SIMBAD references

The 9C survey of radio sources with the Ryle Telescope at 15.2 GHz was set up to survey the fields observed with the cosmic microwave background telescope, the Very Small Array. In our first paper, we described three regions of the survey, constituting a total area of 520 deg² to a completeness limit of ~25mJy. Here we report on a series of deeper regions, amounting to an area of 115 deg² complete to ~10mJy and of 29 deg² complete to ~5.5mJy. We have investigated the source counts and the distributions of the 1.4 to 15.2 GHz spectral indices (α^15.2_1.4) for these deeper samples. The whole catalogue of 643 sources is available online.

Down to our lower limit of 5.5mJy, we detect no evidence for any change in the differential source count from the earlier fitted count above 25mJy, n(S) = 51(S/Jy)^–2.15/ Jy/sr.

We have matched both our new and earlier catalogues with the NRAO VLA Sky Survey (NVSS) catalogue at 1.4 GHz and selected flux-limited samples at both 15 and 1.4 GHz. As expected, we find that the proportions of sources with flat and rising spectra in the samples selected at 15 GHz are significantly higher than those in the samples selected at 1.4 GHz. In addition, for 15-GHz samples selected in three flux density ranges, we detect a significant shift in the median value of α^15.2_1.4: the higher the flux densities the higher the proportions of sources with flat and rising spectra.

In our area complete to ~10mJy, we find five sources between 10 and 15mJy at 15 GHz, amounting to 4.3 per cent of sources in this range, with no counterpart in the NVSS catalogue. This implies that, had we relied on NVSS for locating our sources, we could have missed a significant proportion of them at low flux densities.

Our results illustrate the problems inherent in using a low-frequency catalogue to characterize the source population at a much higher frequency and emphasize the value of our blind 15.2-GHz survey.