The WSRT wide-field HI survey: I. The background galaxy sample

Abstract

We have used the Westerbork array to carry out an unbiased wide-field survey for HI emission features, achieving an RMS sensitivity of about 18 mJy/Beam at a velocity resolution of 17 km/s over 1800 deg^2 and between -1000 < V_Hel<+6500 km/s. The primary data consists of auto-correlation spectra with an effective angular resolution of 49' FWHM. We detect 155 external galaxies in excess of 8 sigma in integrated HI flux density. Plausible optical associations are found within a 30' search radius for all but one of our HI detections in DSS images, although several are not previously cataloged or do not have published red-shift determinations. Twenty-three of our objects are detected in HI for the first time. We classify almost half of our detections as ``confused'', since one or more companions is cataloged within a radius of 30' and a velocity interval of 400 km/s. We identify a handful of instances of significant positional offsets exceeding 10 kpc of unconfused optical galaxies with the associated HI centroid, possibly indicative of severe tidal distortions or uncataloged gas-rich companions. A possible trend is found for an excess of detected HI flux in unconfused galaxies within our large survey beam relative to that detected previously in smaller telescope beams, both as function of increasing distance and increasing gas mass. This may be an indication for a diffuse gaseous component on 100 kpc scales in the environment of massive galaxies or a population of uncataloged low mass companions. We use our galaxy sample to estimate the HI mass function from our survey volume. Good agreement is found with the HIPASS BGC results, but only after explicit correction for galaxy density variations with distance.