arXiv subjects

Tessa Vernstrom

Publications and source records attributed to Tessa Vernstrom.

35 records · Page 2Linked to original sources

arXiv2022

The redshift evolution of extragalactic magnetic fields

Faraday rotation studies of distant radio sources can constrain the evolution and the origin of cosmic magnetism. We use data from the LOFAR Two Metre Sky Survey: Data Release 2 (LoTSS DR2) to study the dependence of the Faraday rotation measure (RM) on redshift. By focusing on radio sources that are close in terms of their projection on the sky, but physically unrelated (random pairs), we measure the RM difference, $Δ$RM, between the two sources. Thus, we isolate the extragalactic contribution to $Δ$RM from other contributions. We present a statistical analysis of the resulting sample of random pairs and find a median absolute RM difference |$Δ$RM| $ = (1.79 \pm 0.09)$ rad/m$^{2}$ , with |$Δ$RM| uncorrelated both with respect to the redshift difference of the pair and the redshift of the nearer source, and a median excess of random pairs over physical pairs of $(1.65 \pm 0.10)$ rad/m$^{2}$. We seek to reproduce this result with Monte Carlo simulations assuming a non vanishing seed cosmological magnetic field and a redshift evolution of the comoving magnetic field strength that varies as $1/(1 + z)^γ$. We find the best fitting results $B_0 \equiv B_{\rm comoving}(z = 0) \lesssim (2.0 \pm 0.2)$ nG and $γ\lesssim 4.5 \pm 0.2$ that we conservatively quote as upper limits due to an unmodelled but non vanishing contribution of local environments to the RM difference. A comparison with cosmological simulations shows our results to be incompatible with primordial magnetogenesis scenarios with uniform seed fields of order nG.

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arXiv2022

GaLactic and Extragalactic All-sky Murchison Widefield Array survey eXtended (GLEAM-X) I: Survey Description and Initial Data Release

We describe a new low-frequency wideband radio survey of the southern sky. Observations covering 72 - 231 MHz and Declinations south of $+30^\circ$ have been performed with the Murchison Widefield Array "extended" Phase II configuration over 2018 - 2020 and will be processed to form data products including continuum and polarisation images and mosaics, multi-frequency catalogues, transient search data, and ionospheric measurements. From a pilot field described in this work, we publish an initial data release covering 1,447 sq. deg over 4h < RA < 13h, -32.7deg < Dec < -20.7deg. We process twenty frequency bands sampling 72 - 231 MHz, with a resolution of $2'$ - $45"$, and produce a wideband source-finding image across 170 - 231MHz with a root-mean-square noise of $1.27\pm0.15$ mJy/beam. Source-finding yields 78,967 components, of which 71,320 are fitted spectrally. The catalogue has a completeness of 98% at $\sim50$mJy, and a reliability of 98.2% at $5σ$ rising to 99.7% at $7σ$. A catalogue is available from Vizier; images are made available on AAO Data Central, SkyView, and the PASA Datastore. This is the first in a series of data releases from the GLEAM-X survey.

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arXiv2022

MeerKAT uncovers the physics of an Odd Radio Circle

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 uJy/bm). In addition to the new images, which reveal a complex internal structure consisting of multiple arcs, we also present polarisation and spectral index maps. Based on these new data, we consider potential mechanisms that may generate the ORCs.

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arXiv2022

The eROSITA view of the Abell 3391/95 field: The Northern Clump. The largest infalling structure in the longest known gas filament observed with eROSITA, XMM-Newton, and Chandra

