E. O'Sullivan

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States

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Publications (83)256.29 Total impact

  • David A. Buote · Ewan O'Sullivan · Trevor J. Ponman
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    ABSTRACT: We discuss some results for mass profiles that we have obtained from hydrostatic studies of the hot gas in isolated, massive elliptical galaxies.
    No preview · Article · Jul 2015 · Proceedings of the International Astronomical Union
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    ABSTRACT: We present Giant Metrewave Radio Telescope (GMRT) 240 MHz observations of the nearby luminous FR I radio source 3C 270, in the group-central elliptical NGC 4261. Combining these data with reprocessed Very Large Array (VLA) 1.55 and 4.8 GHz observations, we produce spectral index maps that reveal a constant spectral index along the jets and a gradual steepening from the ends of the jets through the lobes towards the nucleus. A Jaffe & Perola (JP) model fitted to the integrated spectrum of the source gives an asymptotic low-frequency index of $\alpha_{inj}=0.53_{-0.02}^{+0.01}$, while JP models fitted to the observed spectral index trend along the lobes allow us to estimate radiative ages of $\sim29$ Myr and $\sim37$ Myr for the west and east lobes respectively. Our age estimates are a factor of two lower than the 75-Myr upper limit derived from X-ray data (O'Sullivan et al. 2011). We find unlikely the scenario of an early supersonic phase in which the lobe expanded into the ISM at approximately Mach 6 (3500 km s$^{-1}$), and suggest that either the source underwent multiple AGN outbursts with possible large changes in jet power, or possibly that the source age that we find is due to a backflow that transports young electrons from the jet tips through the lobes toward the nucleus relatively quickly. We calculate that in the lobes the energy ratio of non-radiating to radiating particles is $\sim4-24$ indicating significant gas entrainment. If the lobes are in pressure balance with their surroundings, the total energy required to heat the entrained material is $10^{58}$ erg, $\sim$40% of the total enthalpy of the lobes.
    Full-text · Article · Apr 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present Chandra and XMM-Newton observations of PLCK G036.7+14.9 from the Chandra-Planck Legacy Program. The high resolution X-ray observations reveal two close subclusters, G036N and G036S, which were not resolved by previous ROSAT, optical, or recent Planck observations. We perform detailed imaging and spectral analyses and use a simplified model to study the kinematics of this system. The basic picture is that PLCK G036.7+14.9 is undergoing a major merger (mass ratio close to unity) between the two massive subclusters, with the merger largely along the line-of-sight and probably at an early stage. G036N hosts a small, moderate cool-core, while G036S has at most a very weak cool-core in the central 40 kpc region. The difference in core cooling times is unlikely to be caused by the ongoing merger disrupting a pre-existing cool-core in G036S. G036N also hosts an unresolved radio source in the center, which may be heating the gas if the radio source is extended. The Planck derived mass is higher than the X-ray measured mass of either subcluster, but is lower than the X-ray measured mass of the whole cluster, due to the fact that Planck does not resolve PLCK G036.7+14.9 into subclusters and interprets it as a single cluster. This mass discrepancy could induce significant bias to the mass function if such previously unresolved systems are common in the Planck cluster sample. High resolution X-ray observations are necessary to identify the fraction of such systems and correct such a bias for the purpose of precision cosmological studies.
    Full-text · Article · Mar 2015 · The Astrophysical Journal
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    E. O'Sullivan · F. Combes · S. Hamer · P. Salomé · A. Babul · S. Raychaudhury
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    ABSTRACT: We present IRAM 30m telescope observations of the CO(1-0) and (2-1) lines in a sample of 11 group-dominant elliptical galaxies selected from the CLoGS nearby groups sample. Our observations confirm the presence of molecular gas in 4 of the 11 galaxies at >4sigma significance, and combining these with data from the literature we find a detection rate of 43+-14%, significantly higher than for the wider population of elliptical galaxies, and comparable to the detection rate for nearby radio galaxies. Those group-dominant galaxies which are detected typically contain ~2x10^8 Msol of molecular gas, and although most have low star formation rates (<1 Msol/yr) they have short depletion times, indicating that the gas must be replenished on timescales ~10^8 yr. Almost all of the galaxies contain active nuclei, and we note that while CO detections are more common in the most radio-loud galaxies, the mass of molecular gas required to power the active nuclei through accretion is small compared to the masses observed. We consider the possible origin mechanisms for the gas, through cooling of stellar ejecta within the galaxies, group-scale cooling flows, and gas-rich mergers, and find probable examples of each type within our sample, confirming that a variety of processes act to drive the build up of molecular gas in group-dominant ellipticals.
