A. Robinson

Rochester Institute of Technology, Rochester, NY, United States

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Publications (102)335.42 Total impact

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    ABSTRACT: It remains uncertain which continuum and emission line diagnostics best indicate the bolometric powers of active galactic nuclei (AGN), especially given the attenuation caused by the circum-nuclear material, and the possible contamination by components related to star formation. Here we use mid-IR spectra along with the multi-wavelength data to investigate the merit of various diagnostics of AGN radiative power, including the mid-IR [NeIII]25.89 micron and [OIV]25.89 micron fine structure lines, the optical [OIII]5007 forbidden line, and mid-IR 24 micron, 5GHz radio, and X-ray continuum emission, for complete samples of 46 2Jy radio galaxies (0.05<z<0.7) and 17 3CRR FRII radio galaxies (z<0.1). We find that the mid-IR [OIV] line is the most reliable indicator of AGN power for powerful radio-loud AGN. By assuming that the [OIV] is emitted isotropically, and comparing the [OIII] and 24 micron luminosities of the broad- and narrow-line AGN in our samples at fixed [OIV] luminosity, we show that the [OIII] and 24 micron emission are both mildly attenuated in the narrow-line compared to the broad-line objects by a factor 2. However, despite this attenuation, the [OIII] and 24 micron luminosities are better AGN power indicators for our sample than either the 5 GHz radio or the X-ray continuum luminosities. We also detect the mid-IR 9.7 micron silicate feature in the spectra of many objects but not ubiquitously: at least 40% of the sample show no clear evidence for these features. We conclude that, for the majority of powerful radio galaxies, the mid-IR lines are powered by AGN photoionisation.
    05/2014;
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    ABSTRACT: It remains uncertain which continuum and emission line diagnostics best indicate the bolometric powers of active galactic nuclei (AGN), especially given the attenuation caused by the circum-nuclear material, and the possible contamination by components related to star formation. Here we use mid-IR spectra along with the multi-wavelength data to investigate the merit of various diagnostics of AGN radiative power, including the mid-IR [NeIII]25.89 micron and [OIV]25.89 micron fine structure lines, the optical [OIII]5007 forbidden line, and mid-IR 24 micron, 5GHz radio, and X-ray continuum emission, for complete samples of 46 2Jy radio galaxies (0.05
    04/2014;
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    ABSTRACT: What are the mechanisms that funnel gas from the inner kpc of an active galaxy (AGN) down to the central super-massive black hole (SMBH)? Is there any relation between the SMBH accretion rate and the mass in/outflow rate? To answer these questions we are studying a sample of nearby AGN spanning a wide range of nuclear X-ray luminosity (a proxy for the SMBH accretion rate). Here we present results of a detailed analysis of the ionized gas kinematics in the inner kpc of NGC 1386, a nearby Seyfert 2. Data have been obtained with the GMOS integral field unit on the GEMINI South telescope at a spatial resolution of ~70 pc and a spectral resolution of ~50 km/s. Previous HST narrow-band imaging observations suggested the presence of a bipolar outflow. However, our velocity maps show that these features are consistent with a rotating large-scale disk extending over ~300 pc in radius and illuminated by the AGN radiation cone, whilst the inner 100 pc is dominated by a wide-angle outflow whose axis is roughly perpendicular to the cone.
    01/2014;
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    ABSTRACT: The idea that the central regions of Active Galactic Nuclei (AGN) are obscured by a circum-nuclear torus of dusty molecular gas is a keystone of the AGN Unified Scheme. However, the size and structure of the torus are not well constrained by observations. Here, we present early results from an international campaign to determine the size of the torus in a sample of 12 Seyfert galaxies using reverberation mapping techniques, focussing on the Seyfert 1 galaxy NGC 6418. We have used the Spitzer Space Telescope to acquire mid infrared (3.6 and 4.5 micron) observations for over a year with a high cadence of 3 days. Optical V band observations were obtained concurrently using several ground based telescopes. Cross-correlation of the 3.6 micron and optical (V-band) light curves, indicates that the size for the region 3.6 micron-emitting region to be 30.7 +/- 2.2 light days. We also find a lag of 11.5 +/- 0.9 days between the 3.6 micron and 4.5 micron channels. We discuss the implications of these results.