SRG/eROSITA PV observations revealed the A3391/95 cluster system and the Northern Clump (MCXC J0621.7-5242 galaxy cluster) are aligning along a cosmic filament in soft X-rays, similarly to what has been seen in simulations before. We aim to understand the dynamical state of the Northern Clump as it enters the atmosphere ($3\times R_{200}$) of A3391. We analyzed joint eROSITA, XMM-Newton, and Chandra observations to probe the morphological, thermal, and chemical properties of the Northern Clump from its center out to a radius of 988 kpc ($R_{200}$). We utilized the ASKAP/EMU radio data, DECam optical image, and Planck y-map to study the influence of the WAT radio source on the Northern Clump central ICM. From the Magneticum simulation, we identified an analog of the A3391/95 system along with an infalling group resembling the Northern Clump. The Northern Clump is a WCC cluster centered on a WAT radio galaxy. The gas temperature over $0.2-0.5R_{500}$ is $k_BT_{500}=1.99\pm0.04$ keV. We employed the $M-T$ scaling relation and obtained a mass estimate of $M_{500}=(7.68\pm0.43)\times10^{13}M_{\odot}$ and $R_{500}=(636\pm12)$ kpc. Its atmosphere has a boxy shape and deviates from spherical symmetry. We identify a southern surface brightness edge, likely caused by subsonic motion relative to the filament gas. At $\sim\! R_{500}$, the southern atmosphere appears to be 42% hotter than its northern atmosphere. We detect a downstream tail pointing toward the north with a projected length of $\sim318$ kpc, plausibly the result of ram pressure stripping. The analog group in the Magneticum simulation is experiencing changes in its gas properties and a shift between the position of the halo center and that of the bound gas while approaching the main cluster pair.

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arXiv2021

The EMU view of the Large Magellanic Cloud: Troubles for sub-TeV WIMPs

We present a radio search for WIMP dark matter in the Large Magellanic Cloud (LMC). We make use of a recent deep image of the LMC obtained from observations of the Australian Square Kilometre Array Pathfinder (ASKAP), and processed as part of the Evolutionary Map of the Universe (EMU) survey. LMC is an extremely promising target for WIMP searches at radio frequencies because of the large J-factor and the presence of a substantial magnetic field. We detect no evidence for emission arising from WIMP annihilations and derive stringent bounds on the annihilation rate as a function of the WIMP mass, for different annihilation channels. This work excludes the thermal cross section for masses below 480 GeV and annihilation into quarks.

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arXiv2021

The Galactic Faraday rotation sky 2020

This work gives an update to existing reconstructions of the Galactic Faraday rotation sky by processing almost all Faraday rotation data sets available at the end of the year 2020. Observations of extra-Galactic sources in recent years have, among other regions, further illuminated the previously under-constrained southern celestial sky, as well as parts of the inner disc of the Milky Way. This has culminated in an all-sky data set of 55,190 data points, which is a significant expansion on the 41,330 used in previous works, hence making an updated separation of the Galactic component a promising venture. The increased source density allows us to present our results in a resolution of about $1.3\cdot 10^{-2}\, \mathrm{deg}^2$ ($46.8\,\mathrm{arcmin}^2$), which is a twofold increase compared to previous works. As for previous Faraday rotation sky reconstructions, this work is based on information field theory, a Bayesian inference scheme for field-like quantities which handles noisy and incomplete data. In contrast to previous reconstructions, we find a significantly thinner and pronounced Galactic disc with small-scale structures exceeding values of several thousand $\mathrm{rad}\,\mathrm{m}^{-2}$. The improvements can mainly be attributed to the new catalog of Faraday data, but are also supported by advances in correlation structure modeling within numerical information field theory. We furthermore give a detailed discussion on statistical properties of the Faraday rotation sky and investigate correlations to other data sets.

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arXiv2021

The Evolutionary Map of the Universe Pilot Survey

We present the data and initial results from the first Pilot Survey of the Evolutionary Map of the Universe (EMU), observed at 944 MHz with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The survey covers 270 \sqdeg of an area covered by the Dark Energy Survey, reaching a depth of 25--30 \ujybm\ rms at a spatial resolution of $\sim$ 11--18 arcsec, resulting in a catalogue of $\sim$ 220,000 sources, of which $\sim$ 180,000 are single-component sources. Here we present the catalogue of single-component sources, together with (where available) optical and infrared cross-identifications, classifications, and redshifts. This survey explores a new region of parameter space compared to previous surveys. Specifically, the EMU Pilot Survey has a high density of sources, and also a high sensitivity to low surface-brightness emission. These properties result in the detection of types of sources that were rarely seen in or absent from previous surveys. We present some of these new results here.

