10C continued: a deeper radio survey at 15.7 GHz

Abstract

We present deep 15.7-GHz observations made with the Arcminute Microkelvin Imager Large Array in two fields previously observed as part of the Tenth Cambridge (10C) survey. These observations allow the source counts to be calculated down to 0.1 mJy, a factor of five deeper than achieved by the 10C survey. The new source counts are consistent with the extrapolated fit to the 10C source count, and display no evidence for either steepening or flattening of the counts. There is thus no evidence for the emergence of a significant new population of sources (e.g. starforming) at 15.7 GHz flux densities above 0.1 mJy, the flux density level at which we expect starforming galaxies to begin to contribute. Comparisons with the de Zotti et al. model and the SKADS Simulated Sky show that they both underestimate the observed number of sources by a factor of two at this flux density level. We suggest that this is due to the flatspectrum cores of radio galaxies contributing more significantly to the counts than predicted by the models.

GMRT 610-MHz observations of the faint radio source population – and what these tell us about the higher radio-frequency sky

“…This suggests that star-forming galaxies make no significant contribution to this population. This is in agreement with a recent study of the source counts of this sample (Whittam et al 2016b) and provides further evidence that the SKA Simulated Skies do not accurately model the radio source population at high frequencies.…”

“…This is consistent with the conclusions of Whittam et al (2016b), who used the 10C ultra-deep data to calculate the 15.7-GHz source counts down to 0.1 mJy and found no evidence for the emergence of a significant new population of sources, such as star-forming galaxies. It is in contrast to the predictions made by the SKADS Simulated Skies (Wilman et al 2008), which predicts that 20 per cent of the sources with 0.1 < S 18 GHz /mJy < 0.3 should be star-forming galaxies.…”

“…This showed that the nature of the population was changing as flux density decreased. Using the recent deeper extension to the 10C survey (10C ultra-deep; Whittam et al 2016b), along with the GMRT observations described in this paper, we are able to extend this study to fainter flux densities. …”

“…There are two other catalogues available in this field: the 10C survey (AMI Consortium: Davies et al 2011;AMI Consortium: Franzen et al 2011;Whittam et al 2016b) and the National Radio Astronomy Observatory (NRAO) Very Large Array (VLA) Sky Survey (NVSS; Condon et al 1998). Both of these catalogues are relatively low resolution, with synthesised beam sizes of 30 and 45 arcsec respectively; this means they are not ideal for assessing the astrometric accuracy of our catalogue (this field is not covered by the Faint Images of the Radio Sky and Twenty-one cm).…”

“…Further study has shown that these sources are the cores of faint radio galaxies (Whittam et al 2015;Whittam et al 2016a), which are both more numerous and have flatter spectra than expected. deep, Whittam et al 2016b) has extended the 10C survey to 0.1 mJy in two fields. Due to telescope scheduling, the deeper of these 15.7 GHz surveys is in the AMI001 field, a region of the sky with very little in the way of complementary data as the field was chosen purely on radio grounds.…”