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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).