arXiv subjects

S. P. O'Sullivan

Publications and source records attributed to S. P. O'Sullivan.

At least 55 records · Page 3Linked to original sources

arXiv2020

New constraints on the magnetization of the cosmic web using LOFAR Faraday rotation observations

Measuring the properties of extragalactic magnetic fields through the effect of Faraday rotation provides a means to understand the origin and evolution of cosmic magnetism. Here we use data from the LOFAR Two-Metre Sky Survey (LoTSS) to calculate the Faraday rotation measure (RM) of close pairs of extragalactic radio sources. By considering the RM difference ($Δ$RM) between physical pairs (e.g. double-lobed radio galaxies) and non-physical pairs (i.e. close projected sources on the sky), we statistically isolate the contribution of extragalactic magnetic fields to $Δ$RM along the line of sight between non-physical pairs. From our analysis, we find no significant difference between the $Δ$RM distributions of the physical and non-physical pairs, limiting the excess Faraday rotation contribution to $< 1.9$ rad/m$^2$ ($\sim$$95\%$ confidence). We use this limit with a simple model of an inhomogeneous universe to place an upper limit of 4 nG on the cosmological co-moving magnetic field strength on Mpc scales. We also compare the RM data with a more realistic suite of cosmological MHD simulations, that explore different magnetogenesis scenarios. Both magnetization of the large scale structure by astrophysical processes such as galactic and AGN outflows, and simple primordial scenarios with seed magnetic field strengths $< 0.5$ nG cannot be rejected by the current data; while stronger primordial fields or models with dynamo amplification in filaments are disfavoured.

astro-ph.CO↗

arXiv2020

Low-frequency observations of the Giant Radio Galaxy NGC 6251

We present LOFAR observations at 150 MHz of the borderline FRI/FRII giant radio galaxy NGC 6251. This paper presents the most sensitive and highest-resolution images of NGC 6251 at these frequencies to date, revealing for the first time a low-surface-brightness extension to the northern lobe, and a possible backflow associated with the southern lobe. The integrated spectra of components of NGC 6251 are consistent with previous measurements at higher frequencies, similar to results from other LOFAR studies of nearby radio galaxies. We find the outer structures of NGC 6251 to be either at equipartition or slightly electron dominated, similar to those of FRII sources rather than FRIs; but this conclusion remains tentative because of uncertainties associated with the geometry and the extrapolation of X-ray measurements to determine the external pressure distribution on the scale of the outer lobes. We place lower limits on the ages of the extension of the northern lobe and the backflow of the southern lobe of $t \gtrsim 250$ Myr and $t \gtrsim 210$ Myr respectively. We present the first detection of polarisation at 150 MHz in NGC 6251. Taking advantage of the high Faraday resolution of LOFAR, we place an upper limit on the magnetic field in the group of $B < 0.2 (Λ_B / 10 {\rm kpc})^{-0.5} μ$G for a coherence scale of $Λ_B < 60 {\rm kpc}$ and $B < 13 μ$G for $Λ_B = 240$ kpc.

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arXiv2020

The LOFAR view of intergalactic magnetic fields with giant radio galaxies

Giant radio galaxies (GRGs) are physically large radio sources that extend well beyond their host galaxy environment. Their polarization properties are affected by the poorly constrained magnetic field that permeates the intergalactic medium on Mpc scales. A low frequency ($<$ 200 MHz) polarization study of this class of radio sources is now possible with LOFAR. Here we investigate the polarization properties and Faraday rotation measure (RM) of a catalog of GRGs detected in the LoTSS. This is the first low-frequency polarization study of a large sample of radio galaxies selected on their physical size. We explore the magneto-ionic properties of their under-dense environment and probe intergalactic magnetic fields using the Faraday rotation properties of their radio lobes. We use RM synthesis in the 120-168 MHz band to search for polarized emission and to derive the RM and fractional polarization of each detected source component. We study the depolarization between 1.4 GHz and 144 MHz using images from the NVSS. From a sample of 240 GRGs, we detected 37 sources in polarization, all with a total flux density above 56 mJy. The fractional polarization of the detected GRGs at 1.4 GHz and 144 MHz is consistent with a small amount of Faraday depolarization (a Faraday dispersion $<$ 0.3 rad m$^{-2}$). Our analysis shows that the lobes are expanding into a low-density ($<10^{-5}$ cm$^{-3}$) local environment permeated by weak magnetic fields ($<$0.1 $μ$G) with fluctuations on scales of 3 to 25 kpc. The presence of foreground galaxy clusters appears to influence the polarization detection rate up to 2R$_{500}$. In general, this work demonstrates the ability of LOFAR to quantify the rarefied environments in which these GRGs exist and highlights them as an excellent statistical sample to use as high precision probes of magnetic fields in the intergalactic medium and the Milky Way.

