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Empirical simulations based on extrapolations from well-established low-frequency ($< 5$ GHz) surveys fail to accurately model the faint, high frequency ($>10$~GHz) source population; they under-predict the number of observed sources by a factor of two below $S_{18~\rm GHz} = 10$ mJy and fail to reproduce the observed spectral index distribution. We suggest that this is because the faint radio galaxies are not modelled correctly in the simulations and show that by adding a flat-spectrum core component to the FRI sources in the SKA Simulated Skies, the observed 15-GHz source counts can be reproduced. We find that the observations are best matched by assuming that the fraction of the total 1.4-GHz flux density which originates from the core varies with 1.4-GHz luminosity; sources with 1.4-GHz luminosities $< 10^{25} \rm W \, Hz^{-1}$ require a core fraction $\sim 0.3$, while the more luminous sources require a much smaller core fraction of $5 \times 10^{-4}$. The low luminosity FRI sources with high core fractions which were not included in the original simulation may be equivalent to the compact `FR0' sources found in recent studies., Comment: 6 pages, 5 figures, accepted for publication in MNRAS

A complete sample of 96 faint ($S > 0.5$ mJy) radio galaxies is selected from the Tenth Cambridge (10C) survey at 15.7~GHz. Optical spectra are used to classify 17 of the sources as high-excitation or low-excitation radio galaxies (HERGs and LERGs respectively), for the remaining sources three other methods are used; these are optical compactness, X-ray observations and mid-infrared colour--colour diagrams. 32 sources are HERGs and 35 are LERGs while the remaining 29 sources could not be classified. We find that the 10C HERGs tend to have higher 15.7-GHz flux densities, flatter spectra, smaller linear sizes and be found at higher redshifts than the LERGs. This suggests that the 10C HERGs are more core dominated than the LERGs. Lower-frequency radio images, linear sizes and spectral indices are used to classify the sources according to their radio morphology; 18 are Fanaroff and Riley type I or II sources, a further 13 show some extended emission, and the remaining 65 sources are compact and are referred to as FR0 sources. The FR0 sources are sub-divided into compact, steep-spectrum (CSS) sources (13 sources) or GHz-peaked spectrum (GPS) sources (10 sources) with the remaining 42 in an unclassified class. FR0 sources are more dominant in the subset of sources with 15.7-GHz flux densities $<$1 mJy, consistent with the previous result that the fainter 10C sources have flatter radio spectra. The properties of the 10C sources are compared to the higher-flux density Australia Telescope 20 GHz (AT20G) survey. The 10C sources are found at similar redshifts to the AT20G sources but have lower luminosities. The nature of the high-frequency selected objects change as flux density decreases; at high flux densities the objects are primarily quasars, while at low flux densities radio galaxies dominate., Comment: 16 pages, 13 figures, accepted for publication in MNRAS

We discuss our current understanding of the nature of the faint, high-frequency radio sky. The Tenth Cambridge (10C) survey at 15.7 GHz is the deepest high-frequency radio survey to date, covering 12 square degrees to a completeness limit of 0.5 mJy, making it the ideal starting point from which to study this population. In this work we have matched the 10C survey to several lower-frequency radio catalogues and a wide range of multi-wavelength data (near- and far-infrared, optical and X-ray). We find a significant increase in the proportion of flat-spectrum sources at flux densities below 1 mJy - the median radio spectral index between 15.7 GHz and 610 MHz changes from 0.75 for flux densities greater than 1.5 mJy to 0.08 for flux densities less than 0.8 mJy. The multi-wavelength analysis shows that the vast majority (> 94 percent) of the 10C sources are radio galaxies; it is therefore likely that these faint, flat spectrum sources are a result of the cores of radio galaxies becoming dominant at high frequencies. We have used new observations to extend this study to even fainter flux densities, calculating the 15.7-GHz radio source count down to 0.1 mJy, a factor of five deeper than previous studies. There is no evidence for a new population of sources, showing that the high-frequency sky continues to be dominated by radio galaxies down to at least 0.1 mJy., Comment: 8 pages, 4 figures, to be published in the proceedings of the conference "The Many Facets of Extragalactic Radio Surveys: Towards New Scientific Challenges", Bologna 20-23, 2015

