Publications (27)4.9 Total impact
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Article: Radio Continuum Surveys with Square Kilometre Array Pathfinders
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ABSTRACT: In the lead-up to the Square Kilometre Array (SKA) project, several next-generation radio telescopes and upgrades are already being built around the world. These include APERTIF (The Netherlands), ASKAP (Australia), eMERLIN (UK), VLA (USA), e-EVN (based in Europe), LOFAR (The Netherlands), Meerkat (South Africa), and the Murchison Widefield Array (MWA). Each of these new instruments has different strengths, and coordination of surveys between them can help maximise the science from each of them. A radio continuum survey is being planned on each of them with the primary science objective of understanding the formation and evolution of galaxies over cosmic time, and the cosmological parameters and large-scale structures which drive it. In pursuit of this objective, the different teams are developing a variety of new techniques, and refining existing ones. Here we describe these projects, their science goals, and the technical challenges which are being addressed to maximise the science return.10/2012; -
Article: The ATLAS 5.5 GHz survey of the Extended Chandra Deep Field South: Catalogue, Source Counts and Spectral Indices
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ABSTRACT: Star forming galaxies are thought to dominate the sub-mJy radio population, but recent work has shown that low luminosity AGN can still make a significant contribution to the faint radio source population. Spectral indices are an important tool for understanding the emission mechanism of the faint radio sources. We have observed the extended Chandra Deep Field South at 5.5 GHz using a mosaic of 42 pointings with the Australia Telescope Compact Array (ATCA). Our image reaches an almost uniform sensitivity of ~12 microJy rms over 0.25 deg^2 with a restoring beam of 4.9 x 2.0 arcsec, making it one of the deepest 6cm surveys to date. We present the 5.5 GHz catalogue and source counts from this field. We take advantage of the large amounts of ancillary data in this field to study the 1.4 to 5.5 GHz spectral indices of the sub-mJy population. For the full 5.5 GHz selected sample we find a flat median spectral index, alpha_med = -0.40, which is consistent with previous results. However, the spectral index appears to steepen at the faintest flux density levels (S_{5.5 GHz} < 0.1 mJy), where alpha_med = -0.68. We performed stacking analysis of the faint 1.4 GHz selected sample (40 < S_{1.4 GHz} < 200 microJy) and also find a steep average spectral index, alpha = -0.8, consistent with synchrotron emission. We find a weak trend of steepening spectral index with redshift. Several young AGN candidates are identified using spectral indices, suggesting Gigahertz Peaked Spectrum (GPS) sources are as common in the mJy population as they are at Jy levels.08/2012; -
Article: A wider audience: Turning VLBI into a survey instrument
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ABSTRACT: Radio observations using the Very Long Baseline Interferometry (VLBI) technique typically have fields of view of only a few arcseconds, due to the computational problems inherent in imaging larger fields. Furthermore, sensitivity limitations restrict observations to very compact and bright objects, which are few and far between on the sky. Thus, while most branches of observational astronomy can carry out sensitive, wide-field surveys, VLBI observations are limited to targeted observations of carefully selected objects. However, recent advances in technology have made it possible to carry out the computations required to target hundreds of sources simultaneously. Furthermore, sensitivity upgrades have dramatically increased the number of objects accessible to VLBI observations. The combination of these two developments have enhanced the survey capabilities of VLBI observations such that it is now possible to observe (almost) any point in the sky with milli-arcsecond resolution. In this talk I review the development of wide-field VLBI, which has made significant progress over the last three years.07/2012; -
Article: EMU: Evolutionary Map of the Universe
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ABSTRACT: EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The primary goal of EMU is to make a deep (rms ~ 10 microJy/beam) radio continuum survey of the entire Southern Sky at 1.3 GHz, extending as far North as +30 degrees declination, with a resolution of 10 arcsec. EMU is expected to detect and catalogue about 70 million galaxies, including typical star-forming galaxies up to z~1, powerful starbursts to even greater redshifts, and AGNs to the edge of the visible Universe. It will undoubtedly discover new classes of object. This paper defines the science goals and parameters of the survey, and describes the development of techniques necessary to maximise the science return from EMU.06/2011; -
Article: Are the infrared-faint radio sources pulsars?