    Full-text · Article · Jan 2015 · Astronomy and Astrophysics
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    ABSTRACT: Galaxy groups provide the environment in which the majority of galaxies evolve, with low velocity dispersions and small galaxy separations that are conducive to tidal interactions and mergers between group members. X-ray observations reveal the frequent presence of hot gas in groups, with larger quantities linked to early-type galaxies, whereas cold gas is common in spiral-dominated groups. Clarification of the origin and role of the hot medium is central to the understanding of the evolution of the galaxy population and of all phases of the IGM.We here report on the nuclear activity, star formation and the high luminosity X-ray binary populations of the spiral-dominated, likely not yet virialized, group HCG 16, as well as on its intra-group medium, based principally on deep (150 ks) Chandra X-ray observations of the group, as well as new Giant Metrewave Radio Telescope (GMRT) 610 MHz radio data. We confirm the presence of obscured active nuclei in NGC 833 and NGC 835, and identify what may be a previously unrecognized nuclear source in NGC 838; all are variable. NGC 838 and NGC 839 are both starburst-dominated systems, with galactic superwinds that show X-ray and radio evidence of IGM interaction, but only weak nuclear activity; NGC 848 is also dominated by emission from its starburst.We confirm the existence of a faint, extended low-temperature (0.3 keV) intra-group medium, a subject of some uncertainty in earlier studies. The diffuse emission is strongest in a ridge linking the four principal galaxies, and is at least partly coincident with a large-scale HI tidal filament, indicating that the IGM in the inner part of the group is highly multi-phase. We conclude that starburst winds and shock-heating of stripped HI may play an important role in the early stages of IGM formation, with galactic winds contributing 20-40% of the observed hot gas in the system.
    No preview · Conference Paper · Aug 2014
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    ABSTRACT: We use a combination of deep Chandra X-ray observations and radio continuum imaging to investigate the origin and current state of the intra-group medium in the spiral-rich compact group HCG 16. We confirm the presence of a faint ($L_{X,{\rm bolo}}$=1.87$^{+1.03}_{-0.66}$$\times$10$^{41}$ erg/s), low temperature (0.30$^{+0.07}_{-0.05}$ keV) intra-group medium (IGM) extending throughout the ACIS-S3 field of view, with a ridge linking the four original group members and extending to the southeast, as suggested by previous Rosat and XMM-Newton observations. This ridge contains 6.6$^{+3.9}_{-3.3}$$\times$10$^9$ solar masses of hot gas and is at least partly coincident with a large-scale HI tidal filament, indicating that the IGM in the inner part of the group is highly multi-phase. We present evidence that the group is not yet virialised, and show that gas has probably been transported from the starburst winds of NGC 838 and NGC 839 into the surrounding IGM. Considering the possible origin of the IGM, we argue that material ejected by galactic winds may have played a significant role, contributing 20-40% of the observed hot gas in the system.
    Full-text · Article · Jul 2014 · The Astrophysical Journal
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    ABSTRACT: We present new, deep Chandra X-ray and Giant Metrewave Radio Telescope 610~MHz observations of the spiral-galaxy-rich compact group HCG 16, which we use to examine nuclear activity, star formation and the high luminosity X-ray binary populations in the major galaxies. We confirm the presence of obscured active nuclei in NGC 833 and NGC 835, and identify a previously unrecognized nuclear source in NGC 838. All three nuclei are variable on timescales of months to years, and for NGC 833 and NGC 835 this is most likely caused by changes in accretion rate. The deep Chandra observations allow us to detect for the first time an Fe-K$\alpha$ emission line in the spectrum of the Seyfert 2 nucleus of NGC 835. We find that NGC 838 and NGC 839 are both starburst-dominated systems, with only weak nuclear activity, in agreement with previous optical studies. We estimate the star formation rates in the two galaxies from their X-ray and radio emission, and compare these results with estimates from the infra-red and ultra-violet bands to confirm that star formation in both galaxies is probably declining after galaxy-wide starbursts were triggered ~400-500 Myr ago. We examine the physical properties of their galactic superwinds, and find that both have temperatures of ~0.8 keV. We also examine the X-ray and radio properties of NGC 848, the fifth largest galaxy in the group, and show that it is dominated by emission from its starburst.