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arXiv2020

Coherent radio emission from a quiescent red dwarf indicative of star-planet interaction

Low frequency ($ν\lesssim 150\,$MHz) stellar radio emission is expected to originate in the outer corona at heights comparable to and larger than the stellar radius. Such emission from the Sun has been used to study coronal structure, mass ejections, space-weather conditions around the planets (Schwenn 2006). Searches for low-frequency emission from other stars have only detected a single active flare-star (Lynch et al. 2017) that is not representative of the wider stellar population. Here we report the detection of low-frequency radio emission from a quiescent star, GJ 1151--- a member of the most common stellar type (red dwarf or spectral class M) in the Galaxy. The characteristics of the emission are similar to those of planetary auroral emissions (Zarka 1998) (e.g. Jupiter's decametric emission), suggesting a coronal structure dominated by a global magnetosphere with low plasma density. Our results show that large-scale currents that power radio aurorae operate over a vast range of mass and atmospheric composition, ranging from terrestrial planets to main-sequence stars. The Poynting flux required to produce the observed radio emission cannot be generated by GJ 1151's slow rotation, but can originate in a sub-Alfvénic interaction of its magnetospheric plasma with a short-period exoplanet. The emission properties are consistent with theoretical expectations (Zarka 2007; Lanza 2009; Saur et al. 2013; Turnpenney et al. 2018) for interaction with an Earth-size planet in a $\sim 1-5$ day-long orbit.

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arXiv2019

The Karl G. Jansky Very Large Array Sky Survey (VLASS). Science case and survey design

The Very Large Array Sky Survey (VLASS) is a synoptic, all-sky radio sky survey with a unique combination of high angular resolution ($\approx$2.5"), sensitivity (a 1$σ$ goal of 70 $μ$Jy/beam in the coadded data), full linear Stokes polarimetry, time domain coverage, and wide bandwidth (2-4 GHz). The first observations began in September 2017, and observing for the survey will finish in 2024. VLASS will use approximately 5500 hours of time on the Karl G. Jansky Very Large Array (VLA) to cover the whole sky visible to the VLA (Declination $>-40^{\circ}$), a total of 33,885 deg$^2$. The data will be taken in three epochs to allow the discovery of variable and transient radio sources. The survey is designed to engage radio astronomy experts, multi-wavelength astronomers, and citizen scientists alike. By utilizing an "on the fly" interferometry mode, the observing overheads are much reduced compared to a conventional pointed survey. In this paper, we present the science case and observational strategy for the survey, and also results from early survey observations.

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arXiv2019

Blazar jet evolution revealed by multi-epoch broadband radio polarimetry

We investigate the previously proposed possibility that multi-epoch broadband polarimetry could act as a complement or limited proxy for VLBI observations of blazars, in that the number of polarised emission components in the jet, and some of their properties and those of the foreground environment, might be inferred from the object's time-varying 1D Faraday depth spectrum (FDS) alone. We report on a pilot-scale experiment designed to establish the basic plausibility and utility of this idea. We analyse temporal changes in the complex polarisation spectra of nine spatially unresolved (at arcsecond scales) blazars in two epochs separated by $\sim$5 years, using data taken with the Australia Telescope Compact Array. The data allow for precise modelling, and we demonstrate that all objects in our sample show changes in their polarisation spectrum that cannot be accounted for by uncertainties in calibration or observational effects. By associating polarised emission components across epochs, we infer changes in their number, intrinsic fractional polarisation, intrinsic polarisation angle, rotation measure, and depolarisation characteristics. We attribute these changes to evolution in the structure of the blazar jets, most likely located at distances of up to tens of parsecs from the central active galactic nuclei. Our results suggest that continued work in this area is warranted; in particular, it will be important to determine the frequency ranges and temporal cadence most useful for scientifically exploiting the effects.

