Westerbork Northern Sky Survey

INTRODUCTION

The Westerbork Northern Sky Survey (WENSS) is a radio survey made with the WSRT at wavelengths 92 and 49 cm. At 92 cm the entire sky above declination 30 degrees is covered. The other wavelength regime does not cover the whole sky, but only 30-50 percent, due to the amount of telescope time that is needed to do this. Using a synthesis array like the WSRT means that we get a better resolution than single dish observations, but also has the disadvantage of a smaller field of view, unless we find a way of dealing with wide field imaging. The solution for this problem is mosaicing. Instead of making 1 observation of a large field we make a lot of snapshots of different fields and bind them together with the help of special build software. A typical mosaic contains 80 pointing centres. Each pointing centre is sampled during 20 seconds and then the telescope moves to another centre. This takes about 10 seconds. After 40 minutes all fields are scanned and the procedure starts again. This means that in a 12 hour observation each field is scanned 18 times. To move grating rings of the map 6 different baseline settings are used, so 1 mosaic data block involves 72 hours of observational data. Each pointing centre is located at half half-power maximum beamwidth in order to get a smooth sampling grid of the data. The noise background is uniform up to 5 percent. At 92 cm (declination 30 degrees) a mosaic block is about 10 by 14 degrees large.

THE CATALOGUE

The final product that the WENSS survey produces contain 6 by 6 degrees frames taken from the mosaic blocks. They are centred on the new POSS plate positions (5 degrees grid). The limiting flux density will be about 15 mJy (5 sigma) at both wavelengths. As a result the final catalogue will consist of 300,000 sources at 92 cm and 60,000 at 49 cm. The positional accuracy will be superior to all other all sky surveys (5" for the faint sources to better than 1" for the stronger sources). A large number of sources will have sufficient acurate positions to allow optical identification using a digitized version of the Palomar Sky Survey made with the APM at Cambridge. Final maps will be made at different resolutions in order to make accurate spectral index measurements. At each wavelength we will make high, medium and low resolution maps (1', 2.5' and 4' resolution at 92 cm and 0.5', 1' and 2.5' resolution at 49 cm). Maps will be made with all Stokes parameters (I, Q, U and V). This means that source information will be available both on spectral type and polarization characteristics. This is important for using the catalogue as a database for finding sources based on well known selection criteria, such as steep spectra for high redshift radio galaxies.

SCIENTIFIC GOALS

Just making another fancy catalogue isn't the purpose of this project. Using the WSRT for such a long period of time (about four months per year for the next few years) must be justified by its scientific outcome. Therefore we must know beforehand what type of sources we will see and how we are going to select sources based on selection criteria. In the last decade several of these criteria have been found for finding separate classes of objects.

High redshift radio galaxies:
A very powerful technique for finding high redshift radio galaxies has been developed based on the ultra steep spectrum characteristics of 4C sources. From 33 4C candidates 10 sources had a redshift z > 2. Because the 4C sources are strong sources, only the most luminous high redshift radio galaxies are found. The WENSS data combined with the Cambridge 151 MHz survey will provide many thousands of candidates with intermediate flux densities. Then a more statistically solid sample will be available that contains information about the early universe. Information such as galaxy formation, galaxy distribution, large scale structure and much more cosmological information will be found in the catalogue.
Peaked spectrum sources:
Normal (if one can say that) galaxies usually have a spectrum in the radio domain that follows a powerlaw over a large wavelength regime. Some sources behave differently and are peaked either at 200 MHz (CSS peakers) or 2 GHz (GPS peakers). These rare sources are good candidates for high redshift quasars (50 percent of the strong GPS sources are identified with high $z$ quasars). These sources are not understood very well. Because they are so rare not many sources have been studied in detail and WENSS will provide a large database of these sources in order to study these objects.
Other objects:
Many other objects can be studied with the WENSS catalogue. With the WSRT high quality wide field polarization maps can be made. Even very small reliable polarization structures can be detected and used for instance to study the Laing-Garrington effect (depolarization of radio lobes possibly due to orientation effects) or to map the small scale polarization structure in our own galaxy (as found by M. Wieringa, 1991). Even with the relatively small number of UV points we already found these structures in preliminary maps on scales of several degrees. The polarized emission is interpreted as a result of Faraday rotation of the highly polarized synchrotron radiation background in the diffuse ionized gas in the galactic disk. So gas properties in our galaxy can also be studied in this way.

THE WENSS PROJECT

The WENSS project is devided over two institutes. The data reduction is done in Dwingeloo. We have a dedicated HP 730 workstation with 4 GBytes of diskspace to make maps out of the uv-data. The data are reduced with NEWSTAR (the new version of the R-series reduction package that runs under the DWARF parameter interface). New software has been written by Wim Brouw in order to cope with the mosaic data and its special characteristics. To reduce one mosaic (10 by 14 degrees with 90 cm data at declination 37 degrees) on the HP with all the map making takes about 10 days. These maps are transported to Leiden where the actual catalogue will be built. Using a highly automated source finding package all source parameters like position, flux, size, etc... are determined.

THE WENSS TEAM

The project team consists of the following people:

Project scientists:  A.G. de Bruyn, G.K. Miley 
Project manager:  E. Raimond 
Research assistants:  M.A.R. Bremer (Leiden), Y. Tang (Dwingeloo)
Software design/coding (NEWSTAR): W.N. Brouw 
Software design/coding (Catalogue building / user interfaces): M.A.R. Bremer
WSRT on-line assistance: R. Braun, H. van Someren-Greve 
Dwingeloo off-line processing: G. van Diepen, J.E. Noordam

Several AIO's / OIO's (Ph.D. students) will work with the catalogue, each with their own sources of interest (e.g. milli-second pulsars, high redshift galaxies, GHZ peakers).