=22.5$ W Hz$^{-1}$ and $26.2$ W Hz$^{-1}$. We detect HI in absorption in
$27\% \pm 5.5\%$ of the objects. The distribution and kinematics of the
absorbing HI gas appear to depend on radio power, the properties of the radio
continuum emission, and the dust content of the sources. Among the sources
where HI is detected, gas with kinematics deviating from regular rotation is
more likely found as the radio power increases. In these cases, the HI profile
is often asymmetric with a significant blue-shifted component. This is
particularly common for sources with $\log\,P_{\rm 1.4 \, GHz}>24$ W Hz$^{-1}$,
where the radio emission is small, possibly because these radio sources are
young. The same is found for sources that are bright in the mid-infrared, i.e.
sources rich in heated dust. In these sources, the HI is outflowing likely
under the effect of the interaction with the radio emission. Conversely, in
dust-poor galaxies, and in sources with extended radio emission, at all radio
powers we only detect HI distributed in a rotating disk. Stacking experiments
show that in sources for which we do not detect HI in absorption directly, the
HI has a column density that is lower than $3.5\times 10^{17} (T_{ \rm
spin}/c_f)$ cm$^{-2}$. We use our results to predict the number and type of HI
absorption lines that will be detected by the upcoming surveys of the Square
Kilometre Array precursors and pathfinders (Apertif, MeerKAT, and ASKAP).
