3 Optical spectroscopy

Optical spectra of a large fraction of possible host galaxy of the candidates have been obtained. In order to construct an as complete as possible flux-density limited sample from our candidates, we have tried to obtain spectra and redshifts for all candidate sources in Table .1 with a 325-MHz flux density above 1 Jy. In Table .2 we present the log of the spectroscopic observations. On the 2.5-m INT telescope on La Palma we used the IDS-235 camera with the Ag-Red collimator and the R300V grating. The camera was equipped with a 1k$\times$1k TEK chip. This setup results in a total wavelength coverage of ${\sim}3500$ Å and a pixel scale of ${\sim}3.2$Å/pixel in the dispersion direction and $0\hbox{$.\!\!^{\prime\prime}$ }74$/pixel in the spatial direction. Depending on the magnitude estimated redshift of the host galaxy candidate, the central wavelength of the spectrograph was set at either 5500, 6000 or 6500 Å. The slit-width was held constant at $2\hbox{$^{\prime\prime}$ }$. Only in the case of the source B0809+454 we used a $3\hbox{$^{\prime\prime}$ }$-wide slit because of uncertainties in the optical position at the time of observation. Flat-field observations were made at the beginning of each night by using the internal Tungsten lamp. Wavelength calibration was achieved by internal arc-lamp exposures (from CU-AR and CU-NE lamps) which were taken at the beginning of each night and immediately after each object exposure. For absolute flux calibration and to correct for the wavelength-dependent sensitivity of the instrument, several spectroscopical standard stars were observed each night, using a $5\hbox{$^{\prime\prime}$ }$-wide slit.

The spectra have been reduced using the NOAO IRAF data reduction software. One dimensional spectra have been extracted using a $4\hbox{$^{\prime\prime}$ }$-wide aperture centered on the peak of the spatial profile of the identification. The resulting spectra are shown in Figs. .1-.29. For each object exposure the wavelength calibration has been checked against several bright sky-lines in a non background-subtracted frame.

In addition to these observations, a spectrum of the source B1450+333 was obtained on 8 July 1997 by P.N. Best, using the 4.2-m WHT on La Palma, equipped with the ISIS spectrograph. An optical CCD image and a spectrum of the source B1543+845 were obtained by I.M. Gioia on 4 and 5 March 1998 using the HARIS imaging spectrograph on the 2.2-m University of Hawaii Telescope on Mauna Kea, Hawaii.

Table .5 presents the measured redshifts and the spectral features used to determine these. The optical images and spectra of the observed host galaxies are presented in Figs. .1-.29. For the source B1245+676 a high-quality spectrum and the therefrom derived redshift had been published earlier by Marcha et al. (1996). We have therefore not observed the host galaxy of this radio source again. We have, however, reobserved the source B1310+451 in order to obtain a higher quality spectrum of the host galaxy of this source.

Several other radio and optical properties of the observed candidates, and of the three sources that were found to be GRGs from available data in the NED (B1144+352, B1245+676 and B1310+451) are given in Tables .3 and .4.