ORCID Profile
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Astronomical and Space Sciences | Astronomical and Space Instrumentation | Geomatic Engineering | Galactic Astronomy | Geodesy | Cosmology and Extragalactic Astronomy
Expanding Knowledge in the Physical Sciences | Expanding Knowledge in the Earth Sciences |
Publisher: Oxford University Press (OUP)
Date: 18-10-2012
Publisher: Oxford University Press (OUP)
Date: 30-04-2021
Abstract: We present the discovery of another odd radio circle (ORC) with the Australian Square Kilometre Array Pathfinder (ASKAP) at 944 MHz. The observed radio ring, ORC J0102–2450, has a diameter of ∼70 arcsec or 300 kpc, if associated with the central elliptical galaxy DES J010224.33–245039.5 (z ∼ 0.27). Considering the overall radio morphology (circular ring and core) and lack of ring emission at non-radio wavelengths, we investigate if ORC J0102–2450 could be the relic lobe of a giant radio galaxy seen end on or the result of a giant blast wave. We also explore possible interaction scenarios, for ex le, with the companion galaxy, DES J010226.15–245104.9, located in or projected on to the south-eastern part of the ring. We encourage the search for further ORCs in radio surveys to study their properties and origin.
Publisher: Oxford University Press (OUP)
Date: 11-01-2023
Abstract: We report the discovery of a unique object in the MeerKAT Galaxy Cluster Legacy Survey (MGCLS) using the machine learning anomaly detection framework astronomaly. This strange, ring-like source is 30′ from the MGCLS field centred on Abell 209, and is not readily explained by simple physical models. With an assumed host galaxy at redshift 0.55, the luminosity (1025 W Hz−1) is comparable to powerful radio galaxies. The source consists of a ring of emission 175 kpc across, quadrilateral enhanced brightness regions bearing resemblance to radio jets, two ‘ears’ separated by 368 kpc, and a diffuse envelope. All of the structures appear spectrally steep, ranging from −1.0 to −1.5. The ring has high polarization (25 per cent) except on the bright patches (& per cent). We compare this source to the Odd Radio Circles recently discovered in ASKAP data and discuss several possible physical models, including a termination shock from starburst activity, an end-on radio galaxy, and a supermassive black hole merger event. No simple model can easily explain the observed structure of the source. This work, as well as other recent discoveries, demonstrates the power of unsupervised machine learning in mining large data sets for scientifically interesting sources.
Publisher: Oxford University Press (OUP)
Date: 20-12-2012
DOI: 10.1093/MNRAS/STS470
Publisher: Oxford University Press (OUP)
Date: 13-11-2018
Publisher: Oxford University Press (OUP)
Date: 12-02-2022
Abstract: We present a method for applying spatially resolved adiabatic and radiative loss processes to synthetic radio emission from hydrodynamic simulations of radio sources from active galactic nuclei (AGNs). Lagrangian tracer particles, each representing an ensemble of electrons, are injected into simulations and the position, grid pressure, and time since the last strong shock are recorded. These quantities are used to track the losses of the electron packet through the radio source in a manner similar to the Radio AGN in Semi-analytic Environments formalism, which uses global source properties to calculate the emissivity of each particle ex situ. Freedom in the choice of observing parameters, including redshift, is provided through the post-processing nature of this approach. We apply this framework to simulations of jets in different environments, including asymmetric ones. We find a strong dependence of radio source properties on frequency and redshift, in good agreement with observations and previous modelling work. There is a strong evolution of radio spectra with redshift due to the more prominent inverse-Compton losses at high redshift. Radio sources in denser environments have flatter spectral indices, suggesting that spectral index asymmetry may be a useful environment tracer. We simulate intermediate Mach number jets that disrupt before reaching the tip of the lobe, and find that these retain an edge-brightened Fanaroff–Riley Type II morphology, with the most prominent emission remaining near the tip of the lobes for all environments and redshifts we study.
