C. D. Wilson

McMaster University, Hamilton, Ontario, Canada

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Publications (137)444.04 Total impact

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    ABSTRACT: A correlation between the 24 {\mu}m continuum and the [NII] 205 {\mu}m line emission may arise if both quantities trace the star formation activity on spatially-resolved scales within a galaxy, yet has so far only been observed in the nearby edge-on spiral galaxy NGC 891. We therefore assess whether the [NII] 205 - 24 {\mu}m emission correlation has some physical origin or is merely an artefact of line-of-sight projection effects in an edge-on disc. We search for the presence of a correlation in Herschel and Spitzer observations of two nearby face-on galaxies, M51 and M83, and the interacting Antennae galaxies NGC 4038 and 4039. We show that not only is this empirical relationship also observed in face-on galaxies, but also that the correlation appears to be governed by the star formation rate (SFR). Both the nuclear starburst in M83 and the merger-induced star formation in NGC 4038/9 exhibit less [NII] emission per unit SFR surface density than the normal star-forming discs. These regions of intense star formation exhibit stronger ionization parameters, as traced by the 70/160 {\mu}m far-infrared colour, that suggest the presence of higher ionization lines that may become more important for gas cooling, thereby reducing the observed [NII] 205 {\mu}m line emission in regions with higher star formation rates. Finally, we present a general relation between the [NII] 205 {\mu}m line flux density and SFR density for normal star-forming galaxies, yet note that future studies should extend this analysis by including observations with wider spatial coverage for a larger sample of galaxies.
    Full-text · Article · Jan 2016 · Astronomy and Astrophysics
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    ABSTRACT: We present a study of the molecular gas properties in a sample of 98 H i – flux selected spiral galaxies within ∼25 Mpc, using the CO J = 3 − 2 line observed with the James Clerk Maxwell Telescope. We use the technique of survival analysis to incorporate galaxies with CO upper limits into our results. Comparing the group and Virgo samples, we find a larger mean H2 mass in the Virgo galaxies, despite their lower mean H i mass. This leads to a significantly higher H2 to H i ratio for Virgo galaxies. Combining our data with complementary Hα star formation rate measurements, Virgo galaxies have longer molecular gas depletion times compared to group galaxies, due to their higher H2 masses and lower star formation rates. We suggest that the longer depletion times may be a result of heating processes in the cluster environment or differences in the turbulent pressure. From the full sample, we find that the molecular gas depletion time has a positive correlation with the stellar mass, indicative of differences in the star formation process between low- and high-mass galaxies, and a negative correlation between the molecular gas depletion time and the specific star formation rate.
    Preview · Article · Dec 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present Herschel-SPIRE imaging spectroscopy (194-671 μm) of the bright starburst galaxy M82. We use RADEX and a Bayesian Likelihood Analysis to simultaneously model the temperature, density, column density, and filling factor of both the cool and warm components of molecular gas traced by the entire CO ladder up to J=13-12. The high-J lines observed by SPIRE trace much warmer gas (~500 K) than those observable from the ground. The addition of 13CO (and [C I]) is new and indicates that [C I] may be tracing different gas than 12CO. At such a high temperature, cooling is dominated by molecular hydrogen; we conclude with a discussion on the possible excitation processes in this warm component. Photon-dominated region (PDR) models require significantly higher densities than those indicated by our Bayesian likelihood analysis in order to explain the high-J CO line ratios, though cosmic-ray enhanced PDR models can do a better job reproducing the emission at lower densities. Shocks and turbulent heating are likely required to explain the bright high-J emission.
    No preview · Article · Aug 2015 · Proceedings of the International Astronomical Union
