Enno Middelberg

DiFX-2: A More Flexible, Efficient, Robust, and Powerful Software Correlator

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 to 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 filter-bank 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.

EMU: Evolutionary Map of the Universe

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 μJy/beam) radio continuum survey of the entire Southern sky at 1.3 GHz, extending as far North as +30° 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 active galactic nuclei 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.

Deep Atlas Radio Observations of the Chandra Deep Field – South / Spitzer Wide-area Infrared Extragalactic Field

We present the first results from the Australia Telescope Large Area Survey (ATLAS), which consist of deep radio observations of a 3.7 square degree field surrounding the Chandra Deep Field South, largely coincident with the infrared Spitzer Wide-Area Extragalactic (SWIRE) Survey. We also list cross-identifications to infrared and optical photometry data from SWIRE, and ground-based optical spectroscopy. A total of 784 radio components are identified, corresponding to 726 distinct radio sources, nearly all of which are identified with SWIRE sources. Of the radio sources with measured redshifts, most lie in the redshift range 0.5-2, and include both star-forming galaxies and active galactic nuclei (AGN). We identify a rare population of infrared-faint radio sources which are bright at radio wavelengths but are not seen in the available optical, infrared, or X-ray data. Such rare classes of sources can only be discovered in wide, deep surveys such as this.We present the first results from the Australia Telescope Large Area Survey, which consists of deep radio observations of a 3.7 deg2 field surrounding the Chandra Deep Field-South, largely coincident with the infrared Spitzer Wide-Area Infrared Extragalactic (SWIRE) Survey. We also list cross-identifications to infrared and optical photometry data from SWIRE, and ground-based optical spectroscopy. A total of 784 radio components are identified, corresponding to 726 distinct radio sources, nearly all of which are identified with SWIRE sources. Of the radio sources with measured redshifts, most lie in the redshift range 0.5-2 and include both star-forming galaxies and active galactic nuclei. We identify a rare population of infrared-faint radio sources that are bright at radio wavelengths but are not seen in the available optical, infrared, or X-ray data. Such rare classes of sources can only be discovered in wide, deep surveys such as this.

Motion and properties of nuclear radio components in Seyfert galaxies seen with VLBI

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 NG C 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 � 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 int rinsic to the jet-formation process or results from deceler ation 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 s een 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 ◦ and 90 ◦ . The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium.

Radio Continuum Surveys with Square Kilometre Array Pathfinders

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), e-MERLIN (UK), VLA (USA), e-EVN (based in Europe), LOFAR (The Netherlands), MeerKAT (South Africa), and the Murchison Widefield Array. 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. To achieve these exciting scientific goals, many technical challenges must be addressed by the survey instruments. Given the limited resources of the global radio-astronomical community, it is essential that we pool our skills and knowledge. We do not have sufficient resources to enjoy the luxury of re-inventing wheels. We face significant challenges in calibration, imaging, source extraction and measurement, classification and cross-identification, redshift determination, stacking, and data-intensive research. As these instruments extend the observational parameters, we will face further unexpected challenges in calibration, imaging, and interpretation. If we are to realise the full scientific potential of these expensive instruments, it is essential that we devote enough resources and careful study to understanding the instrumental effects and how they will affect the data. We have established an SKA Radio Continuum Survey working group, whose prime role is to maximise science from these instruments by ensuring we share resources and expertise across the projects. Here we describe these projects, their science goals, and the technical challenges which are being addressed to maximise the science return.

DEEP AUSTRALIA TELESCOPE LARGE AREA SURVEY RADIO OBSERVATIONS OF THE EUROPEAN LARGE AREA ISO SURVEY S1/SPITZER WIDE-AREA INFRARED EXTRAGALACTIC FIELD

We have conducted sensitive (1 σ < 30 μJy) 1.4 GHz radio observations with the Australia Telescope Compact Array of a field largely coincident with infrared observations of the Spitzer Wide-Area Extragalactic Survey. The field is centered on the European Large Area ISO Survey S1 region and has a total area of 3.9°. We describe the observations and calibration, source extraction, and cross-matching to infrared sources. Two catalogs are presented: one of the radio components found in the image and another of radio sources with counterparts in the infrared and extracted from the literature. 1366 radio components were grouped into 1276 sources, 1183 of which were matched to infrared sources. We discover 31 radio sources with no infrared counterpart at all, adding to the class of Infrared-Faint Radio Sources.