Publisher: Oxford University Press (OUP)
Date: 05-10-2016
Publisher: Oxford University Press (OUP)
Date: 11-10-2006
Publisher: Oxford University Press (OUP)
Date: 18-11-2022
Abstract: We present an analytical model for the evolution of extended active galactic nuclei (AGNs) throughout their full lifecycle, including the initial jet expansion, lobe formation, and eventual remnant phases. A particular focus of our contribution is on the early jet expansion phase, which is traditionally not well captured in analytical models. We implement this model within the Radio AGN in Semi-Analytic Environments (RAiSE) framework, and find that the predicted radio source dynamics are in good agreement with hydrodynamic simulations of both low-powered Fanaroff-Riley Type-I and high-powered Type-II radio lobes. We construct synthetic synchrotron surface brightness images by complementing the original RAiSE model with the magnetic field and shock-acceleration histories of a set of Lagrangian tracer particles taken from an existing hydrodynamic simulation. We show that a single set of particles is sufficient for an accurate description of the dynamics and observable features of Fanaroff-Riley Type-II radio lobes with very different jet parameters and ambient density profile normalizations. Our new model predicts that the lobes of young (≲10 Myr) sources will be both longer and brighter than expected at the same age from existing analytical models, which lack a jet-dominated expansion phase this finding has important implications for interpretation of radio galaxy observations. The RAiSE code, written in python, is publicly available on github and pypi.
Publisher: American Astronomical Society
Date: 24-08-2012
Publisher: Oxford University Press (OUP)
Date: 14-12-2021
Abstract: Active galactic nuclei (AGN) are typically identified through radio, mid-infrared, or X-ray emission or through the presence of broad and/or narrow emission lines. AGN can also leave an imprint on a galaxy’s spectral energy distribution (SED) through the re-processing of photons by the dusty torus. Using the SED fitting code ProSpect with an incorporated AGN component, we fit the far-ultraviolet to far-infrared SEDs of ∼494 000 galaxies in the D10-COSMOS field and ∼230 000 galaxies from the GAMA survey. By combining an AGN component with a flexible star formation and metallicity implementation, we obtain estimates for the AGN luminosities, stellar masses, star formation histories, and metallicity histories for each of our galaxies. We find that ProSpect can identify AGN components in 91 per cent of galaxies pre-selected as containing AGN through narrow-emission line ratios and the presence of broad lines. Our ProSpect-derived AGN luminosities show close agreement with luminosities derived for X-ray selected AGN using both the X-ray flux and previous SED fitting results. We show that incorporating the flexibility of an AGN component when fitting the SEDs of galaxies with no AGN has no significant impact on the derived galaxy properties. However, in order to obtain accurate estimates of the stellar properties of AGN host galaxies, it is crucial to include an AGN component in the SED fitting process. We use our derived AGN luminosities to map the evolution of the AGN luminosity function for 0 & z & 2 and find good agreement with previous measurements and predictions from theoretical models.
Publisher: Oxford University Press (OUP)
Date: 30-04-2020
Abstract: Feedback from radio jets associated with active galactic nuclei (AGNs) plays a profound role in the evolution of galaxies. Kinetic power of these radio jets appears to show temporal variation, but the mechanism(s) responsible for this process are not yet clear. Recently, the LOw Frequency ARray (LOFAR) has uncovered large populations of active, remnant, and restarted radio jet populations. By focusing on LOFAR data in the Lockman Hole, in this work we use the Radio AGNs in Semi-Analytic Environments (RAiSE) dynamical model to present the first self-consistent modelling analysis of active, remnant, and restarted radio source populations. Consistent with other recent work, our models predict that remnant radio lobes fade quickly. Any high (& per cent) observed fraction of remnant and restarted sources therefore requires a dominant population of short-lived jets. We speculate that this could plausibly be provided by feedback-regulated accretion.
