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Register a new userThe XXL Survey : XXVI. Optical and near-infrared identifications of the ATCA 2.1 GHz radio sources in the XXL-S Field
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In this paper we present the optical, near-infrared (NIR) and X-ray identifications of the 6287 radio sources detected in the 2.1 GHz deep radio survey down to a median rms of ~ 41microJy/beam obtained with the Australia Telescope Compact Array (ATCA) in the XXL-S field. The goal of this paper is to provide a multi wavelength catalogue of the counterparts of the radio sources to be used in further studies. For the optical and NIR identification of the radio sources, we used the likelihood ratio (LR) technique, slightly modified in order to take into account the presence of a large number of relatively bright counterparts close to the radio sources. This procedure led to the identification of optical/NIR counterparts for 4770 different radio sources (~77% of the whole radio sample), 414 of which also have an X-ray counterpart. This fraction of identification is in agreement with previous radio-optical association studies at a similar optical magnitude depth, but is relatively low in comparison to recent work conducted in other radio fields using deeper optical and NIR data. The analysis of optical and NIR properties of radio sources shows that, regardless of the radio flux limit of a radio survey, the nature of the identified sources is strongly dependent on the depth of the optical/NIR used in the identification process. Only with deep enough optical/NIR data will we be able to identify a significant fraction of radio sources with red (z_{DEC}-K) counterparts whose radio emission is dominated by nuclear activity rather than starburst activity.
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Aims: We investigate the properties of the polarised radio population in the central 6.5 deg$^{2}$ of the XXL-South field observed at 2.1 GHz using the Australia Telescope Compact Array (ATCA) in 81 pointings with a synthesised beam of FWHM 5.2. We also investigate the ATCAs susceptibility to polarisation leakage. Methods: We performed a survey of a 5.6 deg$^{2}$ subregion and calculated the number density of polarised sources. We derived the total and polarised spectral indices, in addition to comparing our source positions with those of X-ray-detected clusters. We measured the polarisation of sources in multiple pointings to examine leakage in the ATCA. Results: We find 39 polarised sources, involving 50 polarised source components, above a polarised flux density limit of 0.2 mJy at 1.332 GHz. The number density of polarised source components is comparable with recent surveys, although there is an indication of an excess at $sim1$ mJy. We find that those sources coincident with X-ray clusters are consistent in their properties with regard to the general population. In terms of the ATCA leakage response, we find that ATCA mosaics with beam separation of $lesssim 2/3$ of the primary beam FWHM have off-axis linear polarisation leakage $lesssim 1.4$ % at 1.332 GHz.
This work is part of a series of studies focusing on the environment and the properties of the X-ray selected active galactic nuclei (AGN) population from the XXL survey. The present survey, given its large area, continuity, extensive multiwavelength coverage, and large-scale structure information, is ideal for this kind of study. Here, we focus on the XXL-South (XXL-S) field. Our main aim is to study the environment of the various types of X-ray selected AGN and investigate its possible role in AGN triggering and evolution. We studied the large-scale (>1 Mpc) environment up to redshift z=1 using the nearest neighbour distance method to compare various pairs of AGN types. We also investigated the small-scale environment (<0.4 Mpc) by calculating the local overdensities of optical galaxies. In addition, we built a catalogue of AGN concentrations with two or more members using the hierarchical clustering method and we correlated them with the X-ray galaxy clusters detected in the XXL survey. It is found that radio detected X-ray sources are more obscured than non-radio ones, though not all radio sources are obscured AGN. We did not find any significant differences in the large-scale clustering between luminous and faint X-ray AGN, or between obscured and unobscured ones, or between radio and non-radio sources. At local scales (<0.4 Mpc), AGN typically reside in overdense regions, compared to non-AGN; however, no differences were found between the various types of AGN. A majority of AGN concentrations with two or more members are found in the neighbourhood of X-ray galaxy clusters within <25-45 Mpc. Our results suggest that X-ray AGN are typically located in supercluster filaments, but they are also found in over- and underdense regions.
We present a catalogue of a large sample of extended radio sources in the SCORPIO field, observed and resolved by the Australia Telescope Compact Array. SCORPIO, a pathfinder project for addressing the early operations of the Australia SKA Pathfinder, is a survey of ~5 square degrees between 1.4 and 3.1 GHz, centered at l=343.5{deg}, b=0.75{deg} and with an angular resolution of about 10 arcsec. It is aimed at understanding the scientific and technical challenges to be faced by future Galactic surveys. With a mean sensitivity around 100 $mu$Jy/beam and the possibility to recover angular scales at least up to 4 arcmin, we extracted 99 extended sources, 35 of them detected for the first time. Among the 64 known sources 55 had at least a tentative classification in literature. Studying the radio morphology and comparing the radio emission with infrared we propose as candidates 6 new H II regions, 2 new planetary nebulae, 2 new luminous blue variable or Wolf--Rayet stars and 3 new supernova remnants. This study provides an overview of the potentiality of future radio surveys in terms of Galactic source extraction and characterization and a discussion on the difficulty to reduce and analyze interferometric data on the Galactic plane.
We present a catalogue containing the redshifts of 3,660 X-ray selected targets in the XXL southern field. The redshifts were obtained with the AAOmega spectrograph and 2dF fibre positioner on the Anglo-Australian Telescope. The catalogue contains 1,515 broad line AGN, 528 stars, and redshifts for 41 out of the 49 brightest X-ray selected clusters in the XXL southern field.
The main goal of this study is to investigate the LF of a sample of 142 X-ray selected clusters, with spectroscopic redshift confirmation and a well defined selection function, spanning a wide redshift and mass range, and to test the LF dependence on cluster global properties, in a homogeneous and unbiased way. Our study is based on the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) photometric galaxy catalogue,associated with photometric redshifts. We constructed LFs inside a scaled radius using a selection in photometric redshift around the cluster spectroscopic redshift in order to reduce projection effects. The width of the photometric redshift selection was carefully determined to avoid biasing the LF and depended on both the cluster redshift and the galaxy magnitudes. The purity was then enhanced by applying a precise background subtraction. We constructed composite luminosity functions (CLFs) by stacking the individual LFs and studied their evolution with redshift and richness, analysing separately the brightest cluster galaxy (BCG) and non-BCG members. We fitted the dependences of the CLFs and BCG distributions parameters with redshift and richness conjointly in order to distinguish between these two effects. We find that the usual photometric redshift selection methods can bias the LF estimate if the redshift and magnitude dependence of the photometric redshift quality is not taken into account. Our main findings concerning the evolution of the galaxy luminosity distribution with redshift and richness are that, in the inner region of clusters and in the redshift-mass range we probe (about $0<z<1$ and $10^{13} M_{odot}<M_{500}<5times10^{14}M_{odot}$), the bright part of the LF (BCG excluded) does not depend much on mass or redshift except for its amplitude, whereas the BCG luminosity increases both with redshift and richness, and its scatter decreases with redshift.
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