arXiv subjects

Elizabeth K. Mahony

Publications and source records attributed to Elizabeth K. Mahony.

At least 37 records · Page 2Linked to original sources

arXiv2021

Characterizing the FRB host galaxy population and its connection to transients in the local and extragalactic Universe

We present the localization and host galaxies of one repeating and two apparently non-repeating Fast Radio Bursts. FRB20180301A was detected and localized with the Karl G. Jansky Very Large Array to a star-forming galaxy at $z=0.3304$. FRB20191228A, and FRB20200906A were detected and localized by the Australian Square Kilometre Array Pathfinder to host galaxies at $z=0.2430$ and $z=0.3688$, respectively. We combine these with 13 other well-localized FRBs in the literature, and analyze the host galaxy properties. We find no significant differences in the host properties of repeating and apparently non-repeating FRBs. FRB hosts are moderately star-forming, with masses slightly offset from the star-forming main-sequence. Star formation and low-ionization nuclear emission-line region (LINER) emission are major sources of ionization in FRB host galaxies, with the former dominant in repeating FRB hosts. FRB hosts do not track stellar mass and star formation as seen in field galaxies (more than 95% confidence). FRBs are rare in massive red galaxies, suggesting that progenitor formation channels are not solely dominated by delayed channels which lag star formation by Gigayears. The global properties of FRB hosts are indistinguishable from core-collapse supernovae (CCSNe) and short gamma-ray bursts (SGRBs) hosts, and the spatial offset (from galaxy centers) of FRBs is mostly inconsistent with that of the Galactic neutron star population (95% confidence). The spatial offsets of FRBs (normalized to the galaxy effective radius) also differ from those of globular clusters (GCs) in late- and early-type galaxies with 95% confidence.

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arXiv2021

HI absorption at z~0.7 against the lobe of the powerful radio galaxy PKS 0409-75

We present results from a search for the HI 21-cm line in absorption towards 16 bright radio sources with the 6-antenna commissioning array of the Australian Square Kilometre Array Pathfinder (ASKAP). Our targets were selected from the 2-Jy sample, a flux-limited survey of the southern radio sky with extensive multi-wavelength follow-up. Two sources were detected in HI absorption including a new detection towards the bright FRII radio galaxy PKS 0409-75 at a redshift of z=0.674. The HI absorption line is blueshifted by ~3300 km/s compared to the optical redshift of the host galaxy of PKS 0409-75 at z=0.693. Deep optical imaging and spectroscopic follow-up with the GMOS instrument on the Gemini-South telescope reveal that the HI absorption is associated with a galaxy in front of the southern radio lobe with a stellar mass of $3.2 - 6.8 \times 10^{11}M_\odot$, a star-formation rate of $\sim 1.24 M_\odot$ yr$^{-1}$ and an estimated HI column density of $2.16\times10^{21}$ cm$^{-2}$, assuming a spin temperature of $T_{\rm spin}=500$ K and source covering factor of $C_{\rm f}=0.3$. Using polarisation measurements of PKS 0409-75 from the literature we estimate the magnetic field of the absorbing galaxy to be ~14.5$μ$G, consistent with field strengths observed in nearby spiral galaxies, but larger than expected for an elliptical galaxy. Results from this pilot study can inform future surveys as new wide-field telescopes allow us to search for 21-cm HI absorption towards all bright radio sources as opposed to smaller targeted samples.

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arXiv2021

The gaseous natal environments of GPS and CSS sources with ASKAP -- FLASH

GPS and CSS sources are thought to represent a young and/or confined sub-population of radio-loud active galactic nuclei (AGN) that are yet to evacuate their surrounding ambient interstellar gas. By studying the gaseous environments of these objects we can gain an insight into the inter-dependent relationship between galaxies and their supermassive black holes (SMBHs). The First Large Absorption Survey in HI (FLASH) will build a census of the neutral atomic hydrogen (HI) gas in galaxies at intermediate cosmological redshifts. FLASH is expected to detect at least several hundred HI absorbers associated with GPS and CSS sources. These absorbers provide an important probe of the abundance and kinematics of line-of-sight neutral gas towards radio AGN, in some cases revealing gas associated with infalling clouds and outflows. Observations are now complete for the first phase of the FLASH Pilot Survey and early analysis has already yielded several detections, including the GPS source PKS2311$-$477. Optical imaging of this galaxy reveals an interacting system that could have supplied the neutral gas seen in absorption and triggered the radio-loud AGN. FLASH will provide a statistically significant sample with which the prevalence of such gas-rich interactions amongst compact radio galaxies can be investigated.

