The WSRT Virgo HI filament survey I; Total Power Data
  Observations of neutral hydrogen can provide a wealth of information about
the kinematics of galaxies. To learn more about the large scale structures and
accretion processes, the extended environment of galaxies have to be observed.
Numerical simulations predict a cosmic web of extended structures and gaseous
filaments. To observe the direct vicinity of galaxies, column densities have to
be achieved that probe the regime of Lyman limit systems. Typically HI
observations are limited to a brightness sensitivity of NHI ~ 10^19 cm-2 but
this has to be improved by ~2 orders of magnitude. With the Westerbork
Synthesis Radio Telescope (WSRT) we map the galaxy filament connecting the
Virgo Cluster with the Local Group. About 1500 square degrees on the sky is
surveyed, with Nyquist sampled pointings. By using the WSRT antennas as single
dish telescopes instead of the more conventional interferometer we are very
sensitive to extended emission. The survey consists of a total of 22,000
pointings and each pointing has been observed for 2 minutes with 14 antennas.
We reach a flux sensitivity of 16 mJy beam-1 over 16 km s-1, corresponding to a
brightness sensitivity of NHI ~ 3.5 \times 10^16 cm-2 for sources that fill the
beam. At a typical distance of 10 Mpc probed by this survey, the beam extent
corresponds to about 145 kpc in linear scale. Although the processed data cubes
are affected by confusion due to the very large beam size, we can identify most
of the galaxies that have been observed in HIPASS. Furthermore we made 20 new
candidate detections of neutral hydrogen. Several of the candidate detections
can be linked to an optical counterpart. The majority of the features however
do not show any signs of stellar emission. Their origin is investigated further
with accompanying HI surveys which will be published in follow up papers.
