arXiv subjects

I. Heywood

Publications and source records attributed to I. Heywood.

125 records · Page 7Linked to original sources

arXiv2013

The preferentially magnified active nucleus in IRAS F10214+4724 - I. Lens model and spatially resolved radio emission

This is the first paper in a series that present a multi-wavelength analysis of the archetype Ultra-Luminous InfraRed Galaxy (ULIRG) IRAS FSC10214+4724, a gravitationally lensed, starburst/AGN at z=2.3. Here we present a new lens model and spatially-resolved radio data, as well as a deep HST F160W map. The lens modelling employs a Bayesian Markov Chain Monte Carlo algorithm with extended-source, forward ray-tracing. Using these high resolution HST, MERLIN and VLA maps, the algorithm allows us to constrain the level of distortion to the continuum spectral energy distribution resulting from emission components with differing magnification factors, due to their size and proximity to the caustic. Our lens model finds the narrow line region (NLR), and by proxy the active nucleus, is preferentially magnified. This supports previous claims that preferential magnification could mask the expected polycyclic aromatic hydrocarbon spectral features in the Spitzer mid-infrared spectrum which roughly trace the star-forming regions. Furthermore, we show the arc-to-counter-image flux ratio is not a good estimate of the magnification in this system, despite its common use in the IRAS FSC10214+4724 literature. Our lens modelling suggests magnifications of μ~ 15-20+-2 for the HST F814W, MERLIN 1.7 GHz and VLA 8 GHz maps, significantly lower than the canonical values of μ= 50-100 often used for this system. Systematic errors such as the dark matter density slope and co-location of stellar and dark matter centroids dominate the uncertainties in the lens model at the 40 percent level.

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arXiv2013

The preferentially magnified active nucleus in IRAS F10214+4724 - III. VLBI observations of the radio core

We report 1.7 GHz Very Long Baseline Interferometry (VLBI) observations of IRAS F10214+4724, a lensed z=2.3 obscured quasar with prodigious star formation. We detect what we argue to be the obscured active nucleus with an effective angular resolution of < 50 pc at z = 2.3 . The S_{1.7} = 210 micro-Jy (9-σ) detection of this unresolved source is located within the HST rest-frame ultraviolet/optical arc, however, >~100 mas northward of the arc centre of curvature. This leads to a source plane inversion that places the European VLBI Network detection to within milli-arcseconds of the modelled cusp caustic, resulting in a very large magnification (μ~70), over an order of magnitude larger than the CO (1-0) derived magnification of a spatially resolved JVLA map, using the same lens model. We estimate the quasar bolometric luminosity from a number of independent techniques and with our X-ray modelling find evidence that the AGN may be close to Compton-thick, with an intrinsic bolometric luminosity log(L_{bol,QSO} / L_sun) = 11.34 +- 0.27 dex. We make the first black hole mass estimate of IRAS F10214+4724 and find log(M_{BH}/M_sun) = 8.36 +- 0.56 which suggests a low black hole accretion rate (λ= \dot{M} / \dot{M}_{Edd} ~ 3\pm^7_2 percent). We find evidence for a M_{BH}/M_{spheroid} ratio that is 1-2 orders of magnitude larger than that of submillimetre galaxies (SMGs) at z~2. At face value, this suggests IRAS F10214+4724 has undergone a different evolutionary path compared to SMGs at the same epoch. A primary result of this work is the demonstration that emission regions of differing size and position can undergo significantly different magnification boosts (> 1 dex) and therefore distort our view of high-redshift, gravitationally lensed galaxies.