Extending the infrared radio correlation

Co-addition of deep (rms ∼ 30 μJy) 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-μm sources in stacked images with rms < 1 μJy. We confirm that the infrared–radio correlation extends to f_(24 μm) = 100 μJy but with a significantly lower coefficient, f_(20 cm) = 0.039f_(24 μm) [q_(24) = log (f_(24 μm)/f20 cm) = 1.39 ± 0.02] than hitherto reported. We postulate that this may be due to a change in the mean q_(24) value ratio for objects with f_(24 μm) < 1 mJy.

Active Galactic Nucleus 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. Subject headings: galaxies: formation — galaxies: evolution — galaxies: active — galaxies: star formation — galaxies: jets — ISM: jets and outflows —Simulations of galaxy growth need to invoke strong negative feedback from active galactic nuclei (AGNs) 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 AGNs 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 AGNs with pronounced radio jets exhibit a much higher star formation rate (SFR) 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 SFR of radio-selected AGNs arises because of jet-induced star formation, as is suggested by the different jet powers among our AGN samples, while the suppressed SFR of X-ray selected AGN is caused by heating and photo-dissociation of molecular gas by the hot AGN accretion disk.

blobcat: software to catalogue flood-filled blobs in radio images of total intensity and linear polarization

We present BLOBCAT, new source extraction software that utilises the flood fill algorithm to detect and catalogue blobs, or islands of pixels representing sources, in twodimensional astronomical images. The software is designed to process radio-wavelength images of both Stokes I intensity and linear polarization, the latter formed through the quadrature sum of Stokes Q and U intensities or as a byproduct of rotation measure synthesis. We discuss an objective, automated method by which estimates of positiondependent background root-mean-square noise may be obtained and incorporated into BLOBCAT’s analysis. We derive and implement within BLOBCAT corrections for two systematic biases to enable the flood fill algorithm to accurately measure flux densities for Gaussian sources. We discuss the treatment of non-Gaussian sources in light of these corrections. We perform simulations to validate the flux density and positional measurement performance of BLOBCAT, and we benchmark the results against those of a standard Gaussian fitting task. We demonstrate that BLOBCAT exhibits accurate measurement performance in total intensity and, in particular, linear polarization. BLOBCAT is particularly suited to the analysis of large survey data.

Wide-field VLBA observations of the Chandra deep field South

Context. Wide-field surveys are a commonly-used method for studying thousands of objects simultaneously, to investigate, e.g., the joint evolution of star-forming galaxies and active galactic nuclei. Very long baseline interferometry (VLBI) observations can yield valuable input to such studies because they are able to identify AGN unambiguously in the moderate to high-redshift Universe. However, VLBI observations of large swaths of the sky are impractical using standard methods, because the fields of view of VLBI observations are of the order of 10 00 or less, and have therefore so far played only a minor role in galaxy evolution studies. Aims. We have embarked on a project to carry out Very Long Baseline Array (VLBA) observations of all 96 known radio sources in one of the best-studied areas in the sky, the Chandra Deep Field South (CDFS). The challenge was to develop methods which could significantly reduce the amount of observing (and post-processing) time, making such a project feasible. Methods. We have developed an extension to the DiFX software correlator which allows one to e ciently correlate up to hundreds of positions within the primary beams of the interferometer antennas. This extension enabled us to target many sources simultaneously, at full resolution and high sensitivity, using only a small amount of observing time. The combination of wide fields-of-view and high sensitivity across the field in this survey is unprecedented. Results. We have observed with the VLBA a single pointing containing the Chandra Deep Field South, in which 96 radio sources were known from previous observations with the Australia Telescope Compact Array (ATCA). From our input sample of 96 sources, 20 were detected with the VLBA, and one more source was tentatively detected. The majority of objects have flux densities in agreement with arcsec-scale observations, implying that their radio emission comes from very small regions. Two objects are visibly resolved. One VLBI-detected object had earlier been classified as a star-forming galaxy. Comparing the VLBI detections to sources found in sensitive, co-located X-ray observations we find that X-ray detections are not a good indicator for VLBI detections. Conclusions. We have successfully demonstrated a new extension to the DiFX software correlator, allowing one to observe hundreds of fields of view simultaneously. In a sensitive observation of the CDFS we detect 21 % of the sources and were able to re-classify 7 sources as AGN which had not been identified as such before. Wide-field VLBI survey science is now coming of age.