R. W. Hunstead

University of Sydney, Sydney, New South Wales, Australia

Are you R. W. Hunstead?

Claim your profile

Publications (280)796.82 Total impact


  • No preview · Article · Mar 2016 · Monthly Notices of the Royal Astronomical Society
  • [Show abstract] [Hide abstract]
    ABSTRACT: We report the first radio interferometric search at 843 MHz for fast transients, particularly Fast Radio Bursts (FRBs). The recently recommissioned Swinburne University of Technology's digital backend for the Molonglo Observatory Synthesis Telescope array (the UTMOST) with its large collecting area (18,000 $\mathrm{m^2}$) and wide instantaneous field of view (7.80 $\mathrm{deg^2}$) is expected to be an efficient tool to detect FRBs. As an interferometer it will be capable of discerning whether the FRBs are truly a celestial population. We show that UTMOST at full design sensitivity can detect an event approximately every few days. We report on 2 preliminary FRB surveys at about 7% and 14% respectively of the array's final sensitivity. Several pulsars have been detected via single pulses and no FRBs were discovered with pulse widths ($W$), in the range 655.36 $\mu$s $< W < 41.9$ ms and dispersion measures (DMs) in the range $100 < $DM$< 2000$ $\mathrm{pc\,cm^{-3}}$. This non-detection sets a 2$\sigma$ upper limit of the sky rate of not more than 1000 events $\mathrm{sky^{-1}}$ $\mathrm{day^{-1}}$ at 843 MHz down to a flux limit of 11 Jy for 1 ms FRBs. We show that this limit is consistent with previous survey limits at 1.4 GHz and 145 MHz and set a lower limit on the mean spectral index of FRBs of $\alpha > -3.2$.
    No preview · Article · Jan 2016
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: High time resolution radio surveys over the last few years have discovered a population of millisecond-duration transient bursts called Fast Radio Bursts (FRBs), which remain of unknown origin. FRBs exhibit dispersion consistent with propagation through a cold plasma and dispersion measures indicative of an origin at cosmological distances. In this paper we perform Monte Carlo simulations of a cosmological population of FRBs, based on assumptions consistent with observations of their energy distribution, their spatial density as a function of redshift and the properties of the interstellar and intergalactic media. We examine whether the dispersion measures, fluences, inferred redshifts, signal-to-noises and effective widths of known FRBs are consistent with a cosmological population. Statistical analyses indicate that at least 50 events at Parkes are required to distinguish between a constant co-moving FRB density, and a FRB density that evolves with redshift like the cosmological star formation rate density.
    Full-text · Article · Dec 2015
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Most successful galaxy formation scenarios now postulate that the intense star formation in massive, high-redshift galaxies during their major growth period was truncated when powerful AGNs launched galaxy-wide outflows of gas that removed large parts of the interstellar medium. The most powerful radio galaxies at z~2 show clear signatures of such winds, but are too rare to be good representatives of a generic phase in the evolution of all massive galaxies at high redshift. Here we present SINFONI imaging spectroscopy of 12 radio galaxies at z~2 that are intermediate between the most powerful radio and vigorous starburst galaxies in radio power, and common enough to represent a generic phase in the early evolution of massive galaxies. The kinematic properties are diverse, with regular velocity gradients with amplitudes of Delta v=200-400 km s^-1 as in rotating disks as well as irregular kinematics with multiple velocity jumps of a few 100 km s^-1. Line widths are generally high, typically around FWHM=800 km s^-1, consistent with wind velocities in hydrodynamic models. A broad H-alpha line in one target implies a black hole mass of a few 10^9 M$_sun. The ratio of line widths, sigma, to bulk velocity, v, is so large that even the gas in galaxies with regular velocity fields is unlikely to be gravitationally bound. It is unclear, however, whether the large line widths are due to turbulence or unresolved, local outflows as are sometimes observed at low redshifts. Comparison of the kinetic energy with the energy supply from the AGN through jet and radiation pressure suggests that the radio source still plays a dominant role for feedback, consistent with low-redshift radio-loud quasars.
