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Register a new userPolarisation of mJy radio sources
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Predictions of the number of faint polarised radio sources that can be detected by SKA pathfinder telescopes and the SKA depend on the polarisation properties of radio sources with a total flux density around 1 mJy. Total intensity source counts suggest a transition in the dominant population from AGN to galaxies around this flux density, and the properties of brighter radio sources may not be representative for this fainter population. We show that unresolved spiral galaxies can be highly polarised radio sources, up to ~ 20% polarised at 4.8 GHz. This result is partly based on observations of nearby galaxies, including galaxies with significant deviations from axial symmetry and other peculiarities. A first analysis of polarised source counts divided into steep-spectrum AGN, flat-spectrum AGN and star forming galaxies is presented, including a prediction of polarised source counts to microjansky levels.
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We investigate the star formation properties of ~800 sources detected in one of the deepest radio surveys at 1.4 GHz. Our sample spans a wide redshift range (~0.1 - 4) and about four orders of magnitude in star formation rate (SFR). It includes both star forming galaxies (SFGs) and active galactic nuclei (AGNs), further divided into radio-quiet and radio-loud objects. We compare the SFR derived from the far infrared luminosity, as traced by Herschel, with the SFR computed from their radio emission. We find that the radio power is a good SFR tracer not only for pure SFGs but also in the host galaxies of RQ AGNs, with no significant deviation with redshift or specific SFR. Moreover, we quantify the contribution of the starburst activity in the SFGs population and the occurrence of AGNs in sources with different level of star formation. Finally we discuss the possibility of using deep radio survey as a tool to study the cosmic star formation history.
The up-turn in Euclidean normalised source counts below 1mJy at 1.4GHz is well established in many deep radio surveys. There are strong reasons, observationally and theoretically, to believe that this up-turn is due to strong evolution of the starforming population up to z=2. However this hypothesis needs further confirmation spectroscopically and the examples in the literature are sparse. Theoretically the up-turn is well modelled by the evolution of the local radio starforming population and is consistent with the up-turn seen in recent mid-infrared source counts at 15um (ISOCAM) and 24um (Spitzer) and the tight correlation of the radio and MIR Luminosities of starforming galaxies.
Inhomogeneities can influence the polarisation emerging from a synchrotron source. However, it is shown that the frequency distribution of circular polarisation is only marginally affected, although its magnitude may change substantially. This is used to argue that the observed properties of compact radio sources imply a radiating plasma in which the characteristic waves are nearly circular. As a result, restrictions can be put on the low energy part of the energy distribution of the relativistic electrons as well as the presence of electron-positron pairs. It is emphasised that this constrains theoretical modelling of the acceleration process for the relativistic electrons; for example, some of the currently popular scenarios seem to need modifications to become consistent with observations.
We present the evolutionary properties and luminosity functions of the radio sources belonging to the Chandra Deep Field South VLA survey, which reaches a flux density limit at 1.4 GHz of 43 microJy at the field center and redshift ~5, and which includes the first radio-selected complete sample of radio-quiet active galactic nuclei (AGN). We use a new, comprehensive classification scheme based on radio, far- and near-IR, optical, and X-ray data to disentangle star-forming galaxies from AGN and radio-quiet from radio-loud AGN. We confirm our previous result that star-forming galaxies become dominant only below 0.1 mJy. The sub-mJy radio sky turns out to be a complex mix of star-forming galaxies and radio-quiet AGN evolving at a similar, strong rate; non-evolving low-luminosity radio galaxies; and declining radio powerful (P > 3 10^24 W/Hz) AGN. Our results suggest that radio emission from radio-quiet AGN is closely related to star formation. The detection of compact, high brightness temperature cores in several nearby radio-quiet AGN can be explained by the co-existence of two components, one non-evolving and AGN-related and one evolving and star-formation-related. Radio-quiet AGN are an important class of sub-mJy sources, accounting for ~30% of the sample and ~60% of all AGN, and outnumbering radio-loud AGN at < 0.1 mJy. This implies that future, large area sub-mJy surveys, given the appropriate ancillary multi-wavelength data, have the potential of being able to assemble vast samples of radio-quiet AGN by-passing the problems of obscuration, which plague the optical and soft X-ray bands.
We present the results of a multi-wavelength study of the 19 most significant sub-mm sources detected in the SCUBA 8-mJy survey. As described in Scott et al. (2001), this survey covers ~260 arcmin^2 using the sub-millimetre camera SCUBA, to a limiting source detection limit S(850um) ~ 8 mJy. One advantage of this relatively bright flux-density limit is that accurate astrometric positions are potentially achievable for every source using existing radio and/or mm-wave interferometers. However, an associated advantage is that SED-based redshift constraints should be more powerful than in fainter sub-mm surveys. Here we therefore exploit the parallel SCUBA 450um data, in combination with existing radio and ISO data at longer and shorter wavelengths to set constraints on the redshift of each source. We also analyse new and existing optical and near-infrared imaging of our SCUBA survey fields to select potential identifications consistent with these constraints. Our derived SED-based redshift constraints, and the lack of statistically significant associations with even moderately bright galaxies allow us to conclude that all 19 sources lie at z > 1, and at least half of them apparently lie at z > 2.
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