Automated Editing of Radio Interferometer Data with Pieflag

Editing of radio interferometer data, a process commonly known as ``flagging'', can be laborious and time-consuming. One quickly tends to flag more data than actually required, sacrificing sensitivity and image fidelity in the process. I describe a program, Pieflag, which can analyse radio interferometer data to filter out measurements which are likely to be affected by interference. Pieflag uses two algorithms to allow for data sets which are either dominated by receiver noise or by source structure. Together, the algorithms detect essentially all affected data whilst the amount of data which is not affected by interference but falsely marked as such is kept to a minimum. The sections marked by Pieflag are very similar to what would be deemed affected by the observer in a visual inspection of the data. Pieflag displays its results concisely and allows the user to add and remove flags interactively. It is written in Python, is easy to install and use, and has a variety of options to adjust its algorithms to a particular observing situation. I describe how Pieflag works and illustrate its effect using data from typical observations.Comment: 6 pages, 5 figures, requires apj.bst, accepted for publication in Publications of the Astronomical Society of Australi

Unveiling the origin of the radio emission in radio-quiet quasars

The origin of the radio emission in radio-quiet quasars (RQQs) has been a matter of debate for a long time. It is not well understood whether the emission is caused by star formation in the host galaxy or by black hole activity of the active galactic nuclei (AGN). We shed some light on these questions using the Very Long Baseline Interferometry (VLBI) technique to search for RQQs in the field of the Cosmological Evolution Survey (COSMOS). The extensive multi-wavelength coverage of the field (from radio to X-rays) was used to classify RQQs, and the milli-arcsecond resolution of VLBI provides a direct way to identify AGNs. In a sample of 18 RQQs we detected 3 using the Very Long Baseline Array (VLBA) at 1.4 GHz. In this letter we report for the first time on a sample of RQQs with a measured lower limit on the fraction of radio emission coming from the AGN, thus demonstrating that the radio emission of at least some RQQs is dominated by an AGN.Comment: 4 pages, letter to the editor, accepted for publication in A&

AGN feedback works both ways

Simulations of galaxy growth need to invoke strong negative feedback from active galactic nuclei (AGN) to suppress the formation of stars and thus prevent the over-production of very massive systems. While some observations provide evidence for such negative feedback, other studies find either no feedback, or even positive feedback, with increased star formation associated with higher AGN luminosities. Here we report an analysis of several hundred AGN and their host galaxies in the Chandra Deep Field South using X-ray and radio data for sample selection. Combined with archival far infrared data as a reliable tracer of star formation activity in the AGN host galaxies, we find that AGN with pronounced radio jets exhibit a much higher star formation rate than the purely X-ray selected ones, even at the same X-ray luminosities. This difference implies that positive AGN feedback plays an important role, too, and therefore has to be accounted for in all future simulation work. We interpret this to indicate that the enhanced star formation rate of radio selected AGN arises because of jet-induced star formation, as is hinted by the different jet powers among our AGN samples, while the suppressed star formation rate of X-ray selected AGN is caused by heating and photo-dissociation of molecular gas by the hot AGN accretion disc.Comment: 21 pages, 9 figures, 3 tables, accepted for publication by Ap

VLBI detection of an Infrared-Faint Radio Source

Infrared-Faint Radio Sources represent a new and unexpected class of object which is bright at radio wavelengths but unusually faint at infrared wavelengths. If, like most mJy radio sources, they were either conventional active or star-forming galaxies in the local Universe, we would expect them to be detectable at infrared wavelengths, and so their non-detection by the Spitzer Space Telescope is surprising. Here we report the detection of one of these sources using Very Long Baseline Interferometry, from which we conclude that the sources are driven by Active Galactic Nuclei. We suggest that these sources are either normal radio-loud quasars at high redshift or abnormally obscured radio galaxies.Comment: accepted by MNRA