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arXiv2013

The preferentially magnified active nucleus in IRAS F10214+4724 - II. Spatially resolved cold molecular gas

We present JVLA observations of the cold (CO (1-0)) molecular gas in IRAS F10214+4724, a lensed ULIRG at z=2.3 with an obscured active nucleus. The galaxy is spatially and spectrally well-resolved in the CO (1-0) emission line. A CO (1-0) counter-image is detected at the 3-sigma level. Five of the 42 km/s channels (with >5-sigma detections) are mapped back into the source plane and their total magnification posterior PDFs sampled. This reveals a roughly linear arrangement, tentatively a rotating disk. We derive a molecular gas mass of M_gas = 1.2 +- 0.2 x 10^10 M_sun, assuming a ULIRG L_{CO}-to-M_{gas} conversion ratio of α= 0.8 M_sun / (K km/s pc^2) that agrees well with the derived range of α= 0.3 - 1.3 for separate dynamical mass estimates at assumed inclinations of i = 90 - 30 degrees. Based on the AGN and CO (1-0) peak emission positions and the lens model, we predict a distortion of the CO Spectral Line Energy Distribution (SLED) where higher order J lines that may be partially excited by AGN heating will be preferentially lensed owing to their smaller solid angles and closer proximity to the AGN and therefore the cusp of the caustic. Comparison with other lensing inversion results shows that the narrow line region and AGN radio core in IRAS F10214+4724 are preferentially lensed by a factor >~ 3 and 11 respectively, relative to the molecular gas emission. This distorts the global continuum emission Spectral Energy Distribution (SED) and suggests caution in unsophisticated uses of IRAS F10214+4724 as an archetype high-redshift ULIRG. We explore two Large Velocity Gradient (LVG) models, incorporating spatial CO (1-0) and (3-2) information and present tentative evidence for an extended, low excitation cold gas component that implies that the total molecular gas mass in IRAS F10214+4724 is a factor >~2 greater than that calculated using spatially unresolved CO observations.

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arXiv2012

The accretion disc, jets and environment of the intermediate mass black hole candidate ESO 243-49 HLX-1

The Ultra Luminous X-ray (ULX) source HLX-1 in the galaxy ESO 243-49 has an observed maximum unabsorbed X-ray luminosity of 1.3e42 erg/s (0.2-10.0 keV). From the conservative assumption that this value exceeds the Eddington limit by at most a factor of 10, the minimum mass is then 500 solar masses. The X-ray luminosity varies by a factor of 40 with an apparent recurrence timescale of approximately one year. This X-ray variability is associated with spectral state transitions similar to those seen in black hole X-ray binaries. Here we discuss our recent modelling of all the X-ray data for HLX-1 and show that it supports the idea that this ULX is powered by sub- and near Eddington accretion onto an intermediate mass black hole. We also present evidence for transient radio emission which is consistent with a discrete jet ejection event as well as comment on the nature of the environment around HLX-1 in light of recent Hubble Space Telescope photometry.

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arXiv2012

Sub-millimetre source identifications and the micro-Jansky source population at 8.4 GHz in the William Herschel Deep Field

[Abridged] Sub-mm observations of the William Herschel Deep Field using LABOCA revealed possible counterparts for 2 X-ray absorbed QSOs. The aim here is to exploit EVLA imaging at 8.4 GHz to establish the QSOs as radio/sub-mm sources. The challenge in reducing the EVLA data was the presence of a strong 4C source in the field. A new calibration algorithm was applied to the data to subtract it. The resulting thermal noise limited radio map covers the 16'x16' Extended WHDF. It contains 41 sources above a 4-sigma limit, 17 of which have primary beam corrected flux. The radio observations show that the absorbed AGN with LABOCA detections are coincident with radio sources, confirming the tendency for X-ray absorbed AGN to be sub-mm bright. These sources show strong ultraviolet excess (UVX) suggesting the nuclear sightline is gas- but not dust-absorbed. Of the 3 remaining LABOCA sources within the ~5' half-power beam width, 1 is identified with a faint nuclear X-ray/radio source in a nearby galaxy, 1 with a faint radio source and 1 is unidentified in any other band. More generally, differential radio source counts are in good agreement with previous observations, showing at S<50 micro-Jy a significant excess over a pure AGN model. In the full area, of 10 sources fainter than this limit, 6 have optical counterparts of which 3 are UVX (i.e. likely QSOs) including the 2 absorbed quasar LABOCA sources. The other faint radio counterparts are not UVX but are only slightly less blue and likely to be star-forming/merging galaxies, predominantly at lower luminosities and redshifts. The 4 faint, optically unidentified radio sources may be either dust obscured QSOs or galaxies. These high-z obscured AGN and lower-z star-forming populations are thus the main candidates to explain the observed excess in faint source counts and hence the excess radio background found previously by the ARCADE2 experiment.

