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Register a new userFaint polarised sources in the Lockman Hole field at 1.4 GHz
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We aim to study the nature of the faint, polarised radio source population whose source composition and redshift dependence contain information about the strength, morphology, and evolution of magnetic fields over cosmic timescales. We use a 15 pointing radio continuum L-band mosaic of the Lockman Hole, observed in full polarisation, generated from archival data of the WSRT. The data were analysed using the RM-Synthesis technique. We achieved a noise of 7 {mu}Jy/beam in polarised intensity, with a resolution of 15. Using infrared and optical images and source catalogues, we were able to cross-identify and determine redshifts for one third of our detected polarised sources. We detected 150 polarised sources, most of which are weakly polarised with a mean fractional polarisation of 5.4 %. With a total area of 6.5 deg^2 and a detection threshold of 6.25 {sigma} we find 23 polarised sources per deg^2. Based on our multi wavelength analysis, we find that our sample consists of AGN only. We find a discrepancy between archival number counts and those present in our data, which we attribute to sample variance. Considering the absolute radio luminosty, to distinguish weak and strong sources, we find a general trend of increased probability to detect weak sources at low redshift and strong sources at high redshift. Further, we find an anti-correlation between fractional polarisation and redshift for our strong sources sample at z{geq}0.6. A decrease in the fractional polarisation of strong sources with increasing redshift cannot be explained by a constant magnetic field and electron density over cosmic scales, however the changing properties of cluster environments over the cosmic timemay play an important role. Disentangling these two effects requires deeper and wider polarisation observations, and better models of the morphology and strength of cosmic magnetic fields.
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We present a study of the 1173 sources brighter than $S_{1.4,rm GHz}= 120,mu$Jy detected over an area of $simeq 1.4,hbox{deg}^{2}$ in the Lockman Hole field. Exploiting the multi-band information available in this field for $sim$79% of the sample, sources have been classified into radio loud (RL) active galactic nuclei (AGNs), star forming galaxies (SFGs) and radio quiet (RQ) AGNs, using a variety of diagnostics available in the literature. Exploiting the observed tight anti-correlations between IRAC band 1 or band 2 and the source redshift we could assign a redshift to 177 sources missing a spectroscopic measurement or a reliable photometric estimate. A Monte Carlo approach was used to take into account the spread around the mean relation. The derived differential number counts and luminosity functions at several redshifts of each population show a good consistency with models and with earlier estimates made using data from different surveys and applying different approaches. Our results confirm that below $sim300,mu$Jy SFGs$+$RQ AGNs overtake RL AGNs that dominate at brighter flux densities. We also confirm earlier indications of a similar evolution of RQ AGNs and SFGs. Finally, we discuss the angular correlation function of our sources and highlight its sensitivity to the criteria used for the classification.
By exploiting the far-infrared(FIR) and radio correlation, we have performed the Likelihood-Ratio analysis to identify optical counterparts to the far-infrared sources in the Lockman Hole. Using the likelihood ratio analysis and the associated reliability, 44 FIR sources have been identified with radio sources. Redshifts have been obtained for 29 out of 44 identified sources. One hyper-luminous infrared galaxy (HyLIRG) with and four ultraluminous infrared galaxies (ULIRGs) are identified in our sample. The space density of the FIR sources at z = 0.3-0.6 is 4.6times 10^{-5}Mpc^{-3}, implying a rapid evolution of the ULIRG population. Most of ISO FIR sources have their FIR-radio ratios similar to star-forming galaxies ARP 220 and M82. At least seven of our FIR sources show evidence for the presence of an active galactic nucleus (AGN) in optical emission lines, radio continuum excess, or X-ray activity. Three out of five (60%) of the ULIRG/HyLIRGs are AGN galaxies. Five of the seven AGN galaxies are within the ROSAT X-ray survey field, and two are within the XMM-Newton survey fields. X-ray emission has been detected in only one source, 1EX030, which is optically classified as a quasar. The non-detection in the XMM-Newton 2-10 keV band suggests a very thick absorption obscuring the central source of the two AGN galaxies. Several sources have an extreme FIR luminosity relative to the optical R-band, L(90mumathrm{m})/L(R) > 500, which is rare even among the local ULIRG population. While source confusion or blending might offer an explanation in some cases, they may represent a new population of galaxies with an extreme activity of star formation in an undeveloped stellar system -- i.e., formation of bulges or young ellipticals.