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ABSTRACT: Infrared-Faint Radio Sources (IFRS) are objects which are strong at radio wavelengths but undetected in sensitive Spitzer observations at infrared wavelengths. Their nature is uncertain and most have not yet been associated with any known astrophysical object. One possibility is that they are radio pulsars. To test this hypothesis we undertook observations of 16 of these sources with the Parkes Radio Telescope. Our results limit the radio emission to a pulsed flux density of less than 0.21 mJy (assuming a 50% duty cycle). This is well below the flux density of the IFRS. We therefore conclude that these IFRS are not radio pulsars.03/2011; -
Article: Spectral index properties of milliJansky radio sources
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ABSTRACT: At the faintest radio flux densities (S 1.4 < 10 mJy), conflicting results have arisen regard-ing whether there is a flattening of the average spectral index between a low radio frequency (325 or 610 MHz), and e.g. 1.4 GHz. We present a new catalogue of 843 MHz radio sources in the ELAIS-S1 field that contains the sources, their ATLAS counterparts, and the spectral index distributions of the sources as a function of flux density. We do not find any statistically significant evidence for a trend towards flatter spectral indices with decreasing flux density. We then investigate the spectral index distribution with redshift for those sources with reli-able redshifts and explore the infrared properties. An initial sample of faint Compact Steep Spectrum sources in ATLAS is also presented, with a brief overview of their properties.Mon. Not. R. Astron. Soc. 02/2011; 000:1-18. -
Article: DiFX2: A more flexible, efficient, robust and powerful software correlator
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ABSTRACT: Software correlation, where a correlation algorithm written in a high-level language such as C++ is run on commodity computer hardware, has become increasingly attractive for small to medium sized and/or bandwidth constrained radio interferometers. In particular, many long baseline arrays (which typically have fewer than 20 elements and are restricted in observing bandwidth by costly recording hardware and media) have utilized software correlators for rapid, cost-effective correlator upgrades to allow compatibility with new, wider bandwidth recording systems and improve correlator flexibility. The DiFX correlator, made publicly available in 2007, has been a popular choice in such upgrades and is now used for production correlation by a number of observatories and research groups worldwide. Here we describe the evolution in the capabilities of the DiFX correlator over the past three years, including a number of new capabilities, substantial performance improvements, and a large amount of supporting infrastructure to ease use of the code. New capabilities include the ability to correlate a large number of phase centers in a single correlation pass, the extraction of phase calibration tones, correlation of disparate but overlapping sub-bands, the production of rapidly sampled filterbank and kurtosis data at minimal cost, and many more. The latest version of the code is at least 15% faster than the original, and in certain situations many times this value. Finally, we also present detailed test results validating the correctness of the new code.01/2011; -
Article: VLBI imaging throughout the primary beam using accurate UV shifting
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ABSTRACT: For Very Long Baseline Interferometry (VLBI), the fringe spacing is extremely narrow compared to the field of view imposed by the primary beam of each element. This means that an extremely large number of resolution units can potentially be imaged from a single observation. We implement and test a technique for efficiently and accurately imaging large VLBI datasets. The DiFX software correlator is used to generate a dataset with extremely high time and frequency resolution. This large dataset is then transformed and averaged multiple times to generate many smaller datasets, each with a phase centre located at a different area of interest. Results of an 8.4 GHz four-station VLBI observation of a field containing multiple sources are presented. Observations of the cali-brator 3C 345 were used for preliminary tests of accuracy of the shifting algorithm. A high level of accuracy was achieved, making the method suitable even for the most demanding astrometric VLBI observations. One target source (1320+299A) was detected and was used as a phase-reference calibrator in searching for further detections. An image containing 13 billion pixels was constructed by independently imaging 782 visibility datasets covering the entire primary beam of the array. Current implementations of this algorithm and possible future developments in VLBI data analysis are discussed.A&A. 01/2011; 526(140). -
Article: Evidence for Infrared-Faint Radio Sources as z > 1 Radio-Loud AGN
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ABSTRACT: Infrared-Faint Radio Sources (IFRSs) are a class of radio objects found in the Australia Telescope Large Area Survey (ATLAS) which have no observable mid-infrared counterpart in the Spitzer Wide-area Infrared Extragalactic (SWIRE) survey. The extended Chandra Deep Field South now has even deeper Spitzer imaging (3.6 to 70 micron) from a number of Legacy surveys. We report the detections of two IFRS sources in IRAC images. The non-detection of two other IFRSs allows us to constrain the source type. Detailed modeling of the SED of these objects shows that they are consistent with high redshift (z > 1) AGN.01/2010; -