Publisher: EDP Sciences
Date: 07-2019
DOI: 10.1051/0004-6361/201935762
Abstract: Context. Gaseous halos play a key role in understanding inflow, feedback, and the overall baryon budget in galaxies. Literature models predict transitions of the state of the gaseous halo between cold and hot accretion, winds, fountains, and hydrostatic halos at certain galaxy masses. Since luminosities of radio AGN are sensitive to halo densities, any significant transition would be expected to show up in the radio luminosities of large s les of galaxies. The LOw Frequency ARray (LOFAR) Two-Metre Sky Survey (LoTSS) has identified a galaxy stellar mass scale, 10 11 M ⊙ , above which the radio luminosities increase disproportionately. Aims. We investigate if radio luminosities of galaxies, especially the marked rise at galaxy masses around 10 11 M ⊙ , can be explained with standard assumptions regarding jet powers, scaling between black hole mass and galaxy mass, and gaseous halos. Methods. Based on observational data and theoretical constraints, we developed models for the radio luminosity of radio AGN in halos under infall, galactic wind, and hydrostatic conditions. We compared these models to LoTSS data for a large s le of galaxies in the mass range between 10 8.5 M ⊙ and 10 12 M ⊙ . Results. Under the assumption that the same characteristic upper limit to jet powers known from high galaxy masses holds at all masses, we find the maximum radio luminosities for the hydrostatic gas halos to lie close to the upper envelope of the distribution of the LOFAR data. The marked rise in radio luminosity at 10 11 M ⊙ is matched in our model and is related to a significant change in halo gas density around this galaxy mass, which is a consequence of lower cooling rates at a higher virial temperature. Wind and infall models overpredict the radio luminosities for small galaxy masses and have no particular steepening of the run of the radio luminosities predicted at any galaxy mass. Conclusions. Radio AGN could have the same characteristic Eddington-scaled upper limit to jet powers in galaxies of all masses in the s le if the galaxies have hydrostatic gas halos in phases when radio AGN are active. We find no evidence of a change of the type of galaxy halo with the galaxy mass. Galactic winds and quasi-spherical cosmological inflow phases cannot frequently occur at the same time as powerful jet episodes unless the jet properties in these phases are significantly different from what we assumed in our model.
Publisher: Springer Science and Business Media LLC
Date: 14-03-2013
Publisher: Oxford University Press (OUP)
Date: 06-10-2015
Publisher: IOP Publishing
Date: 11-09-2019
Publisher: IOP Publishing
Date: 09-09-2019
Publisher: Oxford University Press (OUP)
Date: 15-11-2017
Publisher: Oxford University Press (OUP)
Date: 09-08-2018
Publisher: Oxford University Press (OUP)
Date: 02-08-2017
Publisher: American Astronomical Society
Date: 10-07-2012
Publisher: Oxford University Press (OUP)
Date: 13-12-2022
Abstract: Using the Lyman Dropout technique, we identify 148 candidate radio sources at z ≳ 4–7 from the 887.5 MHz Australian Square Kilometre Array Pathfinder (ASKAP) observations of the GAMA23 field. About 112 radio sources are currently known beyond redshift z ∼ 4. However, simulations predict that hundreds of thousands of radio sources exist in that redshift range, many of which are probably in existing radio catalogues, but do not have measured redshifts, either because their optical emission is too faint or because of the lack of techniques that can identify candidate high-redshift radio sources (HzRSs). Our study addresses these issues using the Lyman Dropout search technique. This newly built s le probes radio luminosities that are 1–2 orders of magnitude fainter than known radio-active galactic nuclei (AGN) at similar redshifts, thanks to ASKAP’s sensitivity. We investigate the physical origin of radio emission in our s le using a set of diagnostics: (i) radio luminosity at 1.4 GHz, (ii) 1.4 GHz to 3.4 μm flux density ratio, (iii) Far-IR detection, (iv) WISE colour, and (v) SED modelling. The radio/IR analysis has shown that the majority of radio emission in the faint and bright end of our s le’s 887.5 MHz flux density distribution originates from AGN activity. Furthermore, ∼10 per cent of our s le are found to have a 250 μm detection, suggesting a composite system. This suggests that some high-z radio-AGNs are hosted by SB galaxies in contrast to low-z radio-AGNs, which are usually hosted by quiescent elliptical galaxies.
Publisher: Frontiers Media SA
Date: 2011
Publisher: Oxford University Press (OUP)
Date: 29-04-2022
Abstract: In this work, we examine the classification accuracy and robustness of a state-of-the-art semi-supervised learning (SSL) algorithm applied to the morphological classification of radio galaxies. We test if SSL with fewer labels can achieve test accuracies comparable to the supervised state of the art and whether this holds when incorporating previously unseen data. We find that for the radio galaxy classification problem considered, SSL provides additional regularization and outperforms the baseline test accuracy. However, in contrast to model performance metrics reported on computer science benchmarking data sets, we find that improvement is limited to a narrow range of label volumes, with performance falling off rapidly at low label volumes. Additionally, we show that SSL does not improve model calibration, regardless of whether classification is improved. Moreover, we find that when different underlying catalogues drawn from the same radio survey are used to provide the labelled and unlabelled data sets required for SSL, a significant drop in classification performance is observed, highlighting the difficulty of applying SSL techniques under data set shift. We show that a class-imbalanced unlabelled data pool negatively affects performance through prior probability shift, which we suggest may explain this performance drop, and that using the Fréchet distance between labelled and unlabelled data sets as a measure of data set shift can provide a prediction of model performance, but that for typical radio galaxy data sets with labelled s le volumes of $\\mathcal {O}(10^3)$, the s le variance associated with this technique is high and the technique is in general not sufficiently robust to replace a train–test cycle.