We present results from simulations of the extragalactic polarized sky at 1.4 GHz. As the basis for our polarization models, we use a semi-empirical simulation of the extragalactic total intensity (Stokes I) continuum sky developed at the University of Oxford (http://scubed.physics.ox.ac.uk) under the European SKA Design Study (SKADS) initiative, and polarization distributions derived from analysis of polarization observations. By considering a luminosity dependence for the polarization of AGN, we are able to fit the 1.4 GHz polarized source counts derived from the NVSS and the DRAO ELAIS N1 deep field survey down to approximately 1 mJy. This trend is confirmed by analysis of the polarization of a complete sample of bright AGN. We are unable to fit the additional flattening of the polarized source counts from the deepest observations of the ELAIS N1 survey, which go down to ~0.5 mJy. Below 1 mJy in Stokes I at 1.4 GHz, starforming galaxies become an increasingly important fraction of all radio sources. We use a spiral galaxy integrated polarization model to make realistic predictions of the number of polarized sources at microJy levels in polarized flux density and hence, realistic predictions of what the next generation radio telescopes such as ASKAP, other SKA pathfinders and the SKA itself will see.
Active Galactic Nuclei (AGN) play a decisive role in galaxy evolution, particularly so when operating in a radiatively inefficient mode, where they launch powerful jets that reshape their surroundings. However, identifying them is difficult, since radio observations commonly have resolutions of between 1 arcsec and 10 arcsec, which is equally sensitive to radio emission from star-forming activity and from AGN. Very Long Baseline Interferometry (VLBI) observations allow one to filter out all but the most compact non-thermal emission from radio survey data. The observational and computational demands to do this in large surveys have been, until recently, too high to make such undertakings feasible. Only the recent advent of wide-field observing techniques have facilitated such observations, and we here present the results from a survey of 217 radio sources in the Lockman Hole/XMM field. We describe in detail some new aspects of the calibration, including primary beam correction, multi-source self-calibration, and mosaicing. As a result, we detected 65 out of the 217 radio sources and were able to construct, for the first time, the source counts of VLBI-detected AGN. They indicate that at least 15%-25% of the sub-mJy radio sources are AGN-driven, consistent with recent findings using other AGN selection techniques. We have used ancillary data to investigate the AGN hosts. We find that among the sources nearby enough to be resolved in the optical images, 88% (23/26) could be classified as early-type or bulge-dominated galaxies. While 50% of these sources are correctly represented by the SED of an early-type galaxy, for the rest the best fit was obtained with a heavily extinct starburst template, an effect we ascribe to a degeneracy in the fit. Overall, the typical hosts of VLBI-detected sources are in good agreement with being early-type or bulge-dominated galaxies.
Ultra Steep Spectrum (USS) radio sources have been successfully used to select powerful radio sources at high redshifts (z>~2). Typically restricted to large-sky surveys and relatively bright radio flux densities, it has gradually become possible to extend the USS search to sub-mJy levels, thanks to the recent appearance of sensitive low-frequency radio facilities. Here a first detailed analysis of the nature of the faintest USS sources is presented. By using Giant Metrewave Radio Telescope and Very Large Array radio observations of the Lockman Hole at 610 MHz and 1.4 GHz, a sample of 58 USS sources, with 610 MHz integrated fluxes above 100 microJy, is assembled. Deep infrared data at 3.6 and 4.5 micron from the Spitzer Extragalactic Representative Volume Survey (SERVS) is used to reliably identify counterparts for 48 (83%) of these sources, showing an average total magnitude of [3.6](AB)=19.8 mag. Spectroscopic redshifts for 14 USS sources, together with photometric redshift estimates, improved by the use of the deep SERVS data, for a further 19 objects, show redshifts ranging from z=0.1 to z=2.8, peaking at z~0.6 and tailing off at high redshifts. The remaining 25 USS sources, with no redshift estimate, include the faintest [3.6] magnitudes, with 10 sources undetected at 3.6 and 4.5 micron (typically [3.6]>22-23 mag, from local measurements), which suggests the likely existence of higher redshifts among the sub-mJy USS population. The comparison with the Square Kilometre Array Design Studies Simulated Skies models indicate that Fanaroff-Riley type I radio sources and radio-quiet Active Galactic Nuclei may constitute the bulk of the faintest USS population, and raises the possibility that the high efficiency of the USS technique for the selection of high redshift sources remains even at the sub-mJy level.
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