We have used recent Very Long Baseline Interferometry (VLBI) observations by Middelberg et al. with a resolution of $\approx 10$ mas to investigate the properties of faint sources selected from the Tenth Cambridge (10C) survey in the Lockman Hole. The 10C survey is complete to 0.5 mJy at 15.7 GHz and has a resolution of 30 arcsec. We have previously shown that this population is dominated by flat-spectrum sources below $\approx 1$ mJy, in disagreement with several models of the faint, high-frequency sky. We find that 33 out of the 51 10C sources in the VLBI field (65 percent) are detected by the VLBI observations. The sources detected by the VLBI observations must have a high brightness temperature, thus ruling out the possibility that this faint, high frequency population is dominated by starbursting or starforming sources and indicating that they must be Active Galactic Nuclei., Comment: Accepted for publication in MNRAS

The Arcminute Microkelvin Imager (AMI) is a telescope specifically designed for high sensitivity measurements of low-surface-brightness features at cm-wavelength and has unique, important capabilities. It consists of two interferometer arrays operating over 13.5-18 GHz that image structures on scales of 0.5-10 arcmin with very low systematics. The Small Array (AMI-SA; ten 3.7-m antennas) couples very well to Sunyaev-Zel'dovich features from galaxy clusters and to many Galactic features. The Large Array (AMI-LA; eight 13-m antennas) has a collecting area ten times that of the AMI-SA and longer baselines, crucially allowing the removal of the effects of confusing radio point sources from regions of low surface-brightness, extended emission. Moreover AMI provides fast, deep object surveying and allows monitoring of large numbers of objects. In this White Paper we review the new science - both Galactic and extragalactic - already achieved with AMI and outline the prospects for much more., Comment: 20 pages, 11 figures; white paper. Revised author list, section IB, section IIIC2, references

In the case of an initially conical jet, we study the relation between jet collimation by the external pressure and large-scale morphology. We first consider the important length-scales in the problem, and then carry out axisymmetric hydrodynamic simulations that include, for certain parameters, all these length-scales. We find three important scales related to the collimation region: (i) where the sideways ram-pressure equals the external pressure, (ii) where the jet density equals the ambient density, and (iii) where the forward ram-pressure falls below the ambient pressure. These scales are set by the external Mach-number and opening angle of the jet. We demonstrate that the relative magnitudes of these scales determine the collimation, Mach-number, density and morphology of the large scale jet. Based on analysis of the shock structure, we reproduce successfully the morphology of Fanaroff-Riley (FR) class I and II radio sources. Within the framework of the model, an FR I radio source must have a large intrinsic opening angle. Entrainment of ambient gas might also be important. We also show that all FR I sources with radio lobes or similar features must have had an earlier FR II phase., Comment: 14 pages, 10 figures, accepted by MNRAS, same as previous version

The first results from the Tenth Cambridge (10C) Survey of Radio Sources, carried out using the AMI Large Array (LA) at an observing frequency of 15.7 GHz, are presented. The survey fields cover an area of approximately 27 sq. degrees to a flux-density completeness of 1 mJy. Results for some deeper areas, covering approximately 12 sq. degrees, 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 source catalogue contains 1897 entries and is available at www.mrao.cam.ac.uk/surveys/10C. It has been combined with that of the 9C Survey to calculate the 15.7-GHz source counts. A broken power law is found to provide a good parameterisation of the differential count between 0.5 mJy and 1 Jy. The measured count has been compared to that predicted by de Zotti et al. (2005). The model displays good agreement with the data at the highest flux densities but under-predicts the integrated count between 0.5 mJy and 1 Jy by about 30 per cent. Entries from the source catalogue have been matched to those contained in the catalogues of NVSS and FIRST (both of which have observing frequencies of 1.4 GHz). This matching provides evidence for a shift in the typical 1.4-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 have been applied to the data; approximately 5 per cent of the sources are found to be extended relative to the LA synthesised beam of approximately 30 arcsec. Investigations using higher-resolution data showed that most of the genuinely extended sources at 16 GHz are classical doubles, although some nearby galaxies and twin-jet sources were also identified., Comment: 15 pages, 12 figures, landscape page (Table 1) is included on final page, submitted to MNRAS. Modified to compare to latest version of the de Zotti model. Added further detail about checks to flux-density scale