Publisher: Oxford University Press (OUP)
Date: 24-03-2022
Abstract: Odd radio circles (ORCs) are recently-discovered faint diffuse circles of radio emission, of unknown cause, surrounding galaxies at moderate redshift (z ∼ 0.2 – 0.6). Here, we present detailed new MeerKAT radio images at 1284 MHz of the first ORC, originally discovered with the Australian Square Kilometre Array Pathfinder, with higher resolution (6 arcsec) and sensitivity (∼ 2.4 μJy/beam). In addition to the new images, which reveal a complex internal structure consisting of multiple arcs, we also present polarization and spectral index maps. Based on these new data, we consider potential mechanisms that may generate the ORCs.
Publisher: Oxford University Press (OUP)
Date: 09-10-2017
Publisher: Oxford University Press (OUP)
Date: 30-06-2021
Abstract: The highly variable BL Lac object PKS B1144 − 379 was monitored at 6.7 GHz using the Ceduna Radio Telescope with high cadence from 2003 to 2011. Intraday variations due to interstellar scintillation (ISS) were observed throughout the period. To complement our earlier analysis of the ISS and variability of this object, we have investigated the physical origin of changes in the modality of flux density distributions, calculated for ∼14 d observing blocks. Our analysis shows that the flux density distribution is primarily bimodal, but it changes to unimodal during the core brightening and jet expansion phases of the source. The presence of unimodal flux density distributions during these two phases is most likely due to the compactness of the scintillating component and the intrinsic evolution of the source. The existence of unimodality in the flux density distributions associated with specific phases of the source evolution also suggests that changes in the modality are unlikely due to multiple scattering screens. We propose that the physical origin of changes in the modality of the flux density distribution for PKS B1144 − 379 is most likely due to the combination of multiple bright jet features with interstellar scintillation along the line of sight between observer and source. This new approach complements our previous investigations of the temporal evolution of PKS B1144 − 379 that used interstellar scintillation and very long baseline interferometry, and the combination of these techniques provides a crucial starting point for understanding the system.
Publisher: Springer Science and Business Media LLC
Date: 18-02-2017
Publisher: MDPI AG
Date: 19-07-2023
Abstract: Analytical models describing the dynamics of lobed radio sources are essential for interpretation of the tens of millions of radio sources that will be observed by the Square Kilometre Array and pathfinder instruments. We propose that historical models can be grouped into two classes in which the forward expansion of the radio source is driven by either the jet momentum flux or lobe internal pressure. The most recent generation of analytical models combines these limiting cases for a more comprehensive description. We extend the mathematical formalism of historical models to describe source expansion in non-uniform environments, and directly compare different model classes with each other and with hydrodynamic numerical simulations. We quantify differences in predicted observable characteristics for lobed radio sources due to the different model assumptions for their dynamics. We have made our code for the historical models analysed in this review openly available to the community.
Publisher: Oxford University Press (OUP)
Date: 09-04-2015
DOI: 10.1093/MNRAS/STV597
Publisher: Oxford University Press (OUP)
Date: 10-12-2021
Abstract: In this work we explore the potential of multidomain multibranch convolutional neural networks (CNNs) for identifying comparatively rare giant radio galaxies from large volumes of survey data, such as those expected for new generation radio telescopes like the SKA and its precursors. The approach presented here allows models to learn jointly from multiple survey inputs, in this case NVSS and FIRST, as well as incorporating numerical redshift information. We find that the inclusion of multiresolution survey data results in correction of 39 per cent of the misclassifications seen from equivalent single domain networks for the classification problem considered in this work. We also show that the inclusion of redshift information can moderately improve the classification of giant radio galaxies.