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arXiv2021

Chronicling the Host Galaxy Properties of the Remarkable Repeating FRB 20201124A

We present the Australian Square Kilometre Array Pathfinder (ASKAP) localization and follow-up observations of the host galaxy of the repeating fast radio burst (FRB) source, FRB20201124A, the fifth such extragalactic repeating FRB with an identified host. From spectroscopic observations using the 6.5-m MMT Observatory, we derive a redshift of $z=0.0979 \pm 0.0001$, a star formation rate inferred from H$α$ emission of SFR(H$α$) $\approx 2.1 M_{\odot}$ yr$^{-1}$, and a gas-phase metallicity of 12+log(O/H)$\approx 9.0$. By jointly modeling the 12-filter optical-mid-infrared (MIR) photometry and spectroscopy of the host, we infer a median stellar mass of $\approx 2 \times 10^{10} M_{\odot}$, internal dust extinction of $A_V\approx 1-1.5$ mag, and a mass-weighted stellar population age of $\approx 5-6$ Gyr. Connecting these data to the radio and X-ray observations, we cannot reconcile the broad-band behavior with strong AGN activity and instead attribute the dominant source of persistent radio emission to star formation, likely originating from the circumnuclear region of the host. The modeling also indicates a hot dust component contributing to the MIR luminosity at a level of $\approx 10-30\%$. We model the host galaxy's star formation and mass assembly histories, finding that the host assembled $>90\%$ of its mass by 1 Gyr ago and exhibited a fairly constant SFR for most of its existence, with no clear evidence of past star-burst activity.

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arXiv2021

The ASKAP Variables and Slow Transients (VAST) Pilot Survey

The Variables and Slow Transients Survey (VAST) on the Australian Square Kilometre Array Pathfinder (ASKAP) is designed to detect highly variable and transient radio sources on timescales from 5 seconds to $\sim 5$ years. In this paper, we present the survey description, observation strategy and initial results from the VAST Phase I Pilot Survey. This pilot survey consists of $\sim 162$ hours of observations conducted at a central frequency of 888~MHz between 2019 August and 2020 August, with a typical rms sensitivity of 0.24~mJy~beam$^{-1}$ and angular resolution of $12-20$ arcseconds. There are 113 fields, \red{each of which was observed for 12 minutes integration time}, with between 5 and 13 repeats, with cadences between 1 day and 8 months. The total area of the pilot survey footprint is 5\,131 square degrees, covering six distinct regions of the sky. An initial search of two of these regions, totalling 1\,646 square degrees, revealed 28 highly variable and/or transient sources. Seven of these are known pulsars, including the millisecond pulsar J2039--5617. Another seven are stars, four of which have no previously reported radio detection (SCR~J0533--4257, LEHPM~2-783, UCAC3~89--412162 and 2MASS J22414436--6119311). Of the remaining 14 sources, two are active galactic nuclei, six are associated with galaxies and the other six have no multiwavelength counterparts and are yet to be identified.

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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

Radio Afterglows from Compact Binary Coalescences: Prospects for Next-Generation Telescopes

The detection of gravitational waves from a neutron star merger, GW170817, marked the dawn of a new era in time-domain astronomy. Monitoring of the radio emission produced by the merger, including high-resolution radio imaging, enabled measurements of merger properties including the energetics and inclination angle. In this work we compare the capabilities of current and future gravitational wave facilities to the sensitivity of radio facilities to quantify the prospects for detecting the radio afterglows of gravitational wave events. We consider three observing strategies to identify future mergers -- widefield follow-up, targeting galaxies within the merger localisation and deep monitoring of known counterparts. We find that while planned radio facilities like the Square Kilometre Array will be capable of detecting mergers at gigaparsec distances, no facilities are sufficiently sensitive to detect mergers at the range of proposed third-generation gravitational wave detectors that would operate starting in the 2030s.

