Optical spectroscopy of two overlapping, flux-density-limited samples of radio sources in the North Ecliptic Cap, selected at 38 and 151 MHz

We present the results of optical spectroscopy of two flux-density-limited samples of radio sources selected at frequencies of 38 and 151 MHz in the s ame region around the North Ecliptic Cap, the 8C-NEC and 7C-III samples res pectively. Both samples are selected at flux density levels approximate to 20 times fainter than samples based on the 3C catalogue, They are amongst t he first low-frequency selected samples with no spectral or angular size se lection for which almost complete redshift information has been obtained, a nd contain many of the lowest-luminosity z > 2 radio galaxies so far discov ered. They will therefore provide a valuable resource for understanding the cosmic evolution of radio sources and their hosts and environments. The 15 1 -MHz 7C-III sample is selected to have S-151 greater than or equal to 0.5 Jy and is the more spectroscopically complete; out of 54 radio sources fai rly reliable redshifts have been obtained for 44 objects. The 8C sample has a flux limit of S-38 greater than or equal to 1.3 JY and contains 58 sourc es of which 46 have fairly reliable redshifts. We discuss possible biases i n the observed redshift distribution, and some interesting individual objec ts, including a number of cases of probable gravitational lensing, Using th e 8C-NEC and 7C-III samples in conjunction, we form the first sample select ed on low-frequency flux in the rest-frame of the source, rather than the u sual selection on flux density in the observed frame. This allows us to rem ove the bias associated with an increasing rest-frame selection frequency w ith redshift, We investigate the difference this selection makes to correla tions of radio source properties with redshift and luminosity by comparing the results from traditional flux-density selection with our new method. We show in particular that flux-density-based selection leads to an overestim ate of the steepness of the correlation of radio source size with redshift.