Infrared-Faint Radio Sources are at high redshifts

Context: Infrared-Faint Radio Sources (IFRS) are characterised by relatively high radio flux densities and associated faint or even absent infrared and optical counterparts. The resulting extremely high radio-to-infrared flux density ratios up to several thousands were previously known only for High-redshift Radio Galaxies (HzRGs), suggesting a link between the two classes of object. Prior to this work, no redshift was known for any IFRS in the Australia Telescope Large Area Survey (ATLAS) fields which would help to put IFRS in the context of other classes of object, especially of HzRGs. Aims: This work aims at measuring the first redshifts of IFRS in the ATLAS fields. Further, we test the hypothesis that IFRS are similar to HzRGs, as higher-redshift or dust-obscured versions of these massive galaxies. Methods: A sample of IFRS was spectroscopically observed using the Focal Reducer and Low Dispersion Spectrograph 2 (FORS2) at the Very Large Telescope (VLT). The data were calibrated based on the Image Reduction and Analysis Facility (IRAF) and redshifts extracted. This information was then used to calculate rest-frame luminosities, and to perform the first spectral energy distribution modelling of IFRS based on redshifts. Results: We found redshifts of 1.84, 2.13, and 2.76, for three IFRS, confirming the suggested high-redshift character of this class of object. These redshifts as well as the resulting luminosities show IFRS to be similar to HzRGs. We found further evidence that fainter IFRS are at even higher redshifts. Conclusions: Considering the similarities between IFRS and HzRGs substantiated in this work, the detection of IFRS, which have a significantly higher sky density than HzRGs, increases the number of Active Galactic Nuclei in the early universe and adds to the problems of explaining the formation of supermassive black holes shortly after the Big Bang.Comment: 7 pages, 4 figures; version in prin

The triggering of local AGN and their role in regulating star formation

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.We explore the processes that trigger local AGN and the role of these AGN in regulating star formation, using ~350 nearby galaxies observed by the mJy Imaging VLBA Exploration at 20cm (mJIVE) survey. The >10^7 K brightness temperature required for an mJIVE detection cannot be achieved via star formation alone, allowing us to unambiguously detect nearby radio AGN and study their role in galaxy evolution. Radio AGN are an order of magnitude more common in early-type galaxies (ETGs) than in their late-type counterparts. The VLBI-detected ETGs in this study have a similar stellar mass distribution to their undetected counterparts, are typically not the central galaxies of clusters and exhibit merger fractions that are significantly higher than in the average ETG. This suggests that these radio AGN (which have VLBI luminosities >10^22 W Hz^-1) are primarily fuelled by mergers, and not by internal stellar mass loss or cooling flows. Our radio AGN are a factor of ~3 times more likely to reside in the UV-optical red sequence than the average ETG. Furthermore, typical AGN lifetimes (a few 10^7 yr) are much shorter than the transit times from blue cloud to red sequence (~1.5 Gyr). This indicates that the AGN are not triggered promptly and appear several dynamical timescales into the associated star formation episode, implying that they typically couple only to residual gas, at a point where star formation has already declined significantly. While evidence for AGN feedback is strong in systems where the black hole is fed by the cooling of hot gas, AGN triggered by mergers appear not to strongly regulate the associated star formation. The inability of the AGN to rapidly quench merger-driven star formation is likely to make merging the dominant mode of star formation in nearby ETGs, in line with evidence for minor mergers being the primary driver of stellar mass growth in these systems.Peer reviewe

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

Context. Very Long Baseline Interferometry (VLBI) data are extremely sensitive to the phase stability of the VLBI array. This is especially important when we reach {\mu}Jy r.m.s. sensitivities. Calibration using standard phase referencing techniques is often used to improve the phase stability of VLBI data but the results are often not optimal. This is evident in blank fields that do not have in-beam calibrators. Aims. We present a calibration algorithm termed Multi-Source Self-Calibration (MSSC) which can be used after standard phase referencing on wide-field VLBI observations. This is tested on a 1.6 GHz wide-field VLBI data set of the Hubble Deep Field-North and the Hubble Flanking Fields. Methods. MSSC uses multiple target sources detected in the field via standard phase referencing techniques and modifies the visibili- ties so that each data set approximates to a point source. These are combined to increase the signal to noise and permit self-calibration. In principle, this should allow residual phase changes caused by the troposphere and ionosphere to be corrected. By means of faceting, the technique can also be used for direction dependent calibration. Results. Phase corrections, derived using MSSC, were applied to a wide-field VLBI data set of the HDF-N comprising of 699 phase centres. MSSC was found to perform considerably better than standard phase referencing and single source self-calibration. All detected sources exhibited dramatic improvements in dynamic range. Using MSSC, one source reached the detection threshold taking the total detected sources to twenty. 60% of these sources can now be imaged with uniform weighting compared to just 45% with standard phase referencing. The Parseltongue code which implements MSSC has been released and made publicly available to the astronomical community (https://github.com/jradcliffe5/multi_self_cal).Comment: 7 pages, 4 figures, accepted to A&