    Full-text · Article · Jul 2014 · The Astrophysical Journal
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    ABSTRACT: A short 30 minute ALMA observation of the early-type galaxy NGC 5044, which resides at the center of an X-ray bright group with a moderate cooling flow, has detected 24 molecular structures within the central 2.5 kpc. The masses of the molecular structures vary from 3e5 to 1e7 Mo3 and the CO(2-1) linewidths vary from 15 to 65 km/s. Given the large CO(2-1) linewidths, the observed structures are likely giant molecular associations (GMAs) and not individual molecular clouds (GMCs). Only a few of the GMAs are spatially resolved with the cycle 0 ALMA beam and the average density of these GMAs yields a GMC volume filling factor of about 15%. The observed masses of the resolved GMAs are insufficient for them to be gravitationally bound, however, the most massive GMA does contain a less massive component with a linewidth of 5.5 km/s (typical of an individual virialized GMC). We also show that the GMAs cannot be pressure confined by the hot gas. Given the observed CO(2-1) linewidths of the GMAs (i.e., the velocity dispersion of the embedded GMCs) they will likely disperse on a timescale of about 12 Myr, which is less than the central cooling time of the hot gas, so the embedded GMCs within a GMA must condense out of the hot gas at the same time and arise from local concentrations of thermally unstable parcels of hot gas. There are no indications of any disk-like molecular structures and all indications suggest that the molecular gas follows ballistic trajectories after condensing out of the thermally unstable hot gas. The 230 GHz luminosity of the central continuum source is 500 times greater than its low frequency radio luminosity and probably reflects a recent accretion event by the central supermassive black hole. The spectrum of the central continuum source also exhibits an absorption feature.
    Full-text · Article · Jul 2014 · The Astrophysical Journal
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    ABSTRACT: The group environment --- the locus of most galaxies in the present-day Universe --- is particularly suitable, owing to high galaxy densities and low velocity dispersions, for study of the merging of spirals to form ellipticals. In particular, little is known about how the resulting ellipticals acquire hot gas halos (whether from stellar mass loss or from accretion of group gas). We here discuss our examination, relying principally on Chandra and GMRT data, of two groups that are at once compact, spiral-dominated, sufficiently X-ray bright, nearby, and in early stages of merging. For HCG92 (Stephan’s Quintet), GMRT 610 and 327 MHz observations show diffuse extended radio emission. X-ray and radio emissions from the group are dominated by a bright north-south ridge associated with the interloper galaxy NGC 7318b, with the low temperature of X-ray gas (0.6 keV) indicating an oblique shock resulting from the interaction. Much of the gas now in the hot phase may have been produced by shock heating of cool material during galaxy interactions, with additional energy and metals injected by star formation. Radio spectral index mapping allows us to trace the relative contributions from the shock and from star formation. Within the ridge spectral information can also be used to estimate the radiative age of the electron population, providing information on the shock age. For HCG16 the nature and even the existence of a hot intragroup medium has been debated. With a combination of a very recent deep (150 ks) Chandra image and radio data already available, we expect to resolve questions about the existence, extent, and nature of hot gas in HCG 16; map the heavy element distribution; determine the hot gas mass compared to the HI deficit; examine the prevalence of shock heating and ram pressure stripping; and study the brightest point sources.