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arXiv2018

The LOFAR Two-metre Sky Survey - II. First data release

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45$^\circ$00$'$00$''$ to 57$^\circ$00$'$00$''$) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325,694 sources are detected with a signal of at least five times the noise, and the source density is a factor of $\sim 10$ higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S$_{\rm 144 MHz} = 71\,μ$Jy beam$^{-1}$ and the point-source completeness is 90% at an integrated flux density of 0.45mJy. The resolution of the images is 6$''$ and the positional accuracy is within 0.2$''$. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further $\sim$20 articles that highlight the scientific potential of LoTSS.

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arXiv2018

The intergalactic magnetic field probed by a giant radio galaxy

Cosmological simulations predict that an intergalactic magnetic field (IGMF) pervades the large scale structure (LSS) of the Universe. Measuring the IGMF is important to determine its origin (i.e. primordial or otherwise). Using data from the LOFAR Two Metre Sky Survey (LoTSS), we present the Faraday rotation measure (RM) and depolarisation properties of the giant radio galaxy J1235+5317, at a redshift of $z = 0.34$ and 3.38 Mpc in size. We find a mean RM difference between the lobes of $2.5\pm0.1$ rad/m$^2$ , in addition to small scale RM variations of ~0.1 rad/m$^2$ . From a catalogue of LSS filaments based on optical spectroscopic observations in the local universe, we find an excess of filaments intersecting the line of sight to only one of the lobes. Associating the entire RM difference to these LSS filaments leads to a gas density-weighted IGMF strength of ~0.3 μG. However, direct comparison with cosmological simulations of the RM contribution from LSS filaments gives a low probability (~5%) for an RM contribution as large as 2.5 rad/m$^2$ , for the case of IGMF strengths of 10 to 50 nG. It is likely that variations in the RM from the Milky Way (on 11' scales) contribute significantly to the mean RM difference, and a denser RM grid is required to better constrain this contribution. In general, this work demonstrates the potential of the LOFAR telescope to probe the weak signature of the IGMF. Future studies, with thousands of sources with high accuracy RMs from LoTSS, will enable more stringent constraints on the nature of the IGMF.

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arXiv2018

Radio-loud AGN in the first LoTSS data release: The lifetimes and environmental impact of jet-driven sources

We constructed a sample of 23,344 radio-loud active galactic nuclei (RLAGN) from the catalogue derived from the LOFAR Two-Metre Sky Survey (LoTSS) survey of the HETDEX Spring field. Although separating AGN from star-forming galaxies remains challenging, the combination of spectroscopic and photometric techniques we used gives us one of the largest available samples of candidate RLAGN. We used the sample, combined with recently developed analytical models, to investigate the lifetime distribution of RLAGN. We show that large or giant powerful RLAGN are probably the old tail of the general RLAGN population, but that the low-luminosity RLAGN candidates in our sample, many of which have sizes $<100$ kpc, either require a very different lifetime distribution or have different jet physics from the more powerful objects. We then used analytical models to develop a method of estimating jet kinetic powers for our candidate objects and constructed a jet kinetic luminosity function based on these estimates. These values can be compared to observational quantities, such as the integrated radiative luminosity of groups and clusters, and to the predictions from models of RLAGN feedback in galaxy formation and evolution. In particular, we show that RLAGN in the local Universe are able to supply all the energy required per comoving unit volume to counterbalance X-ray radiative losses from groups and clusters and thus prevent the hot gas from cooling. Our computation of the kinetic luminosity density of local RLAGN is in good agreement with other recent observational estimates and with models of galaxy formation.