Publisher: Oxford University Press (OUP)
Date: 29-01-2018
DOI: 10.1093/MNRAS/STY228
Publisher: Oxford University Press (OUP)
Date: 12-09-2022
Abstract: We present the discovery of highly collimated radio jets spanning a total of 355 kpc around the nearby elliptical galaxy NGC 2663, and the possible first detection of recollimation on kiloparsec scales. The small distance to the galaxy (∼28.5 Mpc) allows us to resolve portions of the jets to examine their structure. We combine multiwavelength data: radio observations by the Murchison Widefield Array (MWA), the Australian Square Kilometre Array Pathfinder (ASKAP) and the Australia Telescope Compact Array (ATCA), and X-ray data from Chandra, Swift, and SRG/eROSITA. We present intensity, rotation measure, polarization, spectral index, and X-ray environment maps. Regions of the southern jet show simultaneous narrowing and brightening, which can be interpreted as a signature of the recollimation of the jet by external, environmental pressure, though it is also consistent with intermittent active galactic nuclei or complex internal jet structure. X-ray data suggest that the environment is extremely poor if the jet is indeed recollimating, the large recollimation scale (40 kpc) is consistent with a slow jet in a low-density environment.
Publisher: Oxford University Press (OUP)
Date: 05-05-2016
Publisher: Oxford University Press (OUP)
Date: 31-12-2016
Publisher: American Astronomical Society
Date: 03-2019
Publisher: American Astronomical Society
Date: 05-06-2015
Publisher: Cambridge University Press (CUP)
Date: 2020
DOI: 10.1017/PASA.2019.49
Abstract: We present a detailed analysis of the radio galaxy PKS $2250{-}351$ , a giant of 1.2 Mpc projected size, its host galaxy, and its environment. We use radio data from the Murchison Widefield Array, the upgraded Giant Metre-wavelength Radio Telescope, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to model the jet power and age. Optical and IR data come from the Galaxy And Mass Assembly (GAMA) survey and provide information on the host galaxy and environment. GAMA spectroscopy confirms that PKS $2250{-}351$ lies at $z=0.2115$ in the irregular, and likely unrelaxed, cluster Abell 3936. We find its host is a massive, ‘red and dead’ elliptical galaxy with negligible star formation but with a highly obscured active galactic nucleus dominating the mid-IR emission. Assuming it lies on the local M – $\\sigma$ relation, it has an Eddington accretion rate of $\\lambda_{\\rm EDD}\\sim 0.014$ . We find that the lobe-derived jet power (a time-averaged measure) is an order of magnitude greater than the hotspot-derived jet power (an instantaneous measure). We propose that over the lifetime of the observed radio emission ( ${\\sim} 300\\,$ Myr), the accretion has switched from an inefficient advection-dominated mode to a thin disc efficient mode, consistent with the decrease in jet power. We also suggest that the asymmetric radio morphology is due to its environment, with the host of PKS $2250{-}351$ lying to the west of the densest concentration of galaxies in Abell 3936.
Publisher: Oxford University Press (OUP)
Date: 16-09-2020
Abstract: In this paper, we present the identification of five previously unknown giant radio galaxies (GRGs) using Data Release 1 of the Radio Galaxy Zoo citizen science project and a selection method appropriate to the training and validation of deep learning algorithms for new radio surveys. We associate one of these new GRGs with the brightest cluster galaxy (BCG) in the galaxy cluster GMBCG J251.67741+36.45295 and use literature data to identify a further 13 previously known GRGs as BCG candidates, increasing the number of known BCG GRGs by $\\gt 60$ per cent. By examining local galaxy number densities for the number of all known BCG GRGs, we suggest that the existence of this growing number implies that GRGs are able to reside in the centres of rich (∼1014 M⊙) galaxy clusters and challenges the hypothesis that GRGs grow to such sizes only in locally underdense environments.
Publisher: Oxford University Press (OUP)
Date: 23-11-2017
Publisher: Oxford University Press (OUP)
Date: 11-04-2022
Abstract: We present the Australian Square Kilometre Array Pathfinder (ASKAP) observations of the Galaxy and Mass Assembly (GAMA)-23h field. The survey was carried out at 887.5 MHz and covers an ∼83 square deg field. We imaged the calibrated visibility data, taken as part of the Evolutionary Mapping of Universe Early Science Programme, using the latest version of the ASKAPSoft pipeline. The final mosaic has an angular resolution of 10 arcsec and a central rms noise of around 38 $\\mu$Jy beam−1. The derived radio source catalogue has 39 812 entries above a peak flux density threshold of 5σ. We searched for the radio source host galaxy counterparts using the GAMA spectroscopic (with an i-band magnitude limit of 19.2 mag) and multiwavelength catalogues that are available as part of the collaboration. We identified hosts with GAMA spectroscopic redshifts for 5934 radio sources. We describe the data reduction, imaging, and source identification process, and present the source counts. Thanks to the wide area covered by our survey, we obtain very robust counts down to 0.2 mJy. ASKAP’s exceptional survey speed, providing efficient, sensitive, and high-resolution mapping of large regions of the sky in conjunction with the multiwavelength data available for the GAMA23 field, allowed us to discover 63 giant radio galaxies. The data presented here demonstrate the excellent capabilities of ASKAP in the pre-SKA era.