    No preview · Conference Paper · Jan 2014
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    ABSTRACT: Galaxy groups form the environment of the majority of galaxies in the local Universe, and many host an extended hot intra-group medium whose radiative cooling appears to fuel, and be stabilised by, feedback from AGN in group-central galaxies. Unfortunately studies of the physical properties of groups and the influence of AGN on their member galaxies and gaseous haloes have been limited by a lack of reliable representative samples of groups in the local Universe. To address this problem, we have assembled the Complete Local-Volume Groups Sample (CLoGS), an optically-selected statistically-complete sample of 53 groups within 80 Mpc, which we aim to observe in both low-frequency radio and X-ray wavebands. We here describe results from the first half of the sample, for which X-ray and radio observations are complete. Roughly 55% of the groups have group-scale X-ray halos, of which ~65% have cool cores a similar fraction to that found in galaxy clusters. While 25 of the 26 group central galaxies host radio AGN, among the X-ray bright groups only the cool core systems are found to support central jet sources.
    Full-text · Conference Paper · Dec 2013
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    Ewan O'Sullivan · Laurence P. David · Jan M. Vrtilek
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    ABSTRACT: We present temperature and abundance maps of the central 125 kpc of the NGC 5044 galaxy group, based on a deep XMM–Newton observation. The abundance map reveals an asymmetrical abundance structure, with the centroid of the highest abundance gas offset ∼22 kpc north-west of the galaxy centre, and moderate abundances extending almost twice as far to the south-east than in any other direction. The abundance distribution is closely correlated with two previously identified cold fronts and an arc-shaped region of surface brightness excess, and it appears that sloshing, induced by a previous tidal encounter, has produced both the abundance and surface brightness features. Sloshing dominates the uplift of heavy elements from the group core on large scales, and we estimate that the south-east extension (the tail of the sloshing spiral) contains at least 1.2 × 105 M⊙ more iron than would be expected of gas at its radius. Placing limits on the age of the encounter we find that if, as previously suggested, the disturbed spiral galaxy NGC 5054 was the perturber, it must have been moving supersonically when it transited the group core. We also examine the spectral properties of emission from the old, detached radio lobe south-east of NGC 5044, and find that they are consistent with a purely thermal origin, ruling out this structure as a significant source of spectrally hard inverse-Compton emission.
    Preview · Article · Oct 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present the first Herschel spectroscopic detections of the [OI]63µm and [CII]158µm fine-structure transitions, and a single para-H_2O line from the 35 x 15 kpc^2 shocked intergalactic filament in Stephan's Quintet. The filament is believed to have been formed when a high-speed intruder to the group collided with clumpy intergroup gas. Observations with the PACS spectrometer provide evidence for broad (> 1000 km s^(-1)) luminous [CII] line profiles, as well as fainter [OI]63µm emission. SPIRE FTS observations reveal water emission from the p-H_2O (1_(11)-0_(00)) transition at several positions in the filament, but no other molecular lines. The H_2O line is narrow, and may be associated with denser intermediate-velocity gas experiencing the strongest shock-heating. The [CII]/PAH_(tot) and [CII]/FIR ratios are too large to be explained by normal photo-electric heating in PDRs. HII region excitation or X-ray/Cosmic Ray heating can also be ruled out. The observations lead to the conclusion that a large fraction the molecular gas is diffuse and warm. We propose that the [CII], [OI] and warm H_2 line emission is powered by a turbulent cascade in which kinetic energy from the galaxy collision with the IGM is dissipated to small scales and low-velocities, via shocks and turbulent eddies. Low-velocity magnetic shocks can help explain both the [CII]/[OI] ratio, and the relatively high [CII]/H_2 ratios observed. The discovery that [CII] emission can be enhanced, in large-scale turbulent regions in collisional environments has implications for the interpretation of [CII] emission in high-z galaxies.