We present optical spectroscopy, near-infrared (mostly K-band) and radio (151-MHz and 1.4-GHz) imaging of the first complete region (TOOT00) of the TexOx-1000 (TOOT) redshift survey of radio sources. The 0.0015-sr (~ 5 deg^2) TOOT00 region is selected from pointed observations of the Cambridge Low-Frequency Survey Telescope at 151 MHz at a flux density limit of ~= 100 mJy, ~ 5-times fainter than the 7C Redshift Survey (7CRS), and contains 47 radio sources. We have obtained 40 spectroscopic redshifts (~ 85% completeness). Adding redshifts estimated for the 7 other cases yields a median redshift z_med ~ 1.25. We find a significant population of objects with FRI-like radio structures at radio luminosities above both the low-redshift FRI/II break and the break in the radio luminosity function. The redshift distribution and sub-populations of TOOT00 are broadly consistent with extrapolations from the 7CRS/6CE/3CRR datasets underlying the SKADS Simulated Skies Semi-Empirical Extragalactic Database, S^3-SEX., Comment: Accepted for publication in MNRAS; 59 pages, 10 figures, 6 tables

We investigate the potential of the Square Kilometre Array (SKA) for measuring the magnetic fields in clusters of galaxies via Faraday rotation of background polarised sources. [...] We find that about 10 per cent of the sky is covered by a significant extragalactic Faraday screen. Most of it has rotation measures between 10 and 100 rad/m/m. We argue that the cluster centres should have up to about 5000 rad/m/m. We show that the proposed mid frequency aperture array of the SKA as well as the lowest band of the SKA dish array are well suited to make measurements for most of these rotation measure values, typically requiring a signal-to-noise of ten. We calculate the spacing of sources forming a grid for the purpose of measuring foreground rotation measures: it reaches a spacing of 36 arcsec for a 100 hour SKA observation per field. We also calculate the statistics for background RM measurements in clusters of galaxies. We find that a first phase of the SKA would allow us to take stacking experiments out to high redshifts (>1), and provide improved magnetic field structure measurements for individual nearby clusters. The full SKA aperture array would be able to make very detailed magnetic field structure measurements of clusters with more than 100 background sources per cluster up to a redshift of 0.5 and more than 1000 background sources per cluster for nearby clusters, and could for reasonable assumptions about future measurements of electron densities in high redshift clusters constrain the power law index for the magnetic field evolution to better than dm=0.4, if the magnetic field in clusters should follow B ~ (1+z)^m., Comment: 12 pages, 10 figures, 3 tables, accepted by MNRAS, minor correction to eq (5)

We probe the relationship between star formation rate (SFR) and radio synchrotron luminosity in galaxies at 0 < z < 2 within the northern Spitzer Wide-area Infrared Extragalactic survey (SWIRE) fields, in order to investigate some of the assumptions that go into calculating the star formation history of the Universe from deep radio observations. We present new 610-MHz Giant Metrewave Radio Telescope (GMRT) observations of the European Large-Area ISO Survey (ELAIS)-N2 field, and using this data, along with previous GMRT surveys carried out in the ELAIS-N1 and Lockman Hole regions, we construct a sample of galaxies which have redshift and SFR information available from the SWIRE survey. We test whether the local relationship between SFR and radio luminosity is applicable to z = 2 galaxies, and look for evolution in this relationship with both redshift and SFR in order to examine whether the physical processes which lead to synchrotron radiation have remained the same since the peak of star formation in the Universe. We find that the local calibration between radio luminosity and star formation can be successfully applied to radio-selected high-redshift, high-SFR galaxies, although we identify a small number of sources where this may not be the case; these sources show evidence for inaccurate estimations of their SFR, but there may also be some contribution from physical effects such as the recent onset of starburst activity, or suppression of the radio luminosity within these galaxies., Comment: 12 pages, 1 table, 12 figures. Accepted for publication in MNRAS

