Westerbork Ultra-Deep Survey of HI at z=0.2
  In this contribution, we present some preliminary observational results from
the completed ultra-deep survey of 21cm emission from neutral hydrogen at
redshifts z=0.164-0.224 with the Westerbork Synthesis Radio Telescope. In two
separate fields, a total of 160 individual galaxies has been detected in
neutral hydrogen, with HI masses varying from 1.1x10^9 to 4.0x10^10 Msun. The
largest galaxies are spatially resolved by the synthesized beam of 23x37
arcsec^2 while the velocity resolution of 19 km/s allowed the HI emission lines
to be well resolved. The large scale structure in the surveyed volume is traced
well in HI, apart from the highest density regions like the cores of galaxy
clusters. All significant HI detections have obvious or plausible optical
counterparts which are usually blue late-type galaxies that are UV-bright. One
of the observed fields contains a massive Butcher-Oemler cluster but none of
the associated blue galaxies has been detected in HI. The data suggest that the
lower-luminosity galaxies at z=0.2 are more gas-rich than galaxies of similar
luminosities at z=0, pending a careful analysis of the completeness near the
detection limit. Optical counterparts of the HI detected galaxies are mostly
located in the 'blue cloud' of the galaxy population although several galaxies
on the 'red sequence' are also detected in HI. These results hold great promise
for future deep 21cm surveys of neutral hydrogen with MeerKAT, APERTIF, ASKAP,
and ultimately the Square Kilometre Array.