Article: WATs In ATLAS
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ABSTRACT: Wide-Angle-Tailed (WAT) radio sources are known to signpost dynamically unrelaxed or merging clusters. We present a sample of six WATs identified in the 1.4 GHz ATLAS radio survey. Two of the WATs in our sample have sufficient spectroscopic data to confirm the existence of an overdensity, or cluster associated with the WAT. In one case, the overdensity covers an unusually large area ( 12 Mpc) and has a large velocity dispersion (4500 km/s). The WATs all reside in the redshift interval (0.1 < z < 1) that appears to be a transition period between the well-understood low-redshift clusters and the higher-redshift large-scale structures. We also discuss the implications of this for the future large-scale radio surveys planned with SKA pathfinder telescopes such as the Australian SKA Pathfinder (ASKAP) and their value in detecting large scale overdensities at high redshifts.12/2009; 42:517. -
Article: The ATLAS Survey of the CDFS and ELAIS-S1 Fields
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ABSTRACT: The first phase of the ATLAS (Australia Telescope Large Area Survey) project surveyed a total 7 square degrees down to 30 micro Jy rms at 1.4 GHz and is the largest sensitive radio survey ever attempted. We report on the scientific achievements of ATLAS to date and plans to extend the project as a path finder for the proposed EMU (Evolutionary map of the Universe) project which has been designed to use ASKAP (Australian Square Kilometre Array Pathfinder). Comment: 4 pages, 3 figures09/2009; -
Article: Resolving the jet in Cygnus A
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ABSTRACT: Our previous studies revealed a good kinematic model for the jet of Cygnus A, but the counter-jet speed is still not well constrained. The central engine and part of the counter-jet of Cyg A are likely to be obscured by free-free absorbing material, presumably a thick torus. At mm-wavelengths, the absorber becomes optically thin, which provides a more detailed view into the inner nuclear region. Knowing the speed of jet and counter-jet and their flux density ratio allows to determine the jet Lorentz factors and orientation. Therefore we started to monitor Cyg A with global VLBI at 43GHz in Oct. 2007. Our first epoch reveals a previously unseen gap between both jets. This could be either a sign for a new counter-jet component that is slowly separating or we start to see the very inner acceleration region of the jet which is not efficiently radiating at radio wavelengths. Further more the image shows transversely resolved jet structures at distances beyond ~0.5pc which facilitate more detailed investigations addressing jet stratification. Analysis of the resolved jet structure shows that the initially wide jet (opening angle ~10deg) collimates within the first parsec into a edge-brightened jet with an opening angle of ~3deg.01/2009; -
Article: The first VLBI image of an Infrared-Faint Radio Source
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ABSTRACT: Context: To investigate the joint evolution of active galactic nuclei and star formation in the Universe. Aims: In the 1.4 GHz survey with the Australia Telescope Compact Array of the Chandra Deep Field South and the European Large Area ISO Survey - S1 we have identified a class of objects which are strong in the radio but have no detectable infrared and optical counterparts. This class has been called Infrared-Faint Radio Sources, or IFRS. 53 sources out of 2002 have been classified as IFRS. It is not known what these objects are. Methods: To address the many possible explanations as to what the nature of these objects is we have observed four sources with the Australian Long Baseline Array. Results: We have detected and imaged one of the four sources observed. Assuming that the source is at a high redshift, we find its properties in agreement with properties of Compact Steep Spectrum sources. However, due to the lack of optical and infrared data the constraints are not particularly strong. Comment: Accepted for publication in Astronomy and Astrophysics, 5 pages, needs aa.cls09/2008; -
Chapter: A Long Overdue Synthesis Image of Centaurus A
12/2007: pages 287-288; -
Article: Jet–cloud collisions in the jet of the Seyfert galaxy NGC 3079
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ABSTRACT: We report the results from a 6-yr, multi-epoch very long baseline interferometry monitoring of the Seyfert galaxy NGC 3079. We have observed NGC 3079 during eight epochs between 1999 and 2005 predominantly at 5 GHz, but covering the frequency range of 1.7 to 22 GHz. Using our data and observations going back to 1985, we find that the separation of two of the three visible nuclear radio components underwent two decelerations. At the time of these decelerations, the flux density of one of the components increased by factors of 5 and 2, respectively. We interpret these events as a radio jet component undergoing compression, possibly as a result of a collision with interstellar medium material. This interpretation strongly supports the existence of jets surrounded by a clumpy medium of dense clouds within the first few parsec from the central engine in NGC 3079. Moreover, based on recently published simulations of jet interactions with clumpy media, this scenario is able to explain the nature of two additional regions of ageing synchrotron material detected at the lower frequencies as by-products of such interactions, and also the origin of the kpc-scale super-bubble observed in NGC 3079 as the result of the spread of the momentum of the jets impeded from propagating freely. The generalization of this scenario provides an explanation why jets in Seyfert galaxies are not able to propagate to scales of kpc as do jets in radio-loud AGN.Monthly Notices of the Royal Astronomical Society 05/2007; 377(2):731 - 740. · 4.90 Impact Factor -