    Preview · Article · Oct 2015 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the detection of extended warm ionized gas in two powerful high-redshift radio galaxies, NVSS J210626-314003 at z=2.10 and TXS 2353-003 at z=1.49, that does not appear to be associated with the radio jets. This is contrary to what would be expected from the alignment effect, a characteristic feature of distant, powerful radio galaxies at z> 0.6. The gas also has smaller velocity gradients and line widths than most other high-z radio galaxies with similar data. Both galaxies are part of a systematic study of 50 high-redshift radio galaxies with SINFONI, and are the only two that are characterized by the presence of high surface-brightness gas not associated with the jet axis and by the absence of such gas aligned with the jet. Both galaxies are spatially resolved with ISAAC broadband imaging covering the rest-frame R band, and have extended wings that cannot be attributed to line contamination. We argue that the gas and stellar properties of these galaxies are more akin to gas-rich brightest cluster galaxies in cool-core clusters than the general population of high-redshift radio galaxies at z>2. In support of this interpretation, one of our sources, TXS 2353-003, for which we have H\alpha\ narrowband imaging, is associated with an overdensity of candidate H\alpha\ emitters by a factor of 8 relative to the field at z=1.5. We discuss possible scenarios of the evolutionary state of these galaxies and the nature of their emission line gas within the context of cyclical AGN feedback.
    Preview · Article · Jun 2015 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the discovery of a new 21-cm H i absorption system using commissioning data from the Boolardy Engineering Test Array of the Australian Square Kilometre Array Pathfinder (ASKAP). Using the 711.5–1015.5 MHz band of ASKAP we were able to conduct a blind search for the 21-cm line in a continuous redshift range between z = 0.4 and 1.0, which has, until now, remained largely unexplored. The absorption line is detected at z = 0.44 towards the GHz-peaked spectrum radio source PKS B1740−517 and demonstrates ASKAP's excellent capability for performing a future wide-field survey for H i absorption at these redshifts. Optical spectroscopy and imaging using the Gemini-South telescope indicates that the H i gas is intrinsic to the host galaxy of the radio source. The narrow [O iii] emission lines show clear double-peaked structure, indicating either large-scale outflow or rotation of the ionized gas. Archival data from the XMM–Newton satellite exhibit an absorbed X-ray spectrum that is consistent with a high column density obscuring medium around the active galactic nucleus. The H i absorption profile is complex, with four distinct components ranging in width from 5 to 300 km s−1 and fractional depths from 0.2 to 20 per cent. In addition to systemic H i gas, in a circumnuclear disc or ring structure aligned with the radio jet, we find evidence for a possible broad outflow of neutral gas moving at a radial velocity of v ∼ 300 km s−1. We infer that the expanding young radio source (tage ≈ 2500 yr) is cocooned within a dense medium and may be driving circumnuclear neutral gas in an outflow of ∼1 M⊙ yr−1.
    Full-text · Article · Mar 2015 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Recent observations with the Murchison Widefield Array at 185 MHz have serendipitously unveiled a heretofore unknown giant and relatively nearby (z = 0.0178) radio galaxy associated with NGC 1534. The diffuse emission presented here is the first indication that NGC 1534 is one of a rare class of objects (along with NGC 5128 and NGC 612) in which a galaxy with a prominent dust lane hosts radio emission on scales of ∼700 kpc. We present details of the radio emission along with a detailed comparison with other radio galaxies with discs. NGC 1534 is the lowest surface brightness radio galaxy known with an estimated scaled 1.4-GHz surface brightness of just 0.2 mJy arcmin−2. The radio lobes have one of the steepest spectral indices yet observed: α = −2.1 ± 0.1, and the core to lobe luminosity ratio is <0.1 per cent. We estimate the space density of this low brightness (dying) phase of radio galaxy evolution as 7 × 10−7 Mpc−3 and argue that normal AGN cannot spend more than 6 per cent of their lifetime in this phase if they all go through the same cycle.
    Full-text · Article · Dec 2014 · Monthly Notices of the Royal Astronomical Society
  • Source