As part of an ongoing series of deep GMRT surveys we have observed the Spitzer extragalactic First Look Survey field, producing the deepest wide-field 610-MHz survey published to date. We reach an rms noise of 30 microJy/beam before primary beam correction, with a resolution of ~6 arcsec over an area of ~4 square degrees. By combining these observations with the existing 1.4-GHz VLA survey produced by Condon et al. (2003), along with infrared data in up to seven wavebands from the Spitzer Space Telescope, optical photometry from SDSS and a range of spectroscopic redshift surveys, we are able to study the relationship between radio luminosity and star formation rate in star-forming galaxies up to z ~ 1. The large amount of multi-wavelength data available allows accurate k-corrections to be performed in the radio due to the knowledge of the radio spectral index, and in the infrared through the use of a semi-empirical radiative transfer model which models star-forming regions, warm dust surrounding these regions, and diffuse interstellar dust, taking into account the star formation rate, star formation history and hydrogen column density within each galaxy. A strong correlation is seen between radio luminosity and infrared-derived star formation rates, which is best fit by a slightly non-linear power-law. We look for cosmic evolution in the comparative radio brightness of star-forming galaxies by searching for deviations away from the global relationship. Any such deviation would indicate a systematic variation in one or more of the properties controlling synchrotron radiation, in particular an increase in the magnetic field strengths of star-forming galaxies. The data shows no evidence for such an effect, suggesting that there has been little evolution in the magnetic fields of galaxies since z ~ 1., Comment: 5 pages, 3 figures. To Appear in "The Modern Radio Universe: From Planets to Dark Energy" Conference Proceedings (Oct 1-5 2007, The University of Manchester) Editors: Beswick, Diamond & Schilizzi. Updated to reflect resubmission of MNRAS paper

We use a volume- and flux-limited sample of local ($0.03 \leq z \leq 0.1$) radio galaxies with optical counterparts to address the question of how long a typical galaxy spends in radio-active and quiescent states. The length of the active phase has a strong dependence on the stellar mass of the host galaxy. Radio sources in the most massive hosts are also retriggered more frequently. The time spent in the active phase has the same dependence on stellar mass as does the gas cooling rate, suggesting the onset of the quiescent phase is due to fuel depletion. We find radio and emission line AGN activity to be independent, consistent with these corresponding to different accretion states., Comment: accepted for publication in MNRAS; 15 pages, 14 figures

We present observations of the Lockman Hole with the Giant Metrewave Radio Telescope (GMRT). Twelve pointings were observed, covering a total area of ~ five square degrees with a resolution of 6 x 5 arcsec, position angle +45deg. The majority of the pointings have an rms noise of 60 microJy/beam before correction for the attenuation of the GMRT primary beam. Techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic is presented, along with a catalogue of 2845 sources detected above 6 sigma. Radio source counts are calculated at 610 MHz and combined with existing 1.4-GHz source counts, in order to show that pure luminosity evolution of the local radio luminosity functions for active galactic nuclei and starburst galaxies is sufficient to account for the two source counts simultaneously., Comment: Accepted for publication in MNRAS. 8 pages, 8 figures, some with reduced image quality. Table 1 is available in full via http://www.mrao.cam.ac.uk/surveys/