Publisher: Oxford University Press (OUP)
Date: 18-05-2022
Abstract: Quantifying the energetics and lifetimes of remnant radio-loud active galactic nuclei (AGNs) is much more challenging than for active sources due to the added complexity of accurately determining the time since the central black hole switched off. Independent spectral modelling of remnant lobes enables the derivation of the remnant ratio, Rrem (i.e. ‘off-time/source age’) however, the requirement of high-frequency (≳5 GHz) coverage makes the application of this technique over large-area radio surveys difficult. In this work, we propose a new method, which relies on the observed brightness of backflow of Fanaroff–Riley type II lobes, combined with the Radio AGN in Semi-Analytic Environments (RAiSE) code, to measure the duration of the remnant phase. Sensitive radio observations of the remnant radio galaxy J2253-34 are obtained to provide a robust comparison of this technique with the canonical spectral analysis and modelling methods. We find that the remnant lifetimes modelled by each method are consistent spectral modelling yields Rrem = 0.23 ± 0.02, compared to Rrem = 0.26 ± 0.02 from our new method. We examine the viability of applying our proposed technique to low-frequency radio surveys using mock radio source populations, and examine whether the technique is sensitive to any intrinsic properties of radio AGNs. Our results show that the technique can be used to robustly classify active and remnant populations, with the most confident predictions for the remnant ratio, and thus off-time, in the longest lived radio sources (& Myr) and those at higher redshifts (z & 0.1).
Publisher: Oxford University Press (OUP)
Date: 02-09-2020
Abstract: The University of Tasmania Ceduna radio telescope has been used to investigate rapid variability in the radio flux density of the BL Lac object PKS B1144−379 at 6.7 GHz. High-cadence monitoring of this extreme scintillator was carried out over a period of approximately 9 yr, between 2003 and 2011. We have used structure functions created from the intensity time-series to determine the characteristic time-scale of the variability. The characteristic time-scale is consistently observed to increase during certain periods of each year, demonstrating the annual cycle expected for scintillation through an interstellar scattering screen. The best-fitting annual cycle model for each year suggests that the scintillation pattern has an anisotropic structure and that the upper limit of its scattering screen is at a distance of ∼0.84 kpc. Higher anisotropy in some of the annual cycle fits suggests that changes in the intrinsic source structure might be influencing the variability time-scale. We found a prominent annual cycle is only present in the variability time-scale for certain years, where other evidence suggests that the core is compact. From our measurements, we calculated that the core angular size varied between 5.65 and 15.90 μas (0.05–0.13 pc). The core component was found to be at its most compact during two flares in the total flux density, which were observed in 2005 and 2008. We conclude that the long-term variability in the radio flux density of PKS B1144−379 is due to intrinsic changes in the source and that these affect our ability to measure an annual cycle in its variability time-scale.
Publisher: Springer Science and Business Media LLC
Date: 19-03-2014
Publisher: Oxford University Press (OUP)
Date: 22-11-2015
Publisher: Oxford University Press (OUP)
Date: 13-07-2013
Publisher: Oxford University Press (OUP)
Date: 18-05-2018
Publisher: Oxford University Press (OUP)
Date: 24-09-2015
Publisher: Oxford University Press (OUP)
Date: 24-08-2015
Publisher: Oxford University Press (OUP)
Date: 21-09-2018
Publisher: Oxford University Press (OUP)
Date: 30-03-2019
DOI: 10.1093/MNRAS/STZ922
Publisher: Oxford University Press (OUP)
Date: 14-03-2011
Publisher: Oxford University Press (OUP)
Date: 06-10-2020
Abstract: Next-generation radio surveys are expected to detect tens of millions of active galactic nuclei (AGNs) with a median redshift of $z$ ≥ 1. Beyond targeted surveys, the vast majority of these objects will not have spectroscopic redshifts, while photometric redshifts for high-redshift AGNs are of limited quality, and even then require optical and infrared photometry. We propose a new approach to measure the redshifts of lobed radio galaxies based exclusively on radio-frequency imaging and broad-band radio photometry. Specifically, our algorithm uses the lobe flux density, angular size and width, and spectral shape to derive probability density functions for the most likely source redshift based on the Radio AGN in Semi-analytic Environments dynamical model. The full physically based model explains 70 per cent of the variation in the spectroscopic redshifts of a high-redshift (2 & $z$ & 4) s le of radio AGNs, compared to at most 27 per cent for any one of the observed attributes in isolation. We find that upper bounds on the angular size, as expected for unresolved sources, are sufficient to yield accurate redshift measurements at $z$ ≥ 2. The error in the model upon calibration using at least nine sources with known spectroscopic redshifts is & per cent in redshift (as 1 + $z$) across all redshifts. We provide the python code for the calculation and calibration of our radio continuum redshifts in an online library.
