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 1.1×10⁹ to 4.0×10¹⁰ M_☉. The largest galaxies are spatially resolved by the synthesized beam of 23×37 arcsec² 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.