10C survey of radio sources at 15.7 GHz - II. First results★

Abstract

In a previous paper (Paper I), the observational, mapping and source‐extraction techniques used for the Tenth Cambridge (10C) Survey of Radio Sources were described. Here, the first results from the survey, carried out using the Arcminute Microkelvin Imager Large Array (LA) at an observing frequency of 15.7 GHz, are presented. The survey fields cover an area of ≈27 deg2 to a flux‐density completeness of 1 mJy. Results for some deeper areas, covering ≈12 deg2, wholly contained within the total areas and complete to 0.5 mJy, are also presented. The completeness for both areas is estimated to be at least 93 per cent. The 10C survey is the deepest radio survey of any significant extent (≳0.2 deg2) above 1.4 GHz. The 10C source catalogue contains 1897 entries and is available online. The source catalogue has been combined with that of the Ninth Cambridge Survey to calculate the 15.7‐GHz source counts. A broken power law is found to provide a good parametrization of the differential count between 0.5 mJy and 1 Jy. The measured source count has been compared with that predicted by de Zotti et al. – the model is found to display good agreement with the data at the highest flux densities. However, over the entire flux‐density range of the measured count (0.5 mJy to 1 Jy), the model is found to underpredict the integrated count by ≈30 per cent. Entries from the source catalogue have been matched with those contained in the catalogues of the NRAO VLA Sky Survey and the Faint Images of the Radio Sky at Twenty‐cm survey (both of which have observing frequencies of 1.4 GHz). This matching provides evidence for a shift in the typical 1.4‐GHz spectral index to 15.7‐GHz spectral index of the 15.7‐GHz‐selected source population with decreasing flux density towards sub‐mJy levels – the spectra tend to become less steep. Automated methods for detecting extended sources, developed in Paper I, have been applied to the data; ≈5 per cent of the sources are found to be extended relative to the LA‐synthesized beam of ≈30 arcsec. Investigations using higher resolution data showed that most of the genuinely extended sources at 15.7 GHz are classical doubles, although some nearby galaxies and twin‐jet sources were also identified.

The Lockman Hole project: LOFAR observations and spectral index properties of low-frequency radio sources

“…These two fields cover 4.64 deg 2 down to an rms noise level of 0.05 mJy in the central regions of each field and 0.1 mJy in the outskirts. For more details on the observations and data reducion for the 10C survey we refer the reader to Davies et al (2011). When crossmatching with the LOFAR-WSRT catalogue we used a search radius of 15 arcsec to match the analysis carried out by Whittam et al (2013).…”

“…In addition, the Lockman Hole field has an extensive amount of radio data covering a wide range in frequency. This includes the 15-GHz 10C survey (Davies et al 2011;Whittam et al 2013), deep 1.4-GHz observations over 7 square degrees observed with the Westerbork Synthesis Radio Telescope (WSRT; Guglielmino et al 2012, Prandoni et al, 2016a, 610-MHz GMRT observations (Garn et al 2008) and 345-MHz WSRT observations (Prandoni et al, 2016b, in preparation). In this paper we present LOFAR observations of the Lockman Hole field, which extends this multi-frequency information down to 150 MHz, allowing us to study the low-frequency spectral properties of the faint radio source population.…”

10C survey of radio sources at 15.7 GHz - I. Observing, mapping and source extraction★

“…Raw data files from individual observing runs had various checks and calibrations applied using the in-house, data-reduction software REDUCE. A fuller explanation of the steps carried out in REDUCE, including the weighting of data depending on the weather conditions, can be found in AMI Consortium: Davies et al (2009). The data were then output as multisource uv-FITS files, which were, in turn, combined into a single multisource uv-FITS file per field, again using an in-house software.…”

“…The 9C survey (Waldram et al 2003(Waldram et al , 2010 mapped 29 deg 2 of sky to 5.5 mJy completeness at 15 GHz, in addition to several larger and shallower areas. The Ryle Telescope (RT), which carried out the survey, has subsequently been reconfigured and re-equipped to form the Large Array (LA) of the Arcminute Microkelvin Imager (AMI; AMI Consortium: Zwart et al 2008). As part of this metamorphosis, three of the RT's eight antennas were moved to the north of the (almost) east-west line on which they originally stood, providing the telescope with north-south baselines.…”

Comparison of Sunyaev-Zel’dovich measurements fromPlanckand from the Arcminute Microkelvin Imager for 99 galaxy clusters

“…Source-finding is carried out at 4σ on the LA continuum map, as described in Davies et al (2011) andFranzen et al (2011), and sources that are detected at ≥3σ on at least three channel maps and are not extended have a spectral index α fitted across the AMI band. SA data are binned on a grid in uv-space in order to reduce the memory required for subsequent analysis.…”