    Full-text · Article · Jul 2014 · Astronomy and Astrophysics
  • Source
    Adam Schaefer · Richard Hunstead · Helen Johnston
    [Show abstract] [Hide abstract]
    ABSTRACT: Optical positions from the SuperCOSMOS Sky Survey have been compared in detail with accurate radio positions that define the second realisation of the International Celestial Reference Frame (ICRF2). The comparison was limited to the IIIaJ plates from the UK/AAO and Oschin (Palomar) Schmidt telescopes. A total of 1373 ICRF2 sources was used, with the sample restricted to stellar objects brighter than $B_J=20$ and Galactic latitudes $|b|>10^{\circ}$. Position differences showed an rms scatter of $0.16''$ in right ascension and declination. While overall systematic offsets were $<0.1''$ in each hemisphere, both the systematics and scatter were greater in the north.
    Preview · Article · Jan 2014 · Publications of the Astronomical Society of Australia
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Multifibre spectroscopical service observations in the central 2x2deg2 of A3921 were carried out using the Two Degree Field (2dF) system on the AAT (Anglo-Australian Telescope). We used the 400 fibre positioner and double spectrograph system of 2dF to perform two sets of observations both centred at RA=22:49:45.84, DE=-64:22:16.32. We used the same 2dF grating (i.e. 300B) in the two available spectrographs of 2dF and we adopted a central wavelength of 6000Å. Our observations thus gave 200 spectra per observing run and spectrograph with a spectral resolution of 8-9Å and covered the approximate wavelength range 3800-8200Å. Two runs of observations were carried out in October 2004 and November 2004 with a total exposure time of 3600 seconds per run. Table 5 includes observing run, ccd number, target number, coordinates (J2000), redshifts, redshift errors and a redshift quality flag for all 523 galaxies observed with the 2dF device at the AAO in September and November 2004. The combination of observing run, ccd number and target number is unique for each galaxy and can be used to identify the associated spectra (spectra FITS-file names are encoded as "Obs"ccd"ccd number"."target number".fits, e.g. Nov04ccd1.001.fits). The total number of spectra is 658 (including double observations and stars). (4 data files).
    Full-text · Article · Jul 2013
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Continuing our program of spectroscopic observations of International Celestial Reference Frame (ICRF) sources, we present redshifts for 120 quasars and radio galaxies. Data were obtained with five telescopes: the 3.58 m European Southern Observatory New Technology Telescope, the two 8.2 m Gemini telescopes, the 2.5 m Nordic Optical Telescope (NOT), and the 6.0 m Big Azimuthal Telescope of the Special Astrophysical Observatory in Russia. The targets were selected from the International VLBI Service for Geodesy & Astrometry candidate International Celestial Reference Catalog which forms part of an observational very long baseline interferometry (VLBI) program to strengthen the celestial reference frame. We obtained spectra of the potential optical counterparts of more than 150 compact flat-spectrum radio sources, and measured redshifts of 120 emission-line objects, together with 19 BL Lac objects. These identifications add significantly to the precise radio-optical frame tie to be undertaken by Gaia, due to be launched in 2013, and to the existing data available for analyzing source proper motions over the celestial sphere. We show that the distribution of redshifts for ICRF sources is consistent with the much larger sample drawn from Faint Images of the Radio Sky at Twenty cm (FIRST) and Sloan Digital Sky Survey, implying that the ultra-compact VLBI sources are not distinguished from the overall radio-loud quasar population. In addition, we obtained NOT spectra for five radio sources from the FIRST and NRAO VLA Sky Survey catalogs, selected on the basis of their red colors, which yielded three quasars with z > 4.
    Full-text · Article · Jun 2013 · The Astronomical Journal
  • Source
    C. S. Anderson · H. M. Johnston · R. W. Hunstead
    [Show abstract] [Hide abstract]