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    ABSTRACT: We employ SCUBA-2 (Submillimetre Common-User Bolometer Array 2) observations of the Orion A North molecular cloud to derive column density and temperature maps. We apply a novel, Hessian-based structural identification algorithm for detection of prestellar cores to these data, allowing for automated generation of the prestellar mass function. The resulting mass function is observed to peak at 1.39(-0.19)(+0.18) M-circle dot, indicating a star-forming efficiency lower limit of similar to 14 per cent when compared with the Orion nebula Cluster initial mass function (IMF) peak. Additionally, the prestellar mass function is observed to decay with a high-mass power-law exponent alpha = 2.53(-0.14)(+0.16), indicating approximate functional similarity with the Salpeter IMF (alpha = 2.35). This result, when combined with the results of previous investigations suggests a regional dependence of the star-forming efficiency.
    No preview · Article · Mar 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present 850 and 450 μm data from the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey obtained with Submillimetre Common-User Bolometer Array 2 (SCUBA-2) and characterize the dust attributes of Class I, Class II and Class III disc sources in L 1495. We detect 23 per cent of the sample at both wavelengths, with the detection rate decreasing through the Classes from I to III. The median disc mask is 1.6 × 10−3 M⊙, and only 7 per cent of Class II sources have disc masses larger than 20 Jupiter masses. We detect a higher proportion of discs towards sources with stellar hosts of spectral type K than spectral type M. Class II discs with single stellar hosts of spectral type K have higher masses than those of spectral type M, supporting the hypothesis that higher mass stars have more massive discs. Variations in disc masses calculated at the two wavelengths suggest that there may be differences in dust opacity and/or dust temperature between discs with hosts of spectral types K to those with spectral type M.
    Full-text · Article · Feb 2015 · Monthly Notices of the Royal Astronomical Society
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    Full-text · Article · Jan 2015 · Astronomy and Astrophysics
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    ABSTRACT: We present SCUBA-2 450 and 850 μm observations of the Serpens MWC 297 region, part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey of nearby star-forming regions. Simulations suggest that radiative feedback influences the star formation process and we investigate observational evidence for this by constructing temperature maps. Maps are derived from the ratio of SCUBA-2 fluxes and a two-component model of the JCMT beam for a fixed dust opacity spectral index of β = 1.8. Within 40 arcsec of the B1.5Ve Herbig star MWC 297, the submillimetre fluxes are contaminated by free–free emission with a spectral index of 1.03 ± 0.02, consistent with an ultracompact H ii region and polar winds/jets. Contamination accounts for 73 ± 5 per cent and 82 ± 4 per cent of peak flux at 450 μm and 850 μm, respectively. The residual thermal disc of the star is almost undetectable at these wavelengths. Young stellar objects (YSOs) are confirmed where SCUBA-2 850 μm clumps identified by the fellwalker algorithm coincide with Spitzer Gould Belt Survey detections. We identify 23 objects and use Tbol to classify nine YSOs with masses 0.09 to 5.1 M⊙. We find two Class 0, one Class 0/I, three Class I and three Class II sources. The mean temperature is 15 ± 2 K for the nine YSOs and 32 ± 4 K for the 14 starless clumps. We observe a starless clump with an abnormally high mean temperature of 46 ± 2 K and conclude that it is radiatively heated by the star MWC 297. Jeans stability provides evidence that radiative heating by the star MWC 297 may be suppressing clump collapse.
    Full-text · Article · Dec 2014 · Monthly Notices of the Royal Astronomical Society

  • No preview · Article · Nov 2014 · Publications of the Astronomical Society of the Pacific
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    ABSTRACT: We present Herschel PACS and SPIRE spectroscopy of the most important far-infrared cooling lines in the nearby edge-on spiral galaxy, NGC 891: [CII] 158 $\mu$m, [NII] 122, 205 $\mu$m, [OI] 63, 145 $\mu$m, and [OIII] 88 $\mu$m. We find that the photoelectric heating efficiency of the gas, traced via the ([CII]+[OII]63)/$F_{\mathrm{TIR}}$ ratio, varies from a mean of 3.5$\times$10$^{-3}$ in the centre up to 8$\times$10$^{-3}$ at increasing radial and vertical distances in the disc. A decrease in ([CII]+[OII]63)/$F_{\mathrm{TIR}}$ but constant ([CII]+[OI]63)/$F_{\mathrm{PAH}}$ with increasing FIR colour suggests that polycyclic aromