R. W. Hunstead

University of Sydney, Sydney, New South Wales, Australia

Are you R. W. Hunstead?

Claim your profile

Publications (261)799.59 Total impact

  • 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.
    01/2014;
  • [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).
    VizieR Online Data Catalog. 07/2013;
  • [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.
    The Astronomical Journal 06/2013; 146(1):10. · 4.97 Impact Factor
  • 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.
    Monthly Notices of the Royal Astronomical Society 06/2013; 431(4):3269-3281. · 5.52 Impact Factor
  • 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.
    Astronomy and Astrophysics 05/2013; · 5.08 Impact Factor
  • 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.
    05/2013;
  • 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.
    Monthly Notices of the Royal Astronomical Society 02/2013; 431(3). · 5.52 Impact Factor
  • Source
    [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.
    The Astrophysical Journal 12/2012; 767:91. · 6.73 Impact Factor
  • [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).
    VizieR Online Data Catalog. 06/2012;
  • 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.
    The Astrophysical Journal Supplement Series 02/2012; 199(2). · 16.24 Impact Factor
  • 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, 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. A clear trend of increasing velocity dispersion with radius from 0.2 to 1 Mpc/h 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 <~ 1 Mpc/h, 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.
    Monthly Notices of the Royal Astronomical Society 01/2012; 422(1). · 5.52 Impact Factor
  • 01/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the results of spectroscopic observations of the optical counterparts of 47 southern radio sources from the candidate International Celestial Reference Catalogue (ICRC), as part of a Very Long Baseline Interferometry (VLBI) program to strengthen the celestial reference frame, especially in the south. The observations were made with the 3.58-meter European Southern Observatory New Technology Telescope (NTT). We obtained redshifts for 30 quasars and one radio galaxy, with a further 7 objects being probable BL Lac objects with featureless spectra. Of the remainder, four were clear misidentifications with Galactic stars and five had low signal-to-noise spectra and could not be classified. These results, in combination with new VLBI data of the radio sources with redshifts more than 2, add significantly to the existing data needed to refine the distribution of source proper motions over the celestial sphere.
    The Astronomical Journal 09/2011; 142. · 4.97 Impact Factor
  • H. Shi, H. Liang, J. L. Han, R. W. Hunstead
    [Show abstract] [Hide abstract]
    ABSTRACT: A sample of 129 unresolved radio sources with ultrahigh linear polarization (>30%) has been selected from the NRAO VLA Sky Survey. Such high average linear polarization is unusual in extragalactic sources. Higher resolution Australia Telescope Compact Array and Very Large Array observations confirm the high average polarization but find that most of these sources are extended. The Sloan Digital Sky Survey spectroscopy, where available, shows that the optical counterparts are elliptical galaxies with no detectable emission lines. The optical spectra, radio luminosity, linear size and spectral index of these sources are typical of radio-loud active galactic nuclei. (3 data files).
    VizieR Online Data Catalog. 06/2011;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: About 1200 radio quasars are observed regularly by the International VLBI Service, IVS, to define and maintain the International Celestial Reference Frame, ICRF. We have presented evidence for large scale systematic proper motions, dipole and quadrupole effects, at a level in excess of 10 µas/year. This result presents a serious challenge to the standard cosmologies. However, we are concerned that our estimates of the spherical harmonics may be influenced by the paucity of IVS quasars with known redshifts in the south. We estimate that 100–150 new redshifts uniformly distributed in the south will remove these concerns. In this paper we present the results of spectroscopic observations of 47 southern IVS radio sources with the 3.58-meter NTT telescope.
    03/2011;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present a detailed imaging and spectral analysis of the merging environment of the bimodal cluster A3395 using X-ray and radio observations. X-ray images of the cluster show five main constituents of diffuse emission: A3395 NE, A3395 SW, A3395 NW, A3395 W, and a filament connecting NE to W. X-ray surface brightness profiles of the cluster did not show any shock fronts in the cluster. Temperature and entropy maps show high-temperature and high-entropy regions in the W, the NW, the filament, and between the NE and SW subclusters. The NE, SW, and W components have X-ray bolometric luminosities similar to those of rich clusters of galaxies but have relatively higher temperatures. Similarly, the NW component has X-ray bolometric luminosity similar to that of isolated groups but with much higher temperature. It is, therefore, possible that all the components of the cluster have been heated by the ongoing mergers. The NE subcluster is the most massive and luminous constituent and other subclusters are found to be gravitationally bound to it. The W component is most probably either a clump of gas stripped off the SW due to ram pressure or a separate subcluster that has merged or is merging with the SW. No X-ray cavities are seen associated with the wide-angle-tailed (WAT) radio source near the center of the SW subcluster. Minimum energy pressure in the radio emission peaks of the WAT galaxy is comparable with the external thermal pressure. The radio spectrum of the WAT suggests a spectral age of {approx}10 Myr.
    The Astrophysical Journal 01/2011; · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Using X-ray observations from Chandra and XMM-Newton and radio observations from the Australia Telescope Compact Array (ATCA), we have examined the merging environment of the bimodal cluster Abell 3395. From X-ray data we have produced thermodynamic maps of the cluster. The Wide Angle Tail (WAT) galaxy seen in the radio is slightly offset from the X-ray emission peak of the southern part of the cluster. The unsharp masked Chandra image of the cluster does not show any deficit in the X-ray flux near the location of the source possibly because the thermal plasma has leaked into the cavities.
    Journal of Astrophysics and Astronomy 01/2011; · 0.34 Impact Factor
  • Source
    H. Shi, H. Liang, J. L. Han, R. W. Hunstead
    [Show abstract] [Hide abstract]
    ABSTRACT: ABSTRACTA sample of 129 unresolved radio sources with ultrahigh linear polarization (>30 per cent) has been selected from the NRAO VLA Sky Survey. Such high average linear polarization is unusual in extragalactic sources. Higher resolution Australia Telescope Compact Array and Very Large Array observations confirm the high average polarization but find that most of these sources are extended. The Sloan Digital Sky Survey spectroscopy, where available, shows that the optical counterparts are elliptical galaxies with no detectable emission lines. The optical spectra, radio luminosity, linear size and spectral index of these sources are typical of radio-loud active galactic nuclei. Galaxy counts within a 1 Mpc radius of the radio sources show that these highly polarized sources are in environments similar to their low polarization (<2 per cent) counterparts. Similarly, the line-of-sight environments of the ultrahigh polarization sources are on average indistinguishable from those of the low-polarization sources. We conclude that the extraordinarily high average polarization must be due to intrinsic properties of the sources, such as an extremely ordered source magnetic field, low internal thermal plasma density or a preferential orientation of the source magnetic field perpendicular to the line of sight.
    Monthly Notices of the Royal Astronomical Society 11/2010; 409(2):821 - 838. · 5.52 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We performed follow up radio observations of XMMU J115113.3-623730 (ATel #2746, #2777, #2771) with the Australia Telescope Compact Array at two separate epochs (2010 July 20, 22:16 UT and 2010 August 7, 01:00 UT), each for two hours in duration. The array was in the compact east-west configuration (EW352) in the July 20 epoch and the hybrid configuration (H168) in the August 7 epoch. We observed at 5.5 GHz and 9 GHz simultaneously, each with a bandwidth of 2 GHz, using the new Compact Array Broadband Backend.
    The Astronomer's Telegram. 08/2010;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present optical spectra and high-resolution multi-wavelength radio observations of the compact steep-spectrum radio source MRC B1221-423 (z=0.1706). MRC B1221-423 is a very young (~10^5 yr), powerful radio source which is undergoing a tidal interaction with a companion galaxy. We find strong evidence of interaction between the AGN and its environment. The radio morphology is highly distorted, showing a dramatic interaction between the radio jet and the host galaxy, with the jet being turned almost back on itself. H I observations show strong absorption against the nucleus at an infall velocity of ~250 km/s compared to the stellar velocity, as well as a second, broader component which may represent gas falling into the nucleus. Optical spectra show that star formation is taking place across the whole system. Broad optical emission lines in the nucleus show evidence of outflow. Our observations confirm that MRC B1221-423 is a young radio source in a gas-rich nuclear environment, and that there was a time delay of a few x 100 Myr between the onset of star formation and the triggering of the AGN. Comment: 15 pages, 13 figures, to appear in MNRAS
    Monthly Notices of the Royal Astronomical Society 05/2010; · 5.52 Impact Factor

Publication Stats

3k Citations
799.59 Total Impact Points

Institutions

  • 1980–2014
    • University of Sydney
      • • School of Physics
      • • Sydney Institute of Astronomy (SIfA)
      Sydney, New South Wales, Australia
  • 2004
    • University of Bristol
      • School of Physics
      Bristol, England, United Kingdom
    • University of Oxford
      Oxford, England, United Kingdom
  • 2000–2002
    • University of California, Berkeley
      • Department of Astronomy
      Berkeley, California, United States
  • 1996
    • Indian Institute of Astrophysics
      Bengalūru, Karnātaka, India
  • 1983–1988
    • Space Telescope Science Institute
      Baltimore, Maryland, United States