Mosaiced wide-field VLBI observations of the Lockman Hole/XMM

Abstract

Active galactic nuclei (AGN) play a decisive role in galaxy evolution, particularly so when they launch powerful jets, which reshape their surroundings. However, identifying them is difficult, since radio observations typically have a resolution 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 only the most compact non-thermal emission to be filtered from radio survey data. The observational and computational demands to do this in large surveys have, until recently, been too high to make this practical. Only the recent advent of wide-field observing techniques have enabled such observations, and we here present the results of 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, multisource self-calibration, and mosaicing. 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. The source counts 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 optical, infrared and X-ray data to enhance our data set and 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, the best fit for the remainder was obtained with a heavily extinct starburst template. However, this is due to a degeneracy in the fit, as such extinction in the templates is mimicking early-type objects. Our data suggest that the hosts of VLBI-detected sources are typically early-type or bulge-dominated galaxies.

Wide-field observations in the SDSS Stripe 82 with the European VLBI Network

“…The detection rate of 20% is consistent with other wide-field VLBI experiments carried out recently (e.g. Middelberg et al 2013). The project presented here demonstrates the ability of EVN in multiple-phase-centre experiments and paves the way for future large-scale EVN surveys of compact structures in extragalactic radio sources using the multiplephase-centre VLBI technique.…”

“…The result of the project allows us to conclude that the novel multiple-field-centre technique in combination with in-beam and nodding phase referencing offers an efficient tool for massive VLBI survey of extragalactic radio sources (e.g. Frey et al 2008;Middelberg et al 2013).…”

A Large-Scale Structure at Redshift 1.71 in the Lockman Hole

“…This expectation is not realized because there is a degeneracy in the fits: extinguished starburst templates mimic unextinguished early type templates, exactly the degeneracy we are trying to break. Middelberg et al (2013) find that, for a sample of morphologically selected early type or bulge-dominated galaxies, the best fit SED is an extinguished starburst 50% of the time. Instead we use a (U-V)z=0 -(V-J)z=0 color-color diagram to isolate quiescent galaxies in the manner of Wuyts et al (2007) and Williams et al (2009), as shown in Figure 5.…”

Multi-source self-calibration: Unveiling the microJy population of compact radio sources

“…and rotation of asymmetric antenna power patterns (see Rau et al 2009). The MSSC procedure largely follows the techniques outlined in Middelberg et al (2013) and is described below.…”

“…However, Rioja & Porcas (2000) and, in particular, Garrett et al (2004) first demonstrated the potential of employing multiple sources detected across the primary beam as inputs for the self-calibration of large wide-field VLBI data sets. A previous VLBI survey of the Lockman Hole by Middelberg et al (2013) developed the fundamentals of the technique presented here and has recently been employed in the analysis of VLBI observations of the Hubble Deep Field-North (HDF-N).…”