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arXiv2011

The ALMA Observation Support Tool

The ALMA Observation Support Tool (OST) is an ALMA simulator which is interacted with solely via a standard web browser. It is aimed at users who may or may not be experts in interferometry, or those that do not wish to familarise themselves with the simulation components of a data reduction package. It has been designed to offer full imaging simulation capability for an arbitrary ALMA observation while maintaining the accessibility of other online tools such as the ALMA Sensitivity Calculator. Simulation jobs are defined by selecting and entering options on a standard web form. The user can specify the standard parameters that would need to be considered for an ALMA observation (e.g. pointing direction, frequency set up, duration), and there is also the option to upload arbitrary sky models in FITS format. Once submitted, jobs are sequentially processed by a remote server running a CASA-based back-end system. The user is notified by email when the job is complete, and directed to a standard web page which contains the results of the simulation and a range of downloadable data products. The system is currently hosted by the UK ALMA Regional Centre, and can be accessed by directing a web browser to http://almaost.jb.man.ac.uk.

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arXiv2011

Goonhilly: a new site for e-MERLIN and the EVN

The benefits for the e-MERLIN and EVN arrays of using antennae at the satellite communication station at Goonhilly in Cornwall are discussed. The location of this site - new to astronomy - will provide an almost equal distribution of long baselines in the east-west- and north-south directions, and opens up the possibility to get significantly improved observations of equatorial fields with e-MERLIN. These additional baselines will improve the sensitivity on a set of critical spatial scales and will increase the angular resolution of e-MERLIN by a factor of two. e-MERLIN observations, including many allocated under the e-MERLIN Legacy programme, will benefit from the enhanced angular resolution and imaging capability especially for sources close to or below the celestial equator (where ESO facilities such as ALMA will operate) of including the Goonhilly telescopes. Furthermore, the baselines formed between Goonhilly and the existing stations will close the gap between the baselines of e-MERLIN and those of the European VLBI Network (EVN) and therefore enhance the legacy value of e-MERLIN datasets.

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arXiv2011

MESMER: MeerKAT Search for Molecules in the Epoch of Reionization

[Abridged] Observations of molecular gas at all redshifts are critical for measuring the cosmic evolution in molecular gas density and understanding the star-formation history of the Universe. The 12CO molecule (J=1-0 transition = 115.27 GHz) is the best proxy for extragalactic H2, which is the gas reservoir from which star formation occurs, and has been detected out to z~6. Typically, redshifted high-J lines are observed at mm-wavelengths, the most commonly targeted systems exhibiting high SFRs (e.g. submm galaxies), and far-IR-bright QSOs. While the most luminous objects are the most readily observed, detections of more typical galaxies with modest SFRs are essential for completing the picture. ALMA will be revolutionary in terms of increasing the detection rate and pushing the sensitivity limit down to include such galaxies, however the limited FoV when observing at such high frequencies makes it difficult to use ALMA for studies of the large-scale structure traced out by molecular gas in galaxies. This article introduces a strategy for a systematic search for molecular gas during the EoR (z~7 and above), capitalizing on the fact that the J=1-0 transition of 12CO enters the upper bands of cm-wave instruments at high-z. The FoV advantage gained by observing at such frequencies, coupled with modern broadband correlators allows significant cosmological volumes to be probed on reasonable timescales. In this article we present an overview of our future observing programme which has been awarded 6,500 hours as one of the Large Survey Projects for MeerKAT, the forthcoming South African SKA pathfinder instrument. Its large FoV and correlator bandwidth, and high-sensitivity provide unprecedented survey speed for such work. An existing astrophysical simulation is coupled with instrumental considerations to demonstrate the feasibility of such observations and predict detection rates.

