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Published November 2017 | Published + Submitted
Journal Article Open

The faint radio sky: VLBA observations of the COSMOS field


Context. Quantifying the fraction of active galactic nuclei (AGN) in the faint radio population and understanding their relation with star-forming activity are fundamental to studies of galaxy evolution. Very long baseline interferometry (VLBI) observations are able to identify AGN above relatively low redshifts (z> 0.1) since they provide milli-arcsecond resolution. Aims. We have created an AGN catalogue from 2865 known radio sources observed in the Cosmic Evolution Survey (COSMOS) field, which has exceptional multi-wavelength coverage. With this catalogue we intend to study the faint radio sky with statistically relevant numbers and to analyse the AGN – host galaxy co-evolution, making use of the large amount of ancillary data available in the field. Methods. Wide-field VLBI observations were made of all known radio sources in the COSMOS field at 1.4 GHz to measure the AGN fraction, in particular in the faint radio population. We describe in detail the observations, data calibration, source detection and flux density measurements, parts of which we have developed for this survey. The combination of number of sources, sensitivity, and area covered with this project are unprecedented. Results. We have detected 468 radio sources, expected to be AGN, with the Very Long Baseline Array (VLBA). This is, to date, the largest sample assembled of VLBI detected sources in the sub-mJy regime. The input sample was taken from previous observations with the Very Large Array (VLA). We present the catalogue with additional optical, infrared and X-ray information. Conclusions. We find a detection fraction of 20 ± 1%, considering only those sources from the input catalogue which were in principle detectable with the VLBA (2361). As a function of the VLA flux density, the detection fraction is higher for higher flux densities, since at high flux densities a source could be detected even if the VLBI core accounts for a small percentage of the total flux density. As a function of redshift, we see no evolution of the detection fraction over the redshift range 0.5 1.5 we find that spiral galaxies become the most prevalent (48%). The number of detections is high enough to study the faint radio population with statistically significant numbers. We demonstrate that wide-field VLBI observations, together with new calibration methods such as multi-source self-calibration and mosaicing, result in information which is difficult or impossible to obtain otherwise.

Additional Information

© ESO, 2017. Received 12 May 2017; Accepted 13 July 2017; Published online 28 November 2017. The full Tables 2 and 3 are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr ( or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/607/A132. N.H.R. acknowledges support from the Deutsche Forschungsgemeinschaft through project MI 1230/4-1. V.S. and I.D. acknowledge the European Union's Seventh Framework programme under grant agreement 337595 (ERC Starting Grant, "CoSMass"). P.N.B. is grateful for support from STFC via grant ST/M001229/1. We wish to thank the anonymous referee for the helpful comments, which have improved this paper. This research made use of Topcat (Taylor 2005), available at http://www.starlink.ac.uk/topcat/. This research also made use of APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com, and Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013). We wish to thank the staff of the VLBA who greatly supported the experimental observations in this project. The VLBA is operated by the Long Baseline Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

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Published - aa31163-17.pdf

Submitted - 1707.07512.pdf


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