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arXiv2021

ASKAP observations of multiple rapid scintillators reveal a degrees-long plasma filament

We present the results from an Australian Square Kilometre Array Pathfinder search for radio variables on timescales of hours. We conducted an untargeted search over a 30 deg$^2$ field, with multiple 10-hour observations separated by days to months, at a central frequency of 945 MHz. We discovered six rapid scintillators from 15-minute model-subtracted images with sensitivity of $\sim 200\,μ$Jy/beam; two of them are extreme intra-hour variables with modulation indices up to $\sim 40\%$ and timescales as short as tens of minutes. Five of the variables are in a linear arrangement on the sky with angular width $\sim 1$ arcmin and length $\sim 2$ degrees, revealing the existence of a huge plasma filament in front of them. We derived kinematic models of this plasma from the annual modulation of the scintillation rate of our sources, and we estimated its likely physical properties: a distance of $\sim 4$ pc and length of $\sim 0.1$ pc. The characteristics we observe for the scattering screen are incompatible with published suggestions for the origin of intra-hour variability leading us to propose a new picture in which the underlying phenomenon is a cold tidal stream. This is the first time that multiple scintillators have been detected behind the same plasma screen, giving direct insight into the geometry of the scattering medium responsible for enhanced scintillation.

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arXiv2020

Limits on precursor and afterglow radio emission from a fast radio burst in a star-forming galaxy

We present a new fast radio burst (FRB) at 920 MHz discovered during commensal observations conducted with the Australian Square Kilometre Array Pathfinder (ASKAP) as part of the Commensal Real-time ASKAP Fast Transients (CRAFT) survey. FRB 191001 was detected at a dispersion measure (DM) of 506.92(4) pc cm$^{-3}$ and its measured fluence of 143(15) Jy ms is the highest of the bursts localized to host galaxies by ASKAP to date. The subarcsecond localization of the FRB provided by ASKAP reveals that the burst originated in the outskirts of a highly star-forming spiral in a galaxy pair at redshift $z=0.2340(1)$. Radio observations show no evidence for a compact persistent radio source associated with the FRB 191001 above a flux density of $15 μ$Jy. However, we detect diffuse synchrotron radio emission from the disk of the host galaxy that we ascribe to ongoing star formation. FRB 191001 was also detected as an image-plane transient in a single 10 s snapshot with a flux density of 19.3 mJy in the low-time-resolution visibilities obtained simultaneously with CRAFT data. The commensal observation facilitated a search for repeating and slowly varying radio emissions 8 hr before and 1 hr after the burst. We found no variable radio emission on timescales ranging from 1 ms to 1.4 hr. We report our upper limits and briefly review FRB progenitor theories in the literature that predict radio afterglows. Our data are still only weakly constraining of any afterglows at the redshift of the FRB. Future commensal observations of more nearby and bright FRBs will potentially provide stronger constraints.

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arXiv2020

High time resolution and polarisation properties of ASKAP-localised fast radio bursts

Combining high time and frequency resolution full-polarisation spectra of Fast Radio Bursts (FRBs) with knowledge of their host galaxy properties provides an opportunity to study both the emission mechanism generating them and the impact of their propagation through their local environment, host galaxy, and the intergalactic medium. The Australian Square Kilometre Array Pathfinder (ASKAP) telescope has provided the first ensemble of bursts with this information. In this paper, we present the high time and spectral resolution, full polarisation observations of five localised FRBs to complement the results published for the previously studied ASKAP FRB~181112. We find that every FRB is highly polarised, with polarisation fractions ranging from 80 -- 100\%, and that they are generally dominated by linear polarisation. While some FRBs in our sample exhibit properties associated with an emerging archetype (i.e., repeating or apparently non-repeating), others exhibit characteristic features of both, implying the existence of a continuum of FRB properties. When examined at high time resolution, we find that all FRBs in our sample have evidence for multiple sub-components and for scattering at a level greater than expected from the Milky Way. We find no correlation between the diverse range of FRB properties (e.g., scattering time, intrinsic width, and rotation measure) and any global property of their host galaxy. The most heavily scattered bursts reside in the outskirts of their host galaxies, suggesting that the source-local environment rather than the host interstellar medium is likely the dominant origin of the scattering in our sample.

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arXiv2020

A successful search for intervening 21 cm HI absorption in galaxies at 0.4 < z <1.0 with the Australian Square Kilometre Array Pathfinder (ASKAP)