    01/2014;
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    ABSTRACT: We present new CRIRES spectroscopic observations of the Brγ emission line in the nuclear region of the Circinus galaxy, obtained with the aim of measuring the black hole (BH) mass with the spectroastrometric technique. The Circinus galaxy is an ideal benchmark for the spectroastrometric technique given its proximity and secure BH measurement obtained with the observation of its nuclear H 2 O maser disk. The kinematical data have been analyzed both with the classical method based on the analysis of the rotation curves and with the new method developed by us and based on spectroastrometry. The classical method indicates that the gas disk rotates in a gravitational potential resulting from an extended stellar mass distribution and a spatially unresolved dynamical mass of (1.7 ± 0.2) × 10 7 M ⊙ , concentrated within r < 7 pc, corresponding to the seeing-limited resolution of the observations. The new method is capable of probing the gas rotation at scales which are a factor ∼ 3.5 smaller than those probed by the rotation curve analysis, highlighting the potential of spectroastrometry. The dynamical mass which is spatially unresolved with the spectroastrometric method is a factor ∼ 2 smaller, 7.9 +1.4 −1.1 ×10 6 M ⊙ indicating that spectroastrometry has been able to spatially resolve the nuclear mass distribution down to 2 pc scales. This unresolved mass is still a factor ∼ 4.5 larger than the BH mass measurement obtained with the H 2 O maser emission indicating that, even with spectroastrometry, it has not been possible to resolve the sphere of influence of the BH. Based on literature data, this spatially unresolved dynamical mass distribution is likely dominated by warm molecular gas and it has been tentatively identified with the circum-nuclear torus which prevents a direct view of the central BH in Circinus. This mass distribution, with a size of ∼ 2pc, is similar in shape to that of the star cluster of the Milky Way suggesting that a molecular torus, forming stars at a high rate, might be the earlier evolutionary stage of the nuclear star clusters which are common in late type spirals.
    Astronomy and Astrophysics 01/2013; 549. · 5.08 Impact Factor
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    ABSTRACT: We present new CRIRES spectroscopic observations of BrGamma in the nuclear region of the Circinus galaxy, obtained with the aim of measuring the black hole (BH) mass with the spectroastrometric technique. The Circinus galaxy is an ideal benchmark for the spectroastrometric technique given its proximity and secure BH measurement obtained with the observation of its nuclear H2O maser disk. The kinematical data have been analyzed both with the classical method based on the analysis of the rotation curves and with the new method developed by us and based on spectroastrometry. The classical method indicates that the gas disk rotates in the gravitational potential of an extended stellar mass distribution and a spatially unresolved mass of (1.7 +- 0.2) 10^7 Msun, concentrated within r < 7 pc. The new method is capable of probing gas rotation at scales which are a factor ~3.5 smaller than those probed by the rotation curve analysis. The dynamical mass spatially unresolved with the spectroastrometric method is a factor ~2 smaller, 7.9 (+1.4 -1.1) 10^6 Msun indicating that spectroastrometry has been able to spatially resolve the nuclear mass distribution down to 2 pc scales. This unresolved mass is still a factor ~4.5 larger than the BH mass measurement obtained with the H2O maser emission indicating that it has not been possible to resolve the sphere of influence of the BH. Based on literature data, this spatially unresolved dynamical mass distribution is likely dominated by molecular gas and it has been tentatively identified with the circum-nuclear torus which prevents a direct view of the central BH in Circinus. This mass distribution, with a size of ~2pc, is similar in shape to that of the star cluster of the Milky Way suggesting that a molecular torus, forming stars at a high rate, might be the earlier evolutionary stage of the nuclear star clusters which are common in late type spirals.