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arXiv2021

Discovery of Magnetic Fields Along Stacked Cosmic Filaments as Revealed by Radio and X-Ray Emission

Diffuse filaments connect galaxy clusters to form the cosmic web. Detecting these filaments could yield information on the magnetic field strength, cosmic ray population and temperature of intercluster gas, yet, the faint and large-scale nature of these bridges makes direct detections very challenging. Using multiple independent all-sky radio and X-ray maps we stack pairs of luminous red galaxies as tracers for cluster pairs. For the first time, we detect an average surface brightness between the clusters from synchrotron (radio) and thermal (X-ray) emission with $\gtrsim 5σ$ significance, on physical scales larger than observed to date ($\geq 3\,$Mpc). We obtain a synchrotron spectral index of $α\simeq -1.0$ and estimates of the average magnetic field strength of $ 30 \leq B \leq 60 \,$nG, derived from both equipartition and Inverse Compton arguments, implying a 5 to 15$\,$per cent degree of field regularity when compared with Faraday rotation measure estimates. While the X-ray detection is inline with predictions, the average radio signal comes out higher than predicted by cosmological simulations and dark matter annihilation and decay models. This discovery demonstrates that there are connective structures between mass concentrations that are significantly magnetised, and the presence of sufficient cosmic rays to produce detectable synchrotron radiation.

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arXiv2021

Constraints on Large-Scale Magnetic Fields in the Intergalactic Medium Using Cross-Correlation Methods

Large-scale coherent magnetic fields in the intergalactic medium are presumed to play a key role in the formation and evolution of the cosmic web, and in large scale feedback mechanisms. However, they are theorized to be extremely weak, in the nano-Gauss regime. To search for a statistical signature of these weak magnetic fields we perform a cross-correlation between the Faraday rotation measures of 1742 radio galaxies at $z > 0.5$ and large-scale structure at $0.1 < z< 0.5$, as traced by 18 million optical and infrared foreground galaxies. No significant correlation signal was detected within the uncertainty limits. We are able to determine model-dependent $3 σ$ upper limits on the parallel component of the mean magnetic field strength of filaments in the intergalactic medium of $\sim 30 \ \mathrm{nG}$ for coherence scales between $1$ and $2.5 \ \mathrm{Mpc}$, corresponding to a mean upper bound RM enhancement of $\sim 3.8 \ \mathrm{rad/m^{2}}$ due to filaments along all probed sight-lines. These upper bounds are consistent with upper bounds found previously using other techniques. Our method can be used to further constrain intergalactic magnetic fields with upcoming future radio polarization surveys.

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arXiv2020

Low(er) frequency follow-up of 28 candidate, large-scale synchrotron sources

We follow up on a report by Vacca et al. (2018) of 28 candidate large-scale diffuse synchrotron sources in an 8°$\times$8° area of the sky (centred at RA 5h0m0s Dec 5°48'0''). These sources were originally observed at 1.4 GHz using a combination of the single-dish Sardinia Radio Telescope (SRT) and archival NRAO VLA Sky Survey (NVSS) data. They are in an area with nine massive galaxy clusters at z $\approx$ 0.1, and are candidates for the first detection of filaments of the synchrotron cosmic web. We attempt to verify these candidate sources with lower frequency observations at 154 MHz with the Murchison Widefield Array (MWA) and at 887 MHz with the Australian Square Kilometre Array Pathfinder (ASKAP). We use a novel technique to calculate the surface brightness sensitivity of these instruments to show that our lower frequency observations, and in particular those by ASKAP, are ideally suited to detect large-scale, extended synchrotron emission. Nonetheless, we are forced to conclude that none of these sources are likely to be synchrotron in origin or associated with the cosmic web.