    ABSTRACT: Mergers between a massive galaxy and a small gas-rich companion (minor mergers) have been proposed as a viable mechanism for triggering radio emission in an active galaxy. Until now the problem has been catching this sequence of events as they occur. With MRC B1221−423, we have an active radio galaxy that has only recently been triggered, and a companion galaxy that provides the ‘smoking gun’. Using spectroscopic data taken with the VIsible Multi Object Spectrograph (VIMOS) integral field unit detector on the European Southern Observatory's Very Large Telescope, we have examined the distribution, ionization state and kinematics of ionized gas in this interacting system. We have also modelled the stellar continuum with synthesized spectra of stellar populations of different ages. From our study of the ionized gas, we have derived preliminary models for the geometry of the interaction, analysed the kinematic behaviour of the ionized gas, and examined the ionization mechanisms at work throughout the system. Our modelling of the stellar continuum allowed us to identify and date distinct stellar populations within the galaxy pair. We find evidence of multiple episodes of widespread starburst activity, and by dating these populations, we provide tentative insight into the history of the interaction.
    Preview · Article · Jun 2013 · Monthly Notices of the Royal Astronomical Society
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a spectrophotometric analysis of the galaxy pop. in the area of the merging cluster Abell 3921 at redshift 0.093. We investigate the impact of the complex cluster environment on galaxy properties such as morphology or star formation rate. We combine multi-object spectroscopy from the 2dF spectrograph with optical imaging taken with the ESO WFI. We carry out a redshift analysis and determine cluster velocity dispersions using biweight statistics. Applying a Dressler-Shectman (DS-)test we seek evidence for cluster substructure. Cluster and field galaxies are investigated with respect to [OII] and H{\alpha} equivalent width, SFR and morphological descriptors such as concentration index and Gini coefficient. We study these cluster galaxy properties as a function of clustercentric distance and investigate the spatial distribution of various galaxy types. Applying the DS-test we find a 3rd component (A3921-C) in addition to the two main subclusters (A3921-A and A3921-B) already known. The re-determined mass ratio between the main components A and B is approx. 2:1. Similar to prev. studies of galaxy clusters, we find that a large fraction of the disk galaxies close to the cluster core show no detectable star formation. These are likely systems that are quenched due to ram pressure stripping. We also find quenched spirals at rather large distances of 3 to 4 Mpc from the cluster core. A3921-C might be a group of galaxies falling onto the main cluster components. We speculate that the unexpected population of quenched spirals at large clustercentric radii in A3921-A and A3921-B might be an effect of the ongoing cluster merger: shocks in the ICM might give raise to enhanced ram pressure stripping and at least in part be the cause for the quenching of star formation. These quenched spirals might be an interm. stage in the morphological transformation of field spirals into cluster S0s.
    Full-text · Article · May 2013 · Astronomy and Astrophysics
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Continuing our program of spectroscopic observations of ICRF sources, we present redshifts for 120 quasars and radio galaxies. Data were obtained with five telescopes: the 3.58m ESO New Technology Telescope (NTT), the two 8.2m Gemini telescopes, the 2.5m Nordic Optical Telescope (NOT), and the 6.0m Big Azimuthal Telescope (BTA) of the Special Astrophysical Observatory in Russia. The targets were selected from the International VLBI Service for Geodesy & Astrometry (IVS) candidate International Celestial Reference Catalogue which forms part of an observational VLBI program to strengthen the celestial reference frame. We obtained spectra of the potential optical counterparts of more than 150 compact flat-spectrum radio sources, and measured redshifts of 120 emission-line objects, together with 19 BL Lac objects. These identifications add significantly to the precise radio--optical frame tie to be undertaken by Gaia, due to be launched in 2013, and to the existing data available for analysing source proper motions over the celestial sphere. We show that the distribution of redshifts for ICRF sources is consistent with the much larger sample drawn from FIRST and SDSS, implying that the ultra-compact VLBI sources are not distinguished from the overall radio-loud quasar population. In addition, we obtained NOT spectra for five radio sources from the FIRST and NVSS catalogs, selected on the basis of their red colors, which yielded three quasars with z>4.
