6 Summary

Gigahertz Peaked Spectrum objects are an astrophysically significant and important class yet they are still not well understood (O'Dea 1998). They are not necessarily a uniform class, and one can easily name subclasses among the whole GPS ensemble. For example CSOs make one well defined group -- all of them are GPS galaxies with characteristic VLBI morphologies. It is claimed by Readhead et al. (1996b) that CSOs play a key role as an initial stage in the evolutionary scenario of radio-loud AGNs.

Another fascinating subset of GPS class are objects with extreme (z>3) redshifts. There are 9 such objects in the "working sample'' (O'Dea, priv. comm. 1996). Additionally there are $\sim$20 objects with high (1<z<3) redshifts. All the objects with z>1 are identified with quasars.

The above two issues alone are already a good argument to extend the number of known GPS radio sources. With such a goal in mind we have made a search for candidate GPS sources in the part of the Jodrell Bank-VLA Astrometric Survey limited to $35\hbox{$^\circ$}\leq \delta \leq 75\hbox{$^\circ$}$, namely we compared 8.4 GHz flux densities derived from JVAS with respective 1.4 GHz and 5 GHz fluxes in available catalogues. We treated a source as a plausible candidate if its spectrum seemed to be convex according to the criterion we had arbitrarily assumed.

Quite expectedly some of the candidates selected in this manner are already recognised as GPS objects so we did not deal with them here. Using flux densities at low frequencies i.e. 365 MHz and sometimes even 151 MHz in available catalogues we were able to classify 24 objects as GPS sources without any further measurements. For the majority of selected objects the flux densities at frequencies well below 1 GHz were not available. In these cases we performed observations with MERLIN at 408 MHz to establish the low-frequency part of the spectra.

Our final decision on which of our candidates are and which are not GPS sources has been made based on our MERLIN data (or Texas catalogue when available), NVSS, GB6 and JVAS catalogues. Combining flux density measurements made with high resolution both at low (MERLIN, 408 MHz) and high (VLA, 8.4 GHz) frequencies enabled us to eliminate the effects of confusion and any contribution from possible extended "halos''. We regarded a source as a GPS if it had fitted well a "broken power-law'' function and was not variable.

The sample we present here is the largest single contribution to the pool of known GPS sources collected so far. Only 3 of our sources (0513+714, 0758+594, 1946+708) are overlapping with another large collection of GPS sources, namely with the WENSS based sample (SSB98). Seven sources in our sample have large redshifts (); the largest one is z=3.103.

Our approach to finding GPS sources was to search from high frequency to lower ones. WENSS has been equally successful in defining GPS sources but searching from low frequencies to higher ones. We want to stress though that our approach can successfully be applied to the areas of the sky not planned to be covered by WENSS ($\delta < 30\hbox{$^\circ$}$) but already covered by the other catalogues we used (NVSS, GB6, JVAS). For the part of JVAS we used so far (Patnaik et al. 1992) we found that 9% of JVAS sources are GPS; therefore the whole JVAS encompassing around 3000 sources could easily yield 250 - 300 of such objects.

Acknowledgements

The initial stages of the programme described in this paper were completed when AM stayed at MPIfR in Bonn. The Max-Planck-Gesellschaft stipend which supported him in that time is gratefully acknowledged.

AM acknowledges support from the Polish State Committee for Scientific Research grant 2.P304.003.07 and EU grant ERBCIPDCT940087.

HF is supported in part by the DFG, grant Fa 358/1-1&2.

We thank the NVSS team led by Jim Condon for making NVSS public domain data prior its final publication. Special thanks to Bill Cotton for his co-operation.

This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.