Observations of the ELAIS-N1 field taken at 610 MHz with the Giant Metrewave Radio Telescope are presented. Nineteen pointings were observed, covering a total area of 9 square degrees with a resolution of 6" x 5", PA +45 deg. Four of the pointings were deep observations with an rms of 40 microJy before primary beam correction, with the remaining fifteen pointings having an rms of 70 microJy. The techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic is presented, along with a catalogue of 2500 sources detected above 6 sigma. This work complements the large amount of optical and infrared data already available on the region. We calculate 610-MHz source counts down to 270 microJy, and find further evidence for the turnover in differential number counts below 1 mJy, previously seen at both 610 MHz and 1.4 GHz., Comment: 12 pages, 18 figures, two tables. Table 1 can be found in full via http://www.mrao.cam.ac.uk/surveys/ . Accepted for publication in MNRAS

Observations of the Spitzer extragalactic First Look Survey field taken at 610 MHz with the Giant Metrewave Radio Telescope are presented. Seven individual pointings were observed, covering an area of 4 square degrees with a resolution of 5.8'' x 4.7'', PA 60 deg. The r.m.s. noise at the centre of the pointings is between 27 and 30 microJy before correction for the GMRT primary beam. The techniques used for data reduction and production of a mosaicked image of the region are described, and the final mosaic, along with a catalogue of 3944 sources detected above 5 sigma, are presented. The survey complements existing radio and infrared data available for this region., Comment: 11 pages, 13 figures, one table (full version available via http://www.mrao.cam.ac.uk/surveys/). Accepted for publication in MNRAS

We present multi-frequency radio observations of a sample of z~2 obscured (type-2) quasars in the Spitzer extragalactic First Look Survey area. We combine the public data at 1.4 GHz, used in the selection of these sources, with new observations at 610 MHz (GMRT) and at 4.9 GHz (VLA). We find the sample includes sources with steep, flat and gigahertz-peaked spectra. There are no strong correlations between the presence or absence of emission lines in the optical spectra and the radio spectral properties of the sample. However, there are no secure flat-spectrum type-2 quasars with narrow emission lines which would be problematic for unified schemes. Most of the population have straight radio spectra with spectral index alpha~1 as is expected for developed, potentially FRI-like, jets in which continous injection of relativistic electrons is accompanied by inverse-Compton losses against the cosmic microwave background., Comment: 6 pages, 2 colour figures, submitted to MNRAS

We present first results from the study of the TOOT00 region consisting of 47 radio sources brighter than 100 mJy at 151 MHz. We have 81% spectroscopic redshift completeness. From the K-z diagram we deduce that the host galaxies are similar to ~3 L* passively evolved elliptical galaxies and thus estimate the redshifts of the 9 sources without a secure spectroscopic redshift yielding a median redshift of 1.287. Above the RLF break we have a quasar fraction f ~ 0.3 although the quasars appear reddened; below the RLF break f -> 0 if we exclude flat-spectrum radio sources. We present a histogram of the number of TOOT00 radio sources versus their redshift which looks broadly like the Willott et al. (2001) prediction for TOOT, although the observed ratio of high to low redshift objects is somewhat lower than the prediction., Comment: 4 pages, 5 figures, to be published in AN (proceedings of the Granada Workshop on High Redshift Radio Galaxies)

We describe the selection of candidate radio-loud quasars obtained by cross-matching radio source positions from the low-frequency (151 MHz) 7C survey with optical positions from five pairs of EO POSS-I plates scanned with the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied is centred at RA 10 h 28 m, Dec +41 and covers 0.057 sr. We present VLA observations of the quasar candidates, and tabulate various properties derived from the radio maps. We discuss the selection criteria of the resulting `7CQ' sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of the 36 unconfirmed candidates, constitute a filtered sample with the following selection criteria: 151-MHz flux density S151 > 100 mJy; POSS-I E-plate magnitude E approx R < 20; and POSS-I colour (O - E) < 1.8; the effective area of the survey drops significantly below S151 approx 200 mJy. We argue that the colour criterion excludes few if any quasars, but note, on the basis of recent work by Willott et al. (1998b), that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars., Comment: Latex, 30 pages. Accepted by MNRAS