    http://dx.doi.org/10.1051/0004-6361/200912530. 11/2012;
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    ABSTRACT: (Abridged) We present the results from new 15 ks Chandra-ACIS and 4.9 GHz Very Large Array observations of 13 galaxies hosting low luminosity AGN. This completes the multiwavelength study of a sample of 51 nearby early-type galaxies described in Capetti & Balmaverde (2005, 2006); Balmaverde & Capetti (2006). The aim of the three previous papers was to explore the connection between the host galaxies and AGN activity in a radio-selected sample. We detect nuclear X-ray emission in eight sources and radio emission in all but one (viz., UGC6985). The new VLA observations improve the spatial resolution by a factor of ten: the presence of nuclear radio sources in 12 of the 13 galaxies confirms their AGN nature. As previously indicated, the behavior of the X-ray and radio emission in these sources depends strongly on the form of their optical surface brightness profiles derived from Hubble Space Telescope imaging, i.e., on their classification as "core", "power-law" or "intermediate" galaxies. With more than twice the number of "power-law" and "intermediate" galaxies compared to previous work, we confirm with a much higher statistical significance that these galaxies lie well above the radio-X-ray correlation established in FRI radio galaxies and the low-luminosity "core" galaxies. This result highlights the fact that the "radio-loud/radio-quiet" dichotomy is a function of the host galaxy's optical surface brightness profile. We present radio-optical-X-ray spectral indices for all 51 sample galaxies. Survival statistics point to significant differences in the radio-to-optical and radio-to-X-ray spectral indices between the "core" and "power-law" galaxies (Gehan's Generalized Wilcoxon test probability "p" for the two classes being statistically similar is <10^-5), but not in the optical-to-X-ray spectral indices (p=0.25).
    The Astronomical Journal 01/2012; 143(4). · 4.97 Impact Factor
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    ABSTRACT: We present deep Spitzer/IRS spectra for complete samples of 46 2Jy radio galaxies (0.05<z<0.7) and 19 3CRR FRII radio galaxies (z<0.1), and use the detection of polycyclic aromatic hydrocarbon (PAH) features to examine the incidence of contemporaneous star formation and radio-loud AGN activity. Our analysis reveals PAH features in only a minority (30%) of the objects with good IRS spectra. Using the wealth of complementary data available for the 2Jy and 3CRR samples we make detailed comparisons between a range of star formation diagnostics: optical continuum spectroscopy, mid- to far-IR (MFIR) color, far-IR excess and PAH detection. There is good agreement between the various diagnostic techniques: most candidates identified to have star formation activity on the basis of PAH detection are also identified using at least two of the other techniques. We find that only 35% of the combined 2Jy and 3CRR sample show evidence for recent star formation activity (RSFA) at optical and/or MFIR wavelengths. This result argues strongly against the idea of a close link between starburst and powerful radio-loud AGN activity, reinforcing the view that, although a large fraction of powerful radio galaxies may be triggered in galaxy interactions, only a minority are triggered at the peaks of star formation activity in major, gas-rich mergers. However, we find that compact radio sources (D < 15 kpc) show a significantly higher incidence of RSFA (>75%) than their more extended counterparts (=15 -- 25%). We discuss this result in the context of a possible bias towards the selection of compact radio sources triggered in gas-rich environments.
    The Astrophysical Journal 11/2011; 745(2). · 6.73 Impact Factor
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    ABSTRACT: We present high signal-to-noise spectropolarimetry of the Broad Absorption Line QSO (BALQSO) PG 1700+518. Modeling of the polarization in this object, aspects of which were first reported in Young et al. (2007), confirms that in order to reproduce the variations in θ(λ) across Hα, scattering must predominantly take place in a rising hollow cylinder. The main scattering region is rising vertically off the accretion disk at a velocity close to the local Keplerian velocity and rotating about the system axis. Additionally, the scattering region is launched from a region in the accretion disk close to where the BLR originates and extends to a height significantly greater than inner launch radius.