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arXiv2011

Expanding e-MERLIN with the Goonhilly Earth Station

A consortium of universities has recently been formed with the goal of using the decommissioned telecommunications infrastructure at the Goonhilly Earth Station in Cornwall, UK, for astronomical purposes. One particular goal is the introduction of one or more of the ~30-metre parabolic antennas into the existing e-MERLIN radio interferometer. This article introduces this scheme and presents some simulations which quantify the improvements that would be brought to the e-MERLIN system. These include an approximate doubling of the spatial resolution of the array, an increase in its N-S extent with strong implications for imaging the most well-studied equatorial fields, accessible to ESO facilities including ALMA. It also increases the overlap between the e-MERLIN array and the European VLBI Network. We also discuss briefly some niche science areas in which an e-MERLIN array which included a receptor at Goonhilly would be potentially world-leading, in addition to enhancing the existing potential of e-MERLIN in its role as a Square Kilometer Array pathfinder instrument.

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arXiv2010

Herschel-ATLAS: far-infrared properties of radio-selected galaxies

We use the Herschel-ATLAS science demonstration data to investigate the star-formation properties of radio-selected galaxies in the GAMA-9h field as a function of radio luminosity and redshift. Radio selection at the lowest radio luminosities, as expected, selects mostly starburst galaxies. At higher radio luminosities, where the population is dominated by AGN, we find that some individual objects are associated with high far-infrared luminosities. However, the far-infrared properties of the radio-loud population are statistically indistinguishable from those of a comparison population of radio-quiet galaxies matched in redshift and K-band absolute magnitude. There is thus no evidence that the host galaxies of these largely low-luminosity (Fanaroff-Riley class I), and presumably low-excitation, AGN, as a population, have particularly unusual star-formation histories. Models in which the AGN activity in higher-luminosity, high-excitation radio galaxies is triggered by major mergers would predict a luminosity-dependent effect that is not seen in our data (which only span a limited range in radio luminosity) but which may well be detectable with the full Herschel-ATLAS dataset.

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arXiv2010

Further Observations of the Intermediate Mass Black Hole Candidate ESO 243-49 HLX-1

The brightest Ultra-Luminous X-ray source HLX-1 in the galaxy ESO 243-49 currently provides strong evidence for the existence of intermediate mass black holes. Here we present the latest multi-wavelength results on this intriguing source in X-ray, UV and radio bands. We have refined the X-ray position to sub-arcsecond accuracy. We also report the detection of UV emission that could indicate ongoing star formation in the region around HLX-1. The lack of detectable radio emission at the X-ray position strengthens the argument against a background AGN.

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arXiv2010

SKA HI end2end simulation

The current status of the HI simulation efforts is presented, in which a self consistent simulation path is described and basic equations to calculate array sensitivities are given. There is a summary of the SKA Design Study (SKADS) sky simulation and a method for implementing it into the array simulator is presented. A short overview of HI sensitivity requirements is discussed and expected results for a simulated HI survey are presented.

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arXiv2009

Mapping the SKA Simulated Skies with the S3-Tools

The S3-Tools are a set of Python-based routines and interfaces whose purpose is to provide user-friendly access to the SKA Simulated Skies (S3) set of simulations, an effort led by the University of Oxford in the framework of the European Union's SKADS program (http://www.skads-eu.org). The databases built from the S3 simulations are hosted by the Oxford e-Research Center (OeRC), and can be accessed through a web portal at http://s-cubed.physics.ox.ac.uk. This paper focuses on the practical steps involved to make radio images from the S3-SEX and S3-SAX simulations using the S3-Map tool and should be taken as a broad overview. For a more complete description, the interested reader should look up the user's guide. The output images can then be used as input to instrument simulators, e.g. to assess technical designs and observational strategies for the SKA and SKA pathfinders.