Publisher: Oxford University Press (OUP)
Date: 04-02-2022
Abstract: We report the discovery of J0624–6948, a low-surface brightness radio ring, lying between the Galactic Plane and the large magellanic cloud (LMC). It was first detected at 888 MHz with the Australian Square Kilometre Array Pathfinder (ASKAP), and with a diameter of ∼196 arcsec. This source has phenomenological similarities to odd radio circles (ORCs). Significant differences to the known ORCs – a flatter radio spectral index, the lack of a prominent central galaxy as a possible host, and larger apparent size – suggest that J0624–6948 may be a different type of object. We argue that the most plausible explanation for J0624–6948 is an intergalactic supernova remnant due to a star that resided in the LMC outskirts that had undergone a single-degenerate type Ia supernova, and we are seeing its remnant expand into a rarefied, intergalactic environment. We also examine if a massive star or a white dwarf binary ejected from either galaxy could be the supernova progenitor. Finally, we consider several other hypotheses for the nature of the object, including the jets of an active galactic nucleus (30Dor) or the remnant of a nearby stellar super-flare.
Publisher: Oxford University Press (OUP)
Date: 13-10-2021
Abstract: We have carried out relativistic three-dimensional simulations of high-power radio sources propagating i,nto asymmetric cluster environments. We offset the environment by 0 or 1 core radii (equal to 144 kpc), and incline the jets by 0, 15, or 45° away from the environment centre. The different environment encountered by each radio lobe provides a unique opportunity to study the effect of environment on otherwise identical jets. We find that the jets become unstable towards the end of the simulations, even with a Lorentz factor of 5 they nevertheless develop typical Fanaroff–Riley class II radio morphology. The jets propagating into denser environments have consistently shorter lobe lengths and brighter hotspots, while the axial ratio of the two lobes is similar. We reproduce the recently reported observational anticorrelation between lobe length asymmetry and environment asymmetry, corroborating the notion that observed large-scale radio lobe asymmetry can be driven by differences in the underlying environment.
Publisher: Oxford University Press (OUP)
Date: 10-02-2012
Publisher: Springer Science and Business Media LLC
Date: 22-12-2021
Publisher: Cambridge University Press (CUP)
Date: 08-2012
DOI: 10.1017/S1743921314004943
Abstract: We observed the inner filament of NGC 5128 (Centaurus A) with the Hubble Space Telescope Wide Field Camera 3 (WFC3), using the F225W, F657N and F814W filters. We find a young stellar population near the south-west tip of the filament. We constrain the ages of these stars to 1-3 Myrs. No further recent star formation is found along the filament. We propose an updated explanation for the origin of the inner filament. It has been suggested Sutherland et al. 1993 that radio jets can shock the surrounding gas, giving rise to the observed optical line emission. We argue that such shocks can naturally arise due to a weak cocoon-driven bow shock (rather than from the radio jet directly) propagating through the diffuse interstellar medium. We suggest such a shock has overrun a molecular cloud, triggering star formation in the dense molecular core. The outer, more diffuse parts of the cloud are then ablated and shock heated, giving rise to the observed optical line and X-ray emission.
Publisher: Oxford University Press (OUP)
Date: 22-09-2021
Abstract: We have investigated the evolution of the BL Lac object PKS B1144−379 using the University of Tasmania Ceduna 30-m radio telescope at a frequency of 6.7 GHz and very long baseline interferometry (VLBI) data at 8.6 GHz. Variability time-scales associated with two flares detected in 2005 November and 2008 August were derived from long-term variations in total flux density monitored by Ceduna between 2003 and 2011. A kinematic study of the parsec-scale jet of PKS B1144−379 was performed using VLBI data obtained between 1997 and 2018. Quasi-periodic flarings with a period of ∼3–4 yr were observed. Over the 20-yr interval, the average jet position angle was found to be ~150°.