We have used the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope to search for intervening 21 cm neutral hydrogen (HI) absorption along the line of sight to 53 bright radio continuum sources. Our observations are sensitive to HI column densities typical of Damped Lyman Alpha absorbers (DLAs) in cool gas with an HI spin temperature below about 300-500 K. The six-dish Boolardy Engineering Test Array (BETA) and twelve-antenna Early Science array (ASKAP-12) covered a frequency range corresponding to redshift $0.4<z<1.0$ and $0.37<z<0.77$ respectively for the HI line. Fifty of the 53 radio sources observed have reliable optical redshifts, giving a total redshift path $Δz$ = 21.37. This was a spectroscopically-untargeted survey, with no prior assumptions about the location of the lines in redshift space. Four intervening HI lines were detected, two of them new. In each case, the estimated HI column density lies above the DLA limit for HI spin temperatures above 50-80 K, and we estimate a DLA number density at redshift $z\sim0.6$ of $n(z)=0.19\substack{+0.15 \\ -0.09}$. This value lies somewhat above the general trend of $n(z)$ with redshift seen in optical DLA studies. Although the current sample is small, it represents an important proof of concept for the much larger 21cm First Large Absorption Survey in HI (FLASH) project to be carried out with the full 36-antenna ASKAP telescope, probing a total redshift path $Δz\sim\,50,000$.

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arXiv2020

The host galaxies and progenitors of Fast Radio Bursts localized with the Australian Square Kilometre Array Pathfinder

The Australian SKA Pathfinder (ASKAP) telescope has started to localize Fast Radio Bursts (FRBs) to arcsecond accuracy from the detection of a single pulse, allowing their host galaxies to be reliably identified. We discuss the global properties of the host galaxies of the first four FRBs localized by ASKAP, which lie in the redshift range $0.11<z<0.48$. All four are massive galaxies (log( $M_{*}/ M_{\odot}$) $\sim 9.4 -10.4$) with modest star-formation rates of up to $2M_{\odot}$yr$^{-1}$ -- very different to the host galaxy of the first repeating FRB 121102, which is a dwarf galaxy with a high specific star-formation rate. The FRBs localized by ASKAP typically lie in the outskirts of their host galaxies, which appears to rule out FRB progenitor models that invoke active galactic nuclei (AGN) or free-floating cosmic strings. The stellar population seen in these host galaxies also disfavors models in which all FRBs arise from young magnetars produced by superluminous supernovae (SLSNe), as proposed for the progenitor of FRB 121102. A range of other progenitor models (including compact-object mergers and magnetars arising from normal core-collapse supernovae) remain plausible.

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arXiv2019

The low density and magnetization of a massive galaxy halo exposed by a fast radio burst

Present-day galaxies are surrounded by cool and enriched halo gas extending to hundreds of kiloparsecs. This halo gas is thought to be the dominant reservoir of material available to fuel future star formation, but direct constraints on its mass and physical properties have been difficult to obtain. We report the detection of a fast radio burst (FRB 181112) with arcsecond precision, which passes through the halo of a foreground galaxy. Analysis of the burst shows the halo gas has low net magnetization and turbulence. Our results imply predominantly diffuse gas in massive galactic halos, even those hosting active supermassive black holes, contrary to some previous results.

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arXiv2018

Interplanetary Scintillation studies with the Murchison Wide-field Array IV: The hosts of sub-arcsecond compact sources at low radio frequencies

Around 10% of bright low-frequency radio sources observed with the Murchison Widefield Array (MWA) show strong Interplanetary Scintillation (IPS) on timescales of a few seconds, implying that almost all their low-frequency radio emission comes from a compact component less than 0.5 arcsec in angular size. Most of these objects are compact steep-spectrum (CSS) or MHz-peaked spectrum (MPS) radio sources. We have used mid-infrared data from the Wide-field Infrared Survey Explorer (WISE) catalogue to search for the host galaxies of 65 strongly-scintillating MWA sources and compare their properties with those of the overall population of bright low-frequency radio sources. We identified WISE mid-infrared counterparts for 91% of the bright sources in a single 900 square degree MWA field, and found that the hosts of the strongly-scintillating sources were typically at least 1 mag fainter in the WISE W1 (3.4 micron) band than the hosts of weakly-scintillating MWA sources of similar radio flux density. This difference arises mainly because the strongly-scintillating sources are more distant. We estimate that strongly-scintillating MWA sources have a median redshift of z ~ 1.5, and that at least 30% of them are likely to lie at z > 2. The recently-developed wide-field IPS technique therefore has the potential to provide a powerful new tool for identifying high-redshift radio galaxies without the need for radio spectral-index selection.

