THE WENSS SURVEY

The night-time CCI International Time Programme (ITP) observations for the period February 1995 to January 1996 were carried out by a consortium of astronomers following up various aspects of the Westerbork Northern Sky Survey (WENSS). This is a radio survey of the northern sky at the relatively low frequency of 327 MHz. Much of the work in the spring/summer concentrated on the mini-survey region, a 500 square degree area centred on the north ecliptic pole. The radio sources in the survey were split into several subgroups and a high success rate was achieved in following up each one: nearby galaxies, flat (quasars), peaked and ultra-steep (high-redshift galaxies) spectrum radio sources, and gravitational lenses.

The observations were carried out with CCI telescopes, among them, WHT, INT and JKT, both imaging and spectroscopy. These observations have improved the understanding of low-flux radio sources at both low and high redshift. The work at low redshift has allowed the construction of luminosity functions in the optical and in the radio, for nearby weak radio sources. It is clear from the work on flat-spectrum and ultra-steep spectrum radio sources that the WENSS survey allow the study and selection of objects to consistently higher redshifts than have generally been possible with higher flux radio surveys, and is therefore extremely well suited to the study of the high-redshift universe.

During the survey, a good candidate for a giant radio galaxy was found: Mrk 1498 (B1626+5153). These kinds of extragalactic radio sources with dimensions greater than 1.5 Mpc are rare in the cosmos, but provide in principle a good laboratory for studying both the physics of the radio galaxy phenomenon and the nature of the intergalactic medium. It is uncertain whether these sources attain such large sizes because the ratio of jet power to the density of the surrounding medium is unusually large, or because the sources are simply much older than the average radio source of the type and so have had time to expand to unusually large dimensions.

Mrk 1498 is a classical double source which has a maximum dimension of at least 1.6 Mpc, a flux density at 325 MHz of 1.9 Jy and spectral index of –0.66. Optical spectra with the WHT show a narrow line emission spectrum typical of many radio galaxies and yield a redshift of z=0.056. The H-alpha line clearly has a broad line component, making Mrk 1498 the third known giant radio galaxy exhibiting broad permitted lines.

Most available evidence supports the view that the main differences among radio galaxies and radio quasars may be understood as an orientation effect. At some orientations one can see the central source directly, including the broad permitted lines, while at others the center is hidden and only the larger scale narrow emission line gas and large scale radio emission is visible. Of the dozen or so giant radio sources known, three, including Mrk 1498, show broad optical permitted lines, broadly consistent with the predictions of this orientation unification model.