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arXiv2009

Multiwavelength study of Cygnus A III. Evidence for relic lobe plasma

We study the particle energy distribution in the cocoon surrounding Cygnus A, using radio images between 151 MHz and 15 GHz and a 200 ks Chandra ACIS-I image. We show that the excess low frequency emission in the the lobe further from Earth cannot be explained by absorption or excess adiabatic expansion of the lobe or a combination of both. We show that this excess emission is consistent with emission from a relic counterlobe and a relic counterjet that are being re-energized by compression from the current lobe. We detect hints of a relic hotspot at the end of the relic X-ray jet in the more distant lobe. We do not detect relic emission in the lobe nearer to Earth as expected from light travel-time effects assuming intrinsic symmetry. We determine that the duration of the previous jet activity phase was slightly less than that of the current jet-active phase. Further, we explain some features observed at 5 and 15 GHz as due to the presence of a relic jet.

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arXiv2007

V723 Cas (Nova Cassiopeiae 1995): MERLIN observations from 1996 to 2001

MERLIN observations of the unusually slow nova V723 Cas are presented. Nine epochs of 6-cm data between 1996 and 2001 are mapped, showing the initial expansion and brightening of the radio remnant, the development of structure and the final decline. A radio light curve is presented and fitted by the standard Hubble flow model for radio emission from novae in order to determine the values of various physical parameters for the shell. The model is consistent with the overall development of the radio emission. Assuming a distance of 2.39 (+/-0.38) kpc and a shell temperature of 17000 K, the model yields values for expansion velocity of 414 +/- 0.1 km s^-1 and shell mass of 1.13 +/- 0.04 * 10^-4 Msolar. These values are consistent with those derived from other observations although the ejected masses are rather higher than theoretical predictions. The structure of the shell is resolved by MERLIN and shows that the assumption of spherical symmetry in the standard model is unlikely to be correct.

astro-ph↗

arXiv2007

Simulations and interpretation of the 6-cm MERLIN images of the classical nova V723 Cas

We compare the predictions of simple models for the radio emission from classical novae with the MERLIN radio observations of nova V723 Cas (Nova Cas 1995). Spherically symmetric and ellipsoidal radiative transfer models are implemented in order to generate synthetic emission maps. These are then convolved with an accurate representation of the uv coverage of MERLIN. The parameters and geometry of the shell model are based on those returned by fitting models to the observed light curve. This allows direct comparison of the model images with the nine 6-cm MERLIN images of V723 Cas. It is found that the seemingly complex structure (clumping, apparent rotation) evident in the observations can actually be reproduced with a simple spherical emission model. The simulations showthat a 24-h track greatly reduces the instrumental effects and the synthetic radio map is a closer representation of the true (model) sky brightness distribution. It is clear that interferometric arrays with sparse uv coverage (e.g. MERLIN, VLBA) will be more prone to these instrumental effects especially when imaging ring-like objects with time-dependent structure variations. A modelling approach such as that adopted here is essential when interpreting observations.

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arXiv2005

Six months of mass outflow and inclined rings in the ejecta of V1494 Aql

V1494 Aql was a very fast nova which reached a visual maximum of about 4.0 by the end of 1999 December 3. We report observations from 4 to 284 days after discovery, including sub-mm- and cm-band fluxes, a single MERLIN image, and optical spectroscopy in the 410 to 700 nm range. The extent of the radio continuum emission is consistent with a recent lower distance estimate of 1.6 kpc. We conclude that the optical and radio emission arises from the same expanding ejecta. We show that these observations are not consistent with simple kinematical spherical shell models used in the past to explain the rise and fall of the radio flux density in these objects. The resolved remnant structure is consistent with an inclined ring of enhanced density within the ejecta. Optical spectroscopy indicates likely continued mass ejection for over 195 days, with the material becoming optically thin in the visible sometime between 195 and 285 days after outburst.

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