    Full-text · Article · Sep 2013 · The Astrophysical Journal
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    ABSTRACT: (Context) In recent years, our understanding of the cool cores of galaxy clusters has changed. Once thought to be relatively simple places where gas cools and flows toward the centre, now they are believed to be very dynamic places where heating from the central Active Galactic Nucleus (AGN) and cooling, as inferred from active star formation, molecular gas, and Halpha nebulosity, find an uneasy energetic balance. (Aims) We want to characterize the X-ray properties of the nearby cool-core cluster Zw1742+3306, selected because it is bright at X-ray (with a flux greater than 1e-11 erg/s/cm2 in the 0.1-2.4 keV band) and Halpha wavelengths (Halpha luminosity > 1e40 erg/s). (Methods) We used Chandra data to analyze the spatial and spectral properties of the cool core of Zw1742+3306, a galaxy cluster at z=0.0757 that emits in Halpha and presents the brightest central galaxy located in a diffuse X-ray emission with multiple peaks in surface brightness. (Results) We show that the X-ray cool core of the galaxy cluster Zw1742+3306 is thermodynamically very active with evidence of cold fronts and a weak shock in the surface brightness map and of an apparently coherent, elongated structure with metallicity greater than the value measured in the surrounding ambient gas by about 50 per cent. This anisotropic structure is 280 x 90 kpc2 and is aligned with the cold fronts and with the X-ray emission on larger scales. We suggest that all these peculiarities in the X-ray emission of Zw1742+3306 are either a very fine-tuned output of a sloshing gas in the cluster core or the product of a metal-rich outflow from the central AGN.
    Full-text · Article · May 2013 · Astronomy and Astrophysics

  • No preview · Conference Paper · Apr 2013
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    ABSTRACT: HST WFPC2 data for NGC 4261 were obtained with the F450W, F606W and F814W filters (roughly corresponding to the B, V and I bands, respectively; see WFPC2 Instrument Handbook, version 10.0, table 3.1). (1 data file).
    No preview · Article · Mar 2013
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    ABSTRACT: Much of the evolution of galaxies takes place in groups where feedback has the greatest impact on galaxy formation. By using an optically selected, statistically complete sample of 53 nearby groups (CLoGS), observed in Radio (GMRT) and in X-rays (Chandra and XMM-Newton), we aim to characterize the radio-AGN population in groups and examine their impact on the intra-group gas and member galaxies. The sensitivity to older electron populations at 240 MHz and the resolution of 610 MHz is the key to identify past and current AGN activity. Here we will present first results from three different galaxy groups analysed so far. We report an age of ~18 Myr for the radio source 3C270 in NGC 4261 implying that the expansion was supersonic over a large fraction of its lifetime. In NGC 1060 we detect a small-scale (20''/7.4 kpc) jet source with the spectral index of $\alpha_{240}^{610}$=0.9 indicating a remnant of an old outburst. Lastly in NGC 5982 the 610 and 235 MHz observations find a radio point source in the central AGN with no evidence of jets or lobes and diffuse emission from the disks (star formation).
    Full-text · Conference Paper · Dec 2012
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    ABSTRACT: We present a study of the hot gas and stellar content of five optically selected poor galaxy clusters, including a full accounting of the contribution from intracluster light (ICL) and a combined hot gas and hydrostatic X-ray mass analysis with XMM–Newton observations. We find weighted mean stellar (including ICL), gas and total baryon mass fractions within r500 of 0.026 ± 0.003, 0.070 ± 0.005 and 0.096 ± 0.006, respectively, at a corresponding weighted mean M500 of (1.08+ 0.21− 0.18) × 1014 M⊙. Even when accounting for the intracluster stars, four out of five clusters show evidence for a substantial baryon deficit within r500, with baryon fractions (fb) between 50 ± 6 and 59 ± 8 per cent of the universal mean level (i.e. Ωb/Ωm), the remaining cluster having fb = 75±11 per cent. For the three clusters where we can trace the hot halo to r500 we find no evidence for a steepening of the gas density profile in the outskirts with respect to a power law, as seen in more massive clusters. We find that in all cases, the X-ray mass measurements are larger than those originally published on the basis of the galaxy velocity dispersion (σ) and an assumed σ-M500 relation, by a factor of 1.7–5.7. Despite these increased masses, the stellar fractions (in the range 0.016–0.034, within r500) remain consistent with the trend with mass published by Gonzalez et al., from which our sample is drawn.