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arXiv2018

The environments of radio-loud AGN from the LOFAR Two-Metre Sky Survey (LoTSS)

An understanding of the relationship between radio-loud active galaxies and their large-scale environments is essential for realistic modelling of radio-galaxy evolution and environmental impact, for understanding AGN triggering and life cycles, and for calibrating galaxy feedback in cosmological models. We use the LOFAR Two-Metre Sky Survey (LoTSS) Data Release 1 catalogues to investigate this relationship. We cross-matched 8,745 radio-loud AGN with 0.08<z<0.4, selected from LoTSS, with two Sloan Digital Sky Survey cluster catalogues, and find that only 10 percent of LoTSS AGN in this redshift range have an association, so that the majority of low-redshift AGN (including a substantial fraction of the most radio-luminous objects) must inhabit haloes with M < 10^14 M_sun. We find that the probability of a cluster association, and the richness of the associated cluster, is correlated with AGN radio luminosity, and for the cluster population, the number of associated AGN and the radio luminosity of the brightest associated AGN is richness-dependent. We demonstrate that these relations are not driven solely by host-galaxy stellar mass, supporting models in which large-scale environment is influential in driving AGN jet activity. At the lowest radio luminosities we find that the minority of objects with a cluster association are located at larger mean cluster-centre distances than more luminous AGN, which appears to be driven primarily by host-galaxy mass. Finally, we also find that FRI radio galaxies inhabit systematically richer environments than FRIIs. The work presented here demonstrates the potential of LoTSS for AGN environmental studies. In future, the full northern-sky LoTSS catalogue, together with the use of deeper optical/IR imaging data and spectroscopic follow-up with WEAVE-LOFAR, will provide opportunities to extend this type of work to much larger samples and higher redshifts.

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arXiv2018

Faraday rotation study of NGC 612 (PKS 0131-36): a hybrid radio source and its magnetised circumgalactic environment

We present a polarization and Faraday rotation study of the hybrid morphology radio galaxy NGC 612 (PKS 0131-36), using Australian Telescope Compact Array observations from 1 to 3 GHz. In general, the results are consistent with an external Faraday screen close to the radio source. In the eastern FRII lobe, the RM of the hotspot increases in magnitude towards the leading edge, as well as changing sign (compared to the rest of the lobe). The Faraday depolarization is also ~3 times larger at the hotspot than elsewhere. A plausible explanation for this is significant compression of ambient magnetised gas by the bow shock produced by the advancing hotspot. The western FRI lobe also exhibits some evidence of interaction with local magnetised gas, as a transverse band of high RM coincides with a distinct bend in the lobe. Previous observations of NGC 612 revealed an HI bridge of tidal debris along the direction of the eastern lobe towards the gas-rich companion NGC 619. We find no clear evidence that ionised gas associated with this bridge is either mixing with or lies in the foreground of the radio source. This is consistent with the absence of HI absorption against the hotspot, and indicates that the tidal debris must lie mostly behind the eastern lobe.

astro-ph.HE↗

arXiv2018

The magnetic field strength of the Faraday screen surrounding the radio galaxy Coma A

Studying the interaction between AGN jets and lobes and their surrounding environment is important in order to understand how they transfer energy to their environment as well as determining the intrinsic physical properties of the sources themselves. This paper presents broadband VLA polarization and Faraday rotation observations of the radio galaxy Coma A (3C 277.3) from 1 to 4 GHz, including archival VLA observations at 4.9 and 15 GHz. Through broadband polarization model-fitting, we find that an external Faraday screen with a turbulent magnetic field provides an appropriate description to most of the data. By combining the polarization and Faraday rotation results with previous H$α$ observations, we identified the H$α$-emitting gas as the Faraday screen responsible for the observed Faraday depolarization. We were able to derive the magnetic field strength in the H$α$-emitting gas, finding typical field strengths of $\sim1$ $μ$G, which is consistent with studies of the intra-group medium local to other radio galaxies. However, we find a highly depolarized region of the southern lobe coincident with a H$α$ filament that has a field strength comparable to the equipartition field strength in the radio lobe (i.e. $\gtrsim$36 $μ$G). This implies that the H$α$ filament is internal to the radio emitting plasma. Such clear examples of internal Faraday depolarization are rare, thus providing another key insight into the evolution of radio galaxies and their ability to provide significant feedback on the local gas that would otherwise cool and form stars.