    11/2011;
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    ABSTRACT: We summarize results from optical spectropolarimetry of 16 narrow-line Seyfert 1 galaxies (NLS1). Analyzed in the context of our two-component scattering model, our data show that the inclination of a line-emitting accretion disk cannot be the governing parameter, strongly suggesting that the distinctive properties of these objects have a physical rather than a geometrical origin. We also highlight the case of Mkn 1239, whose polarization spectrum reveals signatures of a powerful polar outflow, as might be expected in a source accreting near the Eddington limit.
    11/2011;
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    The Astrophysical Journal 10/2011; 741(2):126. · 6.73 Impact Factor
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    ABSTRACT: We measure the black hole mass in the nearby active galaxy Centaurus A (NGC 5128) using a new method based on spectroastrometry of a rotating gas disk. The spectroastrometric approach consists in measuring the photocenter position of emission lines for different velocity channels. In a previous paper we focused on the basic methodology and the advantages of the spectroastrometric approach with a detailed set of simulations demonstrating the possibilities for black hole mass measurements going below the conventional spatial resolution. In this paper we apply the spectroastrometric method to multiple longslit and integral field near infrared spectroscopic observations of Centaurus A. We find that the application of the spectroastrometric method provides results perfectly consistent with the more complex classical method based on rotation curves: the measured BH mass is nearly independent of the observational setup and spatial resolution and the spectroastrometric method allows the gas dynamics to be probed down to spatial scales of ~0.02", i.e. 1/10 of the spatial resolution and ~1/50 of BH sphere of influence radius. The best estimate for the BH mass based on kinematics of the ionized gas is then log(MBH (sin i)^2/M\odot)=7.5 \pm 0.1 which corresponds to MBH = 9.6(+2.5-1.8) \times 10^7 M\odot for an assumed disk inclination of i = 35deg. The complementarity of this method with the classic rotation curve method will allow us to put constraints on the disk inclination which cannot be otherwise derived from spectroastrometry. With the application to Centaurus A, we have shown that spectroastrometry opens up the possibility of probing spatial scales smaller than the spatial resolution, extending the measured MBH range to new domains which are currently not accessible: smaller BHs in the local universe and similar BHs in more distant galaxies.
    Astronomy and Astrophysics 10/2011; · 5.08 Impact Factor
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    ABSTRACT: The nuclei of Seyfert 1 galaxies exhibit a range of optical polarization characteristics that can be understood in terms of two scattering regions producing orthogonal polarizations: an extended polar scattering region (PSR) and a compact equatorial scattering region (ESR), located within the circum-nuclear torus. Here we present NICMOS 2.0 {mu}m imaging polarimetry of six 'polar-scattered' Seyfert 1 (S1) galaxies, in which the PSR dominates the optical polarization. The unresolved nucleus (<0.''58) is significantly polarized in only three objects, but five of the six exhibit polarization in a 0.''58-1.''5 circum-nuclear annulus. In Fairall 51 and ESO 323-G077, the polarization position angle at 2 {mu}m ({theta}{sub 2}{sub {mu}m}) is consistent with the average for the optical spectrum ({theta}{sub v}), implying that the nuclear polarization is dominated by polar scattering at both wavelengths. The same is probably true for NGC 3227. In both NGC 4593 and Mrk 766, there is a large difference between {theta}{sub 2}{sub {mu}m} and {theta}{sub v} off-nucleus, where polar scattering is expected to dominate. This may be due to contamination by interstellar polarization in NGC 4593, but there is no clear explanation in the case of the strongly polarized Mrk 766. Lastly, in Mrk 1239, a large change ({approx}60{sup 0}) in {theta}{sub 2}{sub {mu}m} between the nucleus and the annulus indicates that the unresolved nucleus and its immediate surroundings have different polarization states at 2 {mu}m, which we attribute to the ESR and PSR, respectively. A further implication is that the source of the scattered 2 {mu}m emission in the unresolved nucleus is the accretion disk, rather than torus hot dust emission.