    Full-text · Article · Dec 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: IRAS 09104+4109 is a rare example of a dust enshrouded type 2 QSO in the centre of a cool-core galaxy cluster. Previous observations of this z=0.44 system showed that as well as powering the hyper-luminous infrared emission of the cluster-central galaxy, the QSO is associated with a double-lobed radio source. However, the steep radio spectral index and misalignment between the jets and ionised optical emission suggested that the orientation of the QSO had recently changed. We use a combination of new, multi-band Giant Metrewave Radio Telescope observations and archival radio data to confirm that the jets are no longer powered by the QSO, and estimate their age to be 120-160 Myr. This is in agreement with the ~70-200 Myr age previously estimated for star-formation in the galaxy. Previously unpublished Very Long Baseline Array data reveal a 200 pc scale double radio source in the galaxy core which is more closely aligned with the current QSO axis and may represent a more recent period of jet activity. These results suggest that the realignment of the QSO, the cessation of jet activity, and the onset of rapid star-formation may have been caused by a gas-rich galaxy merger. A Chandra X-ray observation confirms the presence of cavities associated with the radio jets, and we estimate the energy required to inflate them to be ~7.7x10^60 erg. The mechanical power of the jets is sufficient to balance radiative cooling in the cluster, provided they are efficiently coupled to the intra-cluster medium (ICM). We find no evidence of direct radiative heating and conclude that the QSO either lacks the radiative luminosity to heat the ICM, or that it requires longer than 100-200 Myr to significantly impact its environment. [Abridged]
    Full-text · Article · Aug 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present deep Giant Metrewave Radio Telescope (GMRT) radio observations at 240, 330 and 610 MHz of the complex radio source at the center of the NGC1407 galaxy group. Previous GMRT observations at 240 MHz revealed faint, diffuse emission enclosing the central twin-jet radio galaxy. This has been interpreted as an indication of two possible radio outbursts occurring at different times. Both the inner double and diffuse component are detected in the new GMRT images at high levels of significance. Combining the GMRT observations with archival Very Large Array data at 1.4 and 4.9 GHz, we derive the total spectrum of both components. The inner double has a spectral index alpha=0.7, typical for active, extended radio galaxies, whereas the spectrum of the large-scale emission is very steep, with \alpha=1.8 between 240 MHz and 1.4 GHz. The radiative age of the large-scale component is very long, ~300 Myr, compared to ~30 Myr estimated for the central double, confirming that the diffuse component was generated during a former cycle of activity of the central galaxy. The current activity have so far released an energy which is nearly one order of magnitude lower than that associated with the former outburst. The group X-ray emission in the Chandra and XMM-Newton images and extended radio emission show a similar swept-back morphology. We speculate that the two structures are both affected by the motion of the group core, perhaps due to the core sloshing in response to a recent encounter with the nearby elliptical galaxy NGC1400.
    Full-text · Article · Aug 2012 · The Astrophysical Journal
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    Duncan A. Forbes · Trevor Ponman · Ewan O'Sullivan
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    ABSTRACT: For a sample of nine well-studied giant ellipticals, we compare the projected radial distribution of their red and blue globular cluster (GC) subpopulations with their host galaxy stellar and X-ray surface brightness profiles. We support previous findings that the surface density distribution of red (metal-rich) GCs follows that of the host galaxy starlight. We find good agreement between the outer slope of the blue GC surface density and that of the galaxy X-ray emission. This coincidence of projected radial profiles is likely due to the fact that both blue GCs and X-ray emitting hot gas share the same gravitational potential in equilibrium. When deprojected the X-ray emitting hot gas has a radial density dependence that is the square root of that for the GC density. We further show that the energy per unit mass for blue GCs is roughly half that of the hot gas.
    Preview · Article · May 2012 · Monthly Notices of the Royal Astronomical Society

Publication Stats

1k Citations
256.29 Total Impact Points

Institutions

  • 2003-2015
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2001-2012
    • University of Birmingham
      • School of Physics and Astronomy
      Birmingham, England, United Kingdom
    • Swinburne University of Technology
      • Centre for Astrophysics and Supercomputing
      Melbourne, Victoria, Australia