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arXiv2018

A search for the host galaxy of FRB171020

We report on a search for the host galaxy of FRB171020, the fast radio burst with the smallest recorded dispersion measure (DM $=114$ pc cm$^{-3}$) of our on-ongoing ASKAP survey. The low DM confines the burst location within a sufficiently small volume to rigorously constrain the identity of the host galaxy. We identify 16 candidate galaxies in the search volume and single out ESO 601-G036, a Sc galaxy at redshift $z=0.00867$, as the most likely host galaxy. UV and optical imaging and spectroscopy reveal this galaxy has a star-formation rate of approximately 0.1 M$_\odot$ yr$^{-1}$ and oxygen abundance $12 + \log({\rm O/H}) = 8.3 \pm 0.2$, properties remarkably consistent with the galaxy hosting the repeating FRB121102. However, in contrast to FRB121102, follow-up radio observations of ESO 601-G036 show no compact radio emission above a 5$σ$ limit of $L_{2.1{\rm GHz}}=3.6\times 10^{19}$ W Hz$^{-1}$. Using radio continuum observations of the field, combined with archival optical imaging data, we find no analog to the persistent radio source associated with FRB121102 within the localization region of FRB171020 out to $z=0.06$. These results suggest that FRBs are not necessarily associated with a luminous and compact radio continuum source.

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arXiv2016

FR II radio galaxies at low frequencies I: morphology, magnetic field strength and energetics

Due to their steep spectra, low-frequency observations of FR II radio galaxies potentially provide key insights in to the morphology, energetics and spectrum of these powerful radio sources. However, limitations imposed by the previous generation of radio interferometers at metre wavelengths has meant that this region of parameter space remains largely unexplored. In this paper, the first in a series examining FR IIs at low frequencies, we use LOFAR observations between 50 and 160 MHz, along with complementary archival radio and X-ray data, to explore the properties of two FR II sources, 3C452 and 3C223. We find that the morphology of 3C452 is that of a standard FR II rather than of a double-double radio galaxy as had previously been suggested, with no remnant emission being observed beyond the active lobes. We find that the low-frequency integrated spectra of both sources are much steeper than expected based on traditional assumptions and, using synchrotron/inverse-Compton model fitting, show that the total energy content of the lobes is greater than previous estimates by a factor of around 5 for 3C452 and 2 for 3C223. We go on to discuss possible causes of these steeper than expected spectra and provide revised estimates of the internal pressures and magnetic field strengths for the intrinsically steep case. We find that the ratio between the equipartition magnetic field strengths and those derived through synchrotron/inverse-Compton model fitting remains consistent with previous findings and show that the observed departure from equipartition may in some cases provide a solution to the spectral versus dynamical age disparity.

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arXiv2015

The Lockman Hole with LOFAR - Searching for GPS and CSS sources at low frequencies

The Lockman Hole Project is a wide international collaboration aimed at exploiting the multi-band extensive and deep information available for the Lockman Hole region, with the aim of better characterizing the physical and evolutionary properties of the various source populations detected in deep radio fields. Recent observations with the LOw-Frequency ARray (LOFAR) extends the multi-frequency radio information currently available for the Lockman Hole (from 350 MHz up to 15 GHz) down to 150 MHz, allowing us to explore a new radio spectral window for the faint radio source population. These LOFAR observations allow us to study the population of sources with spectral peaks at lower radio frequencies, providing insight into the evolution of GPS and CSS sources. In this general framework, I present preliminary results from 150 MHz LOFAR observations of the Lockman Hole field.

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arXiv2015

Jet-driven outflows of ionised gas in the nearby radio galaxy 3C293

Fast outflows of gas, driven by the interaction between the radio-jets and ISM of the host galaxy, are being observed in an increasing number of galaxies. One such example is the nearby radio galaxy 3C293. In this paper we present Integral Field Unit (IFU) observations taken with OASIS on the William Herschel Telescope (WHT), enabling us to map the spatial extent of the ionised gas outflows across the central regions of the galaxy. The jet-driven outflow in 3C293 is detected along the inner radio lobes with a mass outflow rate ranging from $\sim 0.05-0.17$ solar masses/yr (in ionised gas) and corresponding kinetic power of $\sim 0.5-3.5\times 10^{40}$ erg/s. Investigating the kinematics of the gas surrounding the radio jets (i.e. not directly associated with the outflow), we find line-widths broader than $300$ km/s up to 5 kpc in the radial direction from the nucleus (corresponding to 3.5 kpc in the direction perpendicular to the radio axis at maximum extent). Along the axis of the radio jet line-widths $>400$ km/s are detected out to 7 kpc from the nucleus and line-widths of $>500$ km/s at a distance of 12 kpc from the nucleus, indicating that the disturbed kinematics clearly extend well beyond the high surface brightness radio structures of the jets. This is suggestive of the cocoon structure seen in simulations of jet-ISM interaction and implies that the radio jets are capable of disturbing the gas throughout the central regions of the host galaxy in all directions.

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