Article: Extending the infrared radio correlation
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ABSTRACT: Co-addition of deep (rms about 30 microJy) 20 cm data obtained with the Australia Telescope Compact Array at the location of Spitzer Wide field survey (SWIRE) sources has yielded statistics of radio source counterparts to faint 24 micron sources in stacked images with rms < 1 microJy. We confirm that the infrared-radio correlation extends to f(24micron) = 100 microJy but with a significantly lower coefficient, f (20cm) = 0.039 f(24micron) (or q24 = 1.39) than hitherto reported. We postulate that this may be due to a change in the mean q24 value ratio for objects with f(24micron) < 1mJy.02/2007; -
Article: VLBI observations of weak sources using fast frequency switching
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ABSTRACT: We have developed a new phase referencing technique for high frequency VLBI observations. In conventional phase referencing, one interleaves short scans on a nearby phase calibrator between the target source observations. In fast frequency switching described here, one observes the target source continuously while switching rapidly between the target frequency and a lower reference frequency. We demonstrate that the technique allows phase calibration almost reaching the thermal noise limit and present the first detection of the AGN in the FR I radio galaxy NGC 4261 at 86 GHz. Although point-like, this is the weakest source ever detected with VLBI at this frequency. Comment: Accepted by A&A, 14 pages, 12 figures, needs aa.cls, aas_macros.sty and amsmath.sty, replaced due reformatting12/2004; -
Article: VLBI detections of a source weaker than 100 mJy at 86 GHz
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ABSTRACT: We use a new phase-calibration strategy to calibrate the phase of 86 GHz VLBI observations of the FR I radio galaxy NGC 4261. Instead of switching between a calibrator source and the target source, the target was observed while rapidly switching between the target frequency and a lower reference frequency. Self-calibration at the reference frequency yielded phase corrections which were multiplied with the frequency ratio and applied to the target frequency visibilities. The resulting detection of NGC 4261 is, to our knowledge, the first of NGC 4261 with 86 GHz VLBI, and it is also the weakest source so far detected with VLBI at that frequency.10/2004; -
Article: Spectral Properties of the Core and the VLBI-Jets of Cygnus A
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ABSTRACT: We present a detailed VLBI study of the spectral properties of the inner core region of the radio galaxy Cygnus A at 5 GHz, 15 GHz, 22 GHz, 43 GHz and 86 GHz. Our observations include an epoch using phase-referencing at 15 GHz and 22 GHz and the first successful VLBI observations of Cygnus A at 86 GHz. We find a pronounced two-sided jet structure, with a steep spectrum along the jet and an inverted spectrum towards the counter-jet. The inverted spectrum and the frequency-dependent jet-to-counter-jet ratio suggest that the inner counter-jet is covered by a circum-nuclear absorber as it is proposed by the unified scheme.10/2004; -
Article: Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI
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ABSTRACT: We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the Seyfert galaxies NGC 7674, NGC 5506, NGC 2110 and Mrk 1210 to study their structure and proper motions on pc scales and to add some constraints on the many possible causes of the radio-quietness of Seyferts. The component configurations in NGC 7674 and NGC 2110 are simple, linear structures, whereas the configurations in NGC 5506 and Mrk 1210 have multiple components with no clear axis of symmetry. We suggest that NGC 7674 is a low-luminosity compact symmetric object. Comparing the images at different epochs, we find a proper motion in NGC 7674 of (0.92+-0.07) c between the two central components separated by 282 pc and, in NGC 5506, we find a 3 sigma upper limit of 0.50 c for the components separated by 3.8 pc. Our results confirm and extend earlier work showing that the outward motion of radio components in Seyfert galaxies is non-relativistic on pc scales. We briefly discuss whether this non-relativistic motion is intrinsic to the jet-formation process or results from deceleration of an initially relativistic jet by interaction with the pc or sub-pc scale interstellar medium. We combined our sample with a list compiled from the literature of VLBI observations made of Seyfert galaxies, and found that most Seyfert nuclei have at least one flat-spectrum component on the VLBI scale, which was not seen in the spectral indices measured at arcsec resolution. We found also that the bimodal alignment of pc and kpc radio structures displayed by radio galaxies and quasars is not displayed by this sample of Seyferts, which shows a uniform distribution of misalignment between 0 deg and 90 deg. The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium. Comment: 21 pages, 12 figures, accepted by A&A02/2004;
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2007
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Spanish National Research Council
- Andalusian Astrophysics Institute
Madrid, Madrid, Spain
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