9C continued: a radio-source survey at 15 GHz
  In our first paper we described three regions of the 9C survey of radio
sources with the Ryle telescope at 15.2 GHz, constituting a total area of 520
deg^2 to a completeness limit of approximately 25 mJy. Here we report on a
series of deeper regions, amounting to an area of 115 deg^2 complete to
approximately 10 mJy and of 29 deg^2 complete to approximately 5.5 mJy. We have
investigated the source counts and the distributions of the 1.4 to 15.2 GHz
spectral index (alpha) for these deeper samples. The whole catalogue of 643
sources is available online.
  Down to our lower limit of 5.5 mJy we detect no evidence for any change in
the differential source count from the earlier fitted count above 25 mJy.
  We have matched both our new and earlier catalogues with the NRAO VLA Sky
Survey (NVSS) catalogue at 1.4 GHz. For samples of sources selected at 15.2
GHz, in three flux density ranges, we detect a significant shift in the median
value of alpha; samples with higher flux densities have higher proportions of
sources with flat and rising spectra. We suggest that this observed shift is
consistent with a model containing two distinct source populations having
differently sloped source counts. Samples selected at 1.4 GHz contain
significantly smaller proportions of sources with flat and rising spectra.
Also, in our area complete to approximately 10 mJy, we find 5 sources between
10 to 15 mJy, amounting to 4.3 per cent of sources in this range, with no
counterpart in the NVSS catalogue. These results illustrate the problems
inherent in using a low frequency catalogue to characterise the source
population at a much higher frequency and emphasise the value of our blind 15.2
GHz survey.