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arXiv2018

Broadband radio polarimetry of Fornax A, I: Depolarized patches generated by advected thermal material from NGC 1316

We present observations and analysis of the polarized radio emission from the nearby radio galaxy Fornax A over 1.28--3.1 GHz, using data from the Australia Telescope Compact Array (ATCA). In this, the first of two associated papers, we use modern broadband polarimetric techniques to examine the nature and origin of conspicuous low-polarization (low-$p$) patches in the lobes. We resolve the low-$p$ patches, and find that their low fractional polarization is associated with complicated frequency-dependent interference in the polarized signal generated by Faraday effects along the line of sight. The low-$p$ patches are spatially correlated with interfaces in the magnetic structure of the lobe, across which the line-of-sight-projected magnetic field changes direction. Spatial correlations with the sky-projected magnetic field orientation and structure in total intensity are also identified and discussed. We argue that the low-$p$ patches, along with associated reversals in the line-of-sight magnetic field and other related phenomena, are best explained by the presence of $\mathcal{O}(10^9)$ $M_\odot$ of magnetized thermal plasma in the lobes, structured in shells or filaments, and likely advected from the ISM of NCG 1316 or its surrounding ICM. Our study underscores the power and utility of spatially-resolved, broadband, full-polarization radio observations to reveal new facets of flow behaviors and magneto-ionic structure in radio lobes and their interplay with the surrounding environment.

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arXiv2018

Revealing the Faraday Depth Structure of Radio Galaxy NGC 612 with Broad-Band Radio Polarimetric Observations

We present full-polarisation, broadband observations of the radio galaxy NGC 612 (PKS B0131-637) from 1.3 to 3.1 GHz using the Australia Telescope Compact Array. The relatively large angular scale of the radio galaxy makes it a good candidate with which to investigate the polarisation mechanisms responsible for the observed Faraday depth structure. By fitting complex polarisation models to the polarised spectrum of each pixel, we find that a single polarisation component can adequately describe the observed signal for the majority of the radio galaxy. While we cannot definitively rule out internal Faraday rotation, we argue that the bulk of the Faraday rotation is taking place in a thin skin that girts the polarised emission. Using minimum energy estimates, we find an implied total magnetic field strength of 4.2 microG.

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arXiv2018

Polarized point sources in the LOFAR Two-meter Sky Survey: A preliminary catalog

The polarization properties of radio sources at very low frequencies (<200 MHz) have not been widely measured, but the new generation of low-frequency radio telescopes, including the Low Frequency Array (LOFAR: a Square Kilometre Array Low pathfinder), now gives us the opportunity to investigate these properties. In this paper, we report on the preliminary development of a data reduction pipeline to carry out polarization processing and Faraday tomography for data from the LOFAR Two-meter Sky Survey (LOTSS) and present the results of this pipeline from the LOTSS preliminary data release region (10h45m - 15h30m right ascension, 45 - 57 degrees declination, 570 square degrees). We have produced a catalog of 92 polarized radio sources at 150 MHz at 4.3 arcminute resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized sources at such low frequencies. We estimate a lower limit to the polarized source surface density at 150 MHz, with our resolution and sensitivity, of 1 source per 6.2 square degrees. We find that our Faraday depth measurements are in agreement with previous measurements and have significantly smaller errors. Most of our sources show significant depolarization compared to 1.4 GHz, but there is a small population of sources with low depolarization indicating that their polarized emission is highly localized in Faraday depth. We predict that an extension of this work to the full LOTSS data would detect at least 3400 polarized sources using the same methods, and probably considerably more with improved data processing.