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arXiv2019

Unveiling the cause of hybrid morphology radio sources (HyMoRS)

Hybrid morphology radio sources (HyMoRS) are a rare group of radio galaxies in which differing Fanaroff & Riley morphologies (FR I/II) are observed for each of the two lobes. While they potentially provide insights into the formation of lobe structure, particle acceleration, and the FR dichotomy, previous work on HyMoRS has mainly been limited to low-resolution studies, searches for new candidates, and milliarcsecond-scale VLBI observations of the core region. In this paper, we use new multi-array configuration Very Large Array (VLA) observations between 1 and 8 GHz to determine the morphology of HyMoRS on arcsecond scales and perform the first well-resolved spectral study of these unusual sources. We find that while the apparent FR I lobe is centre-brightened, this is the result of a compact acceleration region resembling a hotspot with a spectrum more consistent with an FR II ("strong-flavour") jet. We find that the spectra of the apparent FR I lobes are not similar to their classical counterparts and are likely the result of line-of-sight mixing of plasma across a range of spectral ages. We consider possible mechanisms that could lead to the formation of HyMoRS under such conditions, including environment asymmetry and restarted sources, concluding through the use of simple modelling that HyMoRS are the result of orientation effects on intrinsically FR II sources with lobes non-parallel to the inner jet.

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arXiv2017

Radio Polarization Properties of Quasars and Active Galaxies at High Redshifts

We present the largest ever sample of radio polarization properties for $z>4$ sources, with 14 sources having significant polarization detections. Using wideband data from the Karl G. Jansky Very Large Array, we obtained the rest-frame total intensity and polarization properties of 37 radio sources, nine of which have spectroscopic redshifts in the range $1 \le z \le 1.4$, with the other 28 having spectroscopic redshifts in the range $3.5 \le z \le 6.21$. Fits are performed for the Stokes $I$ and fractional polarization spectra, and Faraday rotation measures are derived using Rotation measure synthesis and $QU$ fitting. Using archival data of 476 polarized sources, we compare high redshift ($z>3$) source properties to a $15\,$GHz rest-frame luminosity matched sample of low redshift ($z<3$) sources to investigate if the polarization properties of radio sources at high redshifts are intrinsically different than those at low redshift. We find a mean of the rotation measure absolute values, corrected for Galactic rotation, of $50 \pm 22\,$rad m$^{-2}$ for $z>3$ sources and $57 \pm 4\,$rad m$^{-2}$ for $z<3$. Although there is some indication of lower intrinsic rotation measures at high-$z$ possibly due to higher depolarization from the high density environments, using several statistical tests we detect no significant difference between low and high redshift sources. Larger samples are necessary to determine any true physical difference.

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arXiv2017

Low Frequency Radio Constraints on the Synchrotron Cosmic Web

We present a search for the synchrotron emission from the synchrotron cosmic web by cross correlating 180MHz radio images from the Murchison Widefield Array with tracers of large scale structure (LSS). We use two versions of the radio image covering $21.76\times 21.76$ degrees with point sources brighter than 0.05 Jy subtracted, with and without filtering of Galactic emission. As tracers of the LSS we use the Two-Micron-All-Sky-Survey (2MASS) and the Widefield InfraRed Explorer (WISE) redshift catalogues to produce galaxy number density maps. The cross correlation functions all show peak amplitudes at zero degrees, decreasing with varying slopes towards zero correlation over a range of one degree. The cross correlation signals include components from point source, Galactic, and extragalactic diffuse emission. We use models of the diffuse emission from smoothing the density maps with Gaussians of sizes 1-4 Mpc to find limits on the cosmic web components. From these models we find surface brightness 99.7 per cent upper limits in the range of 0.09-2.20 mJy beam$^{-1}$ (average beam size of 2.6 arcmin), corresponding to 0.01-0.30 mJy arcmin$^{-2}$. Assuming equipartition between energy densities of cosmic rays and the magnetic field, the flux density limits translate to magnetic field strength limits of 0.03-1.98 $μ$G, depending heavily on the spectral index. We conclude that for a 3$σ$ detection of 0.1 $μ$G magnetic field strengths via cross correlations, image depths of sub-mJy to sub-$μ$Jy are necessary. We include discussion on the treatment and effect of extragalactic point sources and Galactic emission, and next steps for building on this work.