    The Astrophysical Journal 09/2011; 738(1). · 6.73 Impact Factor
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    ABSTRACT: We present Spitzer photometric data for a complete sample of 19 low redshift (z<0.1) 3CRR radio galaxies as part of our efforts to understand the origin of the prodigious mid- to far-infrared (MFIR) emission from radio-loud AGN. Our results show a correlation between AGN power (indicated by [OIII] 5007 emission line luminosity) and 24 micron luminosity. This result is consistent with the 24 micron thermal emission originating from warm dust heated directly by AGN illumination. Applying the same correlation test for 70 micron luminosity against [OIII] luminosity we find this relation to suffer from increased scatter compared to that of 24 micron. In line with our results for the higher-radio-frequency-selected 2Jy sample, we are able to show that much of this increased scatter is due to heating by starbursts which boost the far-infrared emission at 70 micron in a minority of objects (17-35%). Overall this study supports previous work indicating AGN illumination as the dominant heating mechanism for MFIR emitting dust in the majority of low to intermediate redshift radio galaxies (0.03<z<0.7), with the advantage of strong statistical evidence. However, we find evidence that the low redshift broad-line objects (z<0.1) are distinct in terms of their positions on the MFIR vs. [OIII] correlations. Comment: 31 pages, 3 figures, accepted for publication to ApJ
    The Astrophysical Journal 08/2010; · 6.73 Impact Factor
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    ABSTRACT: Isophotal analysis of M87, using data from the Advanced Camera for Surveys, reveals a projected displacement of 6.8 +/- 0.8 pc (~ 0.1 arcsec) between the nuclear point source (presumed to be the location of the supermassive black hole, SMBH) and the photo-center of the galaxy. The displacement is along a position angle of 307 +/- 17 degrees and is consistent with the jet axis. This suggests the active SMBH in M87 does not currently reside at the galaxy center of mass, but is displaced in the counter-jet direction. Possible explanations for the displacement include orbital motion of an SMBH binary, gravitational perturbations due to massive objects (e.g., globular clusters), acceleration by an asymmetric or intrinsically one-sided jet, and gravitational recoil resulting from the coalescence of an SMBH binary. The displacement direction favors the latter two mechanisms. However, jet asymmetry is only viable, at the observed accretion rate, for a jet age of >0.1 Gyr and if the galaxy restoring force is negligible. This could be the case in the low density core of M87. A moderate recoil ~1 Myr ago might explain the disturbed nature of the nuclear gas disk, could be aligned with the jet axis, and can produce the observed offset. Alternatively, the displacement could be due to residual oscillations resulting from a large recoil that occurred in the aftermath of a major merger any time in the last 1 Gyr. Comment: ApJ Letters accepted
    The Astrophysical Journal Letters 05/2010; · 6.35 Impact Factor
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    ABSTRACT: This is the first in a series of papers in which we study the application of spectroastrometry in the context of gas kinematical studies aimed at measuring the mass of supermassive black holes. The spectroastrometrical method consists in measuring the photocenter of light emission in different wavelength or velocity channels. In particular we explore the potential of spectroastrometry of gas emission lines in galaxy nuclei to constrain the kinematics of rotating gas disks and to measure the mass of putative supermassive black holes. By means of detailed simulations and test cases, we show that the fundamental advantage of spectroastrometry is that it can provide information on the gravitational potential of a galaxy on scales significantly smaller (~ 1/10) than the limit imposed by the spatial resolution of the observations. We then describe a simple method to infer detailed kinematical informations from spectroastrometry in longslit spectra and to measure the mass of nuclear mass concentrations. Such method can be applied straightforwardly to integral field spectra, which do not have the complexities due to a partial spatial covering of the source in the case of longslit spectra. Comment: Accepted for publication in A&A
    Astronomy and Astrophysics 01/2010; · 5.08 Impact Factor