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arXiv2017

The challenges of low-frequency radio polarimetry: lessons from the Murchison Widefield Array

We present techniques developed to calibrate and correct Murchison Widefield Array (MWA) low frequency (72-300 MHz) radio observations for polarimetry. The extremely wide field-of-view, excellent instantaneous (u, v)-coverage and sensitivity to degree-scale structure that the MWA provides enable instrumental calibration, removal of instrumental artefacts, and correction for ionospheric Faraday rotation through imaging techniques. With the demonstrated polarimetric capabilities of the MWA, we discuss future directions for polarimetric science at low frequencies to answer outstanding questions relating to polarised source counts, source depolarisation, pulsar science, low-mass stars, exoplanets, the nature of the interstellar and intergalactic media, and the solar environment.

astro-ph.IM↗

arXiv2017

Broadband, radio spectro-polarimetric study of 100 radiative-mode and jet-mode AGN

We present the results from a broadband (1 to 3 GHz), spectro-polarimetry study of the integrated emission from 100 extragalactic radio sources with the ATCA, selected to be highly linearly polarized at 1.4 GHz. We use a general purpose, polarization model-fitting procedure that describes the Faraday rotation measure (RM) and intrinsic polarization structure of up to three distinct polarized emission regions or 'RM components' of a source. Overall, 37%/52%/11% of sources are best fit by one/two/three RM components. However, these fractions are dependent on the signal-to-noise ratio (S/N) in polarization (more RM components more likely at higher S/N). In general, our analysis shows that sources with high integrated degrees of polarization at 1.4 GHz have low Faraday depolarization, are typically dominated by a single RM component, have a steep spectral index, and a high intrinsic degree of polarization. After classifying our sample into radiative-mode and jet-mode AGN, we find no significant difference between the Faraday rotation or Faraday depolarization properties of jet-mode and radiative-mode AGN. However, there is a statistically significant difference in the intrinsic degree of polarization between the two types, with the jet-mode sources having more intrinsically ordered magnetic field structures than the radiative-mode sources. We also find a preferred perpendicular orientation of the intrinsic magnetic field structure of jet-mode AGN with respect to the jet direction, while no clear preference is found for the radiative-mode sources.

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arXiv2016

Observed Faraday Effects in Damped Lyman-Alpha Absorbers and Lyman Limit Systems: The Magnetised Environment of Galactic Building Blocks at Redshift=2

Protogalactic environments are typically identified using quasar absorption lines, and these galactic building blocks can manifest as Damped Lyman-Alpha Absorbers (DLAs) and Lyman Limit Systems (LLSs). We use radio observations of Faraday effects to test whether DLAs and LLSs host a magnetised medium, by combining DLA and LLS detections throughout the literature with 1.4 GHz polarization data from the NRAO VLA Sky Survey (NVSS). We obtain a control, a DLA, and a LLS sample consisting of 114, 19, and 27 lines-of-sight respectively - all of which are polarized at $\ge8σ$ to ensure Rician bias is negligible. Using a Bayesian framework, we are unable to detect either coherent or random magnetic fields in DLAs: the regular coherent magnetic fields within the DLAs must be $\le2.8$ $μ$G, and the lack of depolarization is consistent with the weakly magnetised gas in DLAs being non-turbulent and quiescent. However, we find mild suggestive evidence that LLSs have coherent magnetic fields: after controlling for the redshift-distribution of our data, we find a 71.5% probability that LLSs have a higher RM than a control sample. We also find strong evidence that LLSs host random magnetic fields, with a 95.5% probability that LLS lines-of-sight have lower polarized fractions than a control sample. The regular coherent magnetic fields within the LLSs must be $\le2.4$ $μ$G, and the magnetised gas must be highly turbulent with a typical scale on the order of $\approx5$-20 pc, which is similar to that of the Milky Way. This is consistent with the standard dynamo pedagogy, whereby magnetic fields in protogalaxies increase in coherence and strength as a function of cosmic time. Our results are consistent with a hierarchical galaxy formation scenario, with the DLAs, LLSs, and strong magnesium II (MgII) systems exploring three different stages of magnetic field evolution in galaxies.

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