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arXiv2016

Deep 3 GHz Observations of the Lockman Hole North with the Very Large Array -2. Catalogue and μJy source properties

This is the second of two papers describing the observations and source catalogues derived from sensitive 3-GHz images of the Lockman Hole North using the Karl G. Jansky Very Large Array. We describe the reduction and cataloguing process, which yielded an image with 8 arcsec resolution and instrumental noise of σ$_{\rm n}$=1.0 μJy beam$^{-1}$ rms (before primary beam corrections) and a catalogue of 558 sources detected above 5σ$_{\rm n}$. We include details of how we estimate source spectral indices across the 2-GHz VLA bandwidth, finding a median index of -0.76$\pm$0.04. Stacking of source spectra reveals a flattening of spectral index with decreasing flux density. We present a source count derived from the catalogue. We show a traditional count estimate compared with a completely independent estimate made via a P(D) confusion analysis, and find very good agreement. Cross-matches of the catalogue with X-ray, optical, infrared, radio, and redshift catalogues are also presented. The X-ray, optical and infrared data, as well as AGN selection criteria allow us to classify 10 per cent as radio-loud AGN, 28 percent as radio-quiet AGN, and 58 per cent as star-forming galaxies, with only 4 per cent unclassified.

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arXiv2016

Deep 3 GHz Observations of the Lockman Hole North with the Very Large Array - 1. Source extraction and uncertainty analysis

This is the first of two papers describing the observations and cataloguing of deep 3-GHz observations of the Lockman Hole North using the Karl G. Jansky Very Large Array. The aim of this paper is to investigate, through the use of simulated images, the uncertainties and accuracy of source-finding routines, as well as to quantify systematic effects due to resolution, such as source confusion and source size. While these effects are not new, this work is intended as a particular case study that can be scaled and translated to other surveys. We use the simulations to derive uncertainties in the fitted parameters, as well as bias corrections for the actual catalogue (presented in Paper 2). We compare two different source-finding routines, OBIT and AEGEAN, and two different effective resolutions, 8 and 2.75 arcsec. We find that the two routines perform comparably well, with OBIT being slightly better at de-blending sources, but slightly worse at fitting resolved sources. We show that 30 to 70 per cent of sources are missed or fit inaccurately once the source size becomes larger than the beam, possibly explaining source count errors in high-resolution surveys. We also investigate the effect of blending, finding that any sources with separations smaller than the beam size are fit as single sources. We show that the use of machine-learning techniques can correctly identify blended sources up to 90 per cent of the time, and prior-driven fitting can lead to a 70 per cent improvement in the number of de-blended sources.

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arXiv2014

Astronomy below the Survey Threshold

Astronomy at or below the 'survey threshold' has expanded significantly since the publication of the original 'Science with the Square Kilometer Array' in 1999 and its update in 2004. The techniques in this regime may be broadly (but far from exclusively) defined as 'confusion' or 'P(D)' analyses (analyses of one-point statistics), and 'stacking', accounting for the flux-density distribution of noise-limited images co-added at the positions of objects detected/isolated in a different waveband. Here we discuss the relevant issues, present some examples of recent analyses, and consider some of the consequences for the design and use of surveys with the SKA and its pathfinders.

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arXiv2011

Contribution to the Diffuse Radio Background from Extragalactic Radio Sources

We examine the brightness of the Cosmic Radio Background (CRB) by comparing the contribution from individual source counts to absolute measurements. We use a compilation of radio counts to estimate the contribution of detected sources to the CRB in several different frequency bands.We apply a Monte Carlo Markov Chain technique to estimate the brightness values and uncertainties, paying attention to various sources of systematic error. We compare our results to absolute measurements from the ARCADE 2 experiment. At v = 150 MHz, 325 MHz, 408 MHz, 610 MHz, 1.4 GHz, 4.8 GHz, and 8.4 GHz our calculated contributions to the background sky temperature are 18, 2.8, 1.6, 0.71, 0.11, 0.0032, 0.0059 K, respectively. If the ARCADE 2 measurements are correct and come from sources, then there must be an additional population of radio galaxies, fainter than where current data are probing. More specifically, the Euclidean-normalized counts at 1.4 GHz have to have an additional bump below about 10 μJy.

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