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    ABSTRACT: The mid-infrared spectral energy distributions (SEDs) of 83 active galaxies, mostly Seyfert galaxies, selected from the extended 12 mum sample are presented. The data were collected using all three instruments, Infrared Array Camera (IRAC), Infrared Spectrograph (IRS), and Multiband Imaging Photometer for Spitzer (MIPS), aboard the Spitzer Space Telescope. The IRS data were obtained in spectral mapping mode, and the photometric data from IRAC and IRS were extracted from matched, 20'' diameter circular apertures. The MIPS data were obtained in SED mode, providing very low-resolution spectroscopy (R ~ 20) between ~55 and 90 mum in a larger, 20'' × 30'' synthetic aperture. We further present the data from a spectral decomposition of the SEDs, including equivalent widths and fluxes of key emission lines; silicate 10 mum and 18 mum emission and absorption strengths; IRAC magnitudes; and mid-far-infrared spectral indices. Finally, we examine the SEDs averaged within optical classifications of activity. We find that the infrared SEDs of Seyfert 1s and Seyfert 2s with hidden broad line regions (HBLRs, as revealed by spectropolarimetry or other technique) are qualitatively similar, except that Seyfert 1s show silicate emission and HBLR Seyfert 2s show silicate absorption. The infrared SEDs of other classes within the 12 mum sample, including Seyfert 1.8-1.9, non-HBLR Seyfert 2 (not yet shown to hide a type 1 nucleus), LINER, and H II galaxies, appear to be dominated by star formation, as evidenced by blue IRAC colors, strong polycyclic aromatic hydrocarbon emission, and strong far-infrared continuum emission, measured relative to mid-infrared continuum emission.
    The Astrophysical Journal Supplement Series 01/2010; 187(1):172-211. · 16.24 Impact Factor
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    ABSTRACT: We have observed the composite active galactic nucleus (AGN)-starburst galaxy NGC 6764 with the Very Long Baseline Array at 1.6 and 4.9 GHz. These observations have detected a "core-jet" structure and a possible weak counterjet component at 1.6 GHz. The upper limits to the core and jet (1.6-4.9 GHz) spectral index are 0.6 and 0.3, respectively. Taken together with the high brightness temperature of ~107 K for the core region, the radio emission appears to be coming from a synchrotron jet. At a position angle of ~25°, the parsec-scale jet seems to be pointing closely toward the western edge of the southern kpc-scale bubble in NGC 6764. A real connection between the parsec- and sub-kpc-scale emission would not only suggest the presence of a curved jet, but also a close link between the AGN jet and the radio bubbles in NGC 6764. We demonstrate that a precessing jet model can explain the radio morphology from parsec to sub-kpc scales, and the model best-fit parameters of jet speed and orientation are fully consistent with the observed jet-to-counterjet surface brightness ratio. The jet however appears to be disrupted on scales of hundreds of parsecs, possibly due to interaction with and entrainment of the interstellar medium gas, which subsequently leads to the formation of bubbles. The jet energetics in NGC 6764 suggest that it would take 12-21 Myr to inflate the (southern) bubble. This timescale corresponds roughly to the starburst episode that took place in NGC 6764 about 15-50 Myr ago, and could be indicative of a close connection between jet formation and the starburst activity in this galaxy.
    The Astrophysical Journal 01/2010; 723:580-586. · 6.73 Impact Factor

Publication Stats

975 Citations
335.42 Total Impact Points

Institutions

  • 2005–2013
    • Rochester Institute of Technology
      • • School of Physics and Astronomy
      • • Center for Imaging Science
      Rochester, NY, United States
  • 2012
    • National Institute of Astrophysics
      • Institute of Space Astrophysics and Cosmic Physics IASF - Rome
      Roma, Latium, Italy
  • 2010–2011
    • University of Sussex
      • Department of Physics and Astronomy
      Brighton, England, United Kingdom
  • 1996–2008
    • University of Hertfordshire
      • Centre for Astrophysics Research (CAR)
      Hatfield, ENG, United Kingdom
  • 2001
    • Liverpool John Moores University
      Liverpool, England, United Kingdom
  • 1996–2000
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 1993
    • Cancer Research UK Cambridge Institute
      Cambridge, England, United Kingdom
  • 1989
    • Victoria University Sydney
      Sydney, New South Wales, Australia
  • 1987
    • European Southern Observatory
      Arching, Bavaria, Germany