Publisher: Oxford University Press (OUP)
Date: 13-03-2020
Abstract: We model the X-ray surface brightness distribution of emission associated with Fanaroff & Riley type-II radio galaxies. Our approach builds on the RAiSE dynamical model which describes broad-band radio frequency synchrotron evolution of jet-inflated lobes in a wide range of environments. The X-ray version of the model presented here includes: (1) inverse-Compton upscattering of cosmic microwave background radiation (2) the dynamics of the shocked gas shell and associated bremsstrahlung radiation and (3) emission from the surrounding ambient medium. We construct X-ray surface brightness maps for a mock catalogue of extended FR-IIs based on the technical characteristics of the eRosita telescope. The integrated X-ray luminosity function at low redshifts (z ≤ 1) is found to strongly correlate with the density of the ambient medium in all but the most energetic sources, whilst at high-redshift (z & 1) the majority of objects are dominated by inverse-Compton lobe emission due to the stronger cosmic microwave background radiation. By inspecting our mock spatial brightness distributions, we conclude that any extended X-ray detection can be attributed to AGN activity at redshifts z ≥ 1. We compare the expected detection rates of active and remnant high-redshift radio AGNs for eRosita and LOFAR, and future more sensitive surveys. We find that a factor of ten more remnants can be detected using X-ray wavelengths over radio frequencies at z & 2.2, increasing to a factor of 100 for redshifts z & 3.1.
Publisher: Oxford University Press (OUP)
Date: 06-09-2015
Publisher: Oxford University Press (OUP)
Date: 03-2005
Publisher: Oxford University Press (OUP)
Date: 11-04-2023
Abstract: We present a novel natural language processing (NLP) approach to deriving plain English descriptors for science cases otherwise restricted by obfuscating technical terminology. We address the limitations of common radio galaxy morphology classifications by applying this approach. We experimentally derive a set of semantic tags for the Radio Galaxy Zoo EMU (Evolutionary Map of the Universe) project and the wider astronomical community. We collect 8486 plain English annotations of radio galaxy morphology, from which we derive a taxonomy of tags. The tags are plain English. The result is an extensible framework, which is more flexible, more easily communicated, and more sensitive to rare feature combinations, which are indescribable using the current framework of radio astronomy classifications.
Publisher: Oxford University Press (OUP)
Date: 23-10-2018
Publisher: Cambridge University Press (CUP)
Date: 08-2012
DOI: 10.1017/S1743921313001762
Abstract: The effects of active galactic nucleus (AGN) feedback on group and cluster galaxies are investigated. We examine the colors of non -AGN hosts (i.e. satellite galaxies) by comparing galaxies overrun by radio AGN with similar galaxies located outside the radio AGN contours. We find that powerful Fanaroff-Riley type II (edge-brightened) radio AGN truncate star formation in the galaxies overrun by AGN-driven bow shocks. On the other hand, the ubiquitous Fanaroff-Riley type I (core-dominated) AGN do not affect neighboring galaxies. This result shows that, despite their rarity, feedback from powerful radio AGN is an important factor in the evolution of group/cluster galaxies.
Publisher: Oxford University Press (OUP)
Date: 08-07-2015
No related organisations have been discovered for Stanislav Shabala.
Start Date: 2015
End Date: 2015
Funder: Australian Research Council
View Funded ActivityStart Date: 2015
End Date: 2015
Funder: Smithsonian Institution
View Funded ActivityStart Date: 2015
End Date: 2015
Funder: CSIRO-Commonwealth Scientific & Industrial Research Organisation
View Funded ActivityStart Date: 2015
End Date: 2015
Funder: Auckland University of Technology
View Funded ActivityStart Date: 2015
End Date: 2015
Funder: Geoscience Australia
View Funded ActivityStart Date: 2015
End Date: 2015
Funder: Max Planck Institute for Radio Astronomy
View Funded ActivityStart Date: 07-2013
End Date: 06-2018
Amount: $370,799.00
Funder: Australian Research Council
View Funded ActivityStart Date: 06-2015
End Date: 10-2018
Amount: $570,000.00
Funder: Australian Research Council
View Funded ActivityStart Date: 08-2020
End Date: 08-2024
Amount: $530,000.00
Funder: Australian Research Council
View Funded Activity