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 same region around the North Ecliptic Cap, the 8C‐NEC and 7C‐ iii samples respectively. Both samples are selected at flux density levels ≈20 times fainter than samples based on the 3C catalogue. They are amongst the first low‐frequency selected samples with no spectral or angular size selection for which almost complete redshift information has been obtained, and contain many of the lowest‐luminosity z >2 radio galaxies so far discovered. They will therefore provide a valuable resource for understanding the cosmic evolution of radio sources and their hosts and environments. The 151‐MHz 7C‐ iii sample is selected to have S 151 ≥0.5 Jy and is the more spectroscopically complete; out of 54 radio sources fairly reliable redshifts have been obtained for 44 objects. The 8C sample has a flux limit of S 38 ≥1.3 Jy and contains 58 sources of which 46 have fairly reliable redshifts. We discuss possible biases in the observed redshift distribution, and some interesting individual objects, including a number of cases of probable gravitational lensing. Using the 8C‐NEC and 7C‐ iii samples in conjunction, we form the first sample selected on low‐frequency flux in the rest‐frame of the source, rather than the usual selection on flux density in the observed frame. This allows us to remove the bias associated with an increasing rest‐frame selection frequency with redshift. We investigate the difference this selection makes to correlations 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 overestimate of the steepness of the correlation of radio source size with redshift.