    Full-text · Article · May 2013
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present archival radio observations of SN1996cr from ATCA and MOST, and model its radio lightcurves using X-ray constrained hydrodynamical simulations of the interaction between the SN ejecta and the CSM. The early radio data show signs of free-free absorption (FFA), which decreases gradually and is minimal above 1.4 GHz after day ~3000. FFA optical depth constraints provide estimates of the CSM free electron density, which allows insight into the ionisation of SN1996cr's CSM and offers a test on the density distribution adopted by the hydrodynamical simulation. The intrinsic spectral index of the radiation shows evidence for spectral flattening, which is characterised by alpha = 0.852 +/- 0.002 at day 3000 and a decay rate of d_alpha = -0.014 +/- 0.001 yr^-1. The similarity in the spectral flattening of SN1987A, SN1993J, and SN1996cr suggests this may be a relatively common feature of SNe/CSM shocks. We adopt this spectral index variation to model the synchrotron radio emission of the shock, and consider several scalings that relate the parameters of the hydrodynamical simulation to the magnetic field and electron distribution. The simulated light curves match the large-scale features of the observed light curves, but fail to match certain tightly constraining sections. This suggests that simple energy density scalings may not be able to account for the complexities of the true physical processes at work, or alternatively, that the parameters of the simulation require modification in order to accurately represent SN1996cr's CSM.
    Full-text · Article · Feb 2013 · Monthly Notices of the Royal Astronomical Society
  • Source
    Kiran Lakhchaura · K. P. Singh · D. J. Saikia · R. W. Hunstead
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a detailed study of a close pair of clusters of galaxies, A3532 and A3530, and their environments. The \textit{Chandra} X-ray image of A3532 reveals presence of substructures on scales of $\sim$20$^{\prime\prime}$ in its core. XMM-Newton maps of the clusters show excess X-ray emission from an overlapping region between them. Spectrally determined projected temperature and entropy maps do not show any signs of cluster scale mergers either in the overlapping region or in any of the clusters. In A3532, however, some signs of the presence of galaxy scale mergers are visible e.g., anisotropic temperature variations in the projected thermodynamic maps, a wide angled tailed (WAT) radio source in the brighter nucleus of its dumbbell Brightest Cluster Galaxy (BCG), and a candidate X-ray cavity coincident with the northwestern extension of the WAT source in the low-frequency radio observations. The northwestern extension in A3532 seems either a part of the WAT or an unrelated diffuse source in A3532 or in the background. There is an indication that the cool core in A3532 has been disrupted by the central AGN activity. A reanalysis of the redshift data reinforces the close proximity of the clusters. The excess emission in the overlapping region appears to be a result of tidal interactions as the two clusters approach each other for the first time. However, we can not rule out the possibility of the excess being due to the chance superposition of their X-ray halos.
    Full-text · Article · Dec 2012 · The Astrophysical Journal
  • [Show abstract] [Hide abstract]
    ABSTRACT: The ATLBS radio images were made at 1388MHz using the Australia Telescope Compact Array (ATCA) by mosaic observing 38 adjacent pointing positions covering about 8.4deg2 of sky area. ATLBS images made with the synthesized beam of 50 arcsec were used as the basic resource for compiling the ATLBS-ESS sample. In the two, 2deg mosaic images, only "islands" of image pixels with peaks exceeding five times the image rms noise were considered. As described in Subrahmanyan et al. (2010, Cat. J/MNRAS/402/2792), the integrated flux density in compact components within these source islands were computed from images made with 4 arcsec FWHM beam using exclusively interferometer baselines to the 6km antenna. The ATLBS survey regions were also observed in optical g, r, and z band with the MOSAICII imager on the CTIO NOAO 4m Blanco telescope. (2 data files).
    No preview · Article · Jun 2012
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Based on the ATLBS survey we present a sample of extended radio sources and derive morphological properties of faint radio sources. 119 radio galaxies form the ATLBS-Extended Source Sample (ATLBS-ESS) consisting of all sources exceeding 30" in extent and integrated flux densities exceeding 1 mJy. We give structural details along with information on galaxy identifications and source classifications. The ATLBS-ESS, unlike samples with higher flux-density limits, has almost equal fractions of FR-I and FR-II radio galaxies with a large fraction of the FR-I population exhibiting 3C31-type structures. Significant asymmetry in lobe extents appears to be a common occurrence in the ATLBS-ESS FR-I sources compared to FR-II sources. We present a sample of 22 FR-Is at z>0.5 with good structural information. The detection of several giant radio sources, with size exceeding 0.7 Mpc, at z>1 suggests that giant radio sources are not less common at high redshifts. The ESS also includes a sample of 28 restarted radio galaxies. The relative abundance of dying and restarting sources is indicative of a model where radio sources undergo episodic activity in which an active phase is followed by a brief dying phase that terminates with restarting of the central activity; in any massive elliptical a few such activity cycles wherein adjacent events blend may constitute the lifetime of a radio source and such bursts of blended activity cycles may be repeated over the age of the host. The ATLBS-ESS includes a 2-Mpc giant radio galaxy with the lowest surface brightness lobes known to date.
    Preview · Article · Feb 2012 · The Astrophysical Journal Supplement Series
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have developed a multiscale structure identification algorithm for the detection of overdensities in galaxy data that identifies structures having radii within a user-defined range. Our ‘multiscale probability mapping’ technique combines density estimation with a shape statistic to identify local peaks in the density field. This technique takes advantage of a user-defined range of scale sizes, which are used in constructing a coarse-grained map of the underlying fine-grained galaxy distribution, from which overdense structures are then identified. In this study we have compiled a catalogue of groups and clusters at 0.025 < z < 0.24 based on the Sloan Digital Sky Survey (SDSS), Data Release 7, quantifying their significance and comparing with other catalogues. Most measured velocity dispersions for these structures lie between 50 and 400 km s−1. A clear trend of increasing velocity dispersion with radius from 0.2 to 1 h−1 Mpc is detected, confirming the lack of a sharp division between groups and clusters. A method for quantifying elongation is also developed to measure the elongation of group and cluster environments. By using our group and cluster catalogue as a coarse-grained representation of the galaxy distribution for structure sizes of Mpc, we identify 53 filaments (from an algorithmically derived set of 100 candidates) as elongated unions of groups and clusters at 0.025 < z < 0.13. These filaments have morphologies that are consistent with previous samples studied.
    Preview · Article · Jan 2012 · Monthly Notices of the Royal Astronomical Society

  • No preview · Article · Jan 2012

Publication Stats

5k Citations
796.82 Total Impact Points

Institutions

  • 1980-2015
    • University of Sydney
      • School of Physics
      Sydney, New South Wales, Australia
  • 2008
    • The Commonwealth Scientific and Industrial Research Organisation
      Canberra, Australian Capital Territory, Australia
  • 2004
    • University of Bristol
      • School of Physics
      Bristol, England, United Kingdom
  • 2003
    • University of Oxford
      Oxford, England, United Kingdom
  • 1976-2002
    • University of California, Berkeley
      • Department of Astronomy
      Berkeley, California, United States
  • 1999
    • Massachusetts Institute of Technology
      Cambridge, Massachusetts, United States
  • 1997
    • York University
      • Department of Physics and Astronomy
      Toronto, Ontario, Canada
    • National Research Council
      Roma, Latium, Italy
  • 1996
    • Indian Institute of Astrophysics
      Bengalūru, Karnātaka, India
    • Raman Research Institute
      Bengalūru, Karnataka, India
    • Lawrence Livermore National Laboratory
      Livermore, California, United States
  • 1994
    • Max Planck Institute for Extraterrestrial Physics
      Arching, Bavaria, Germany
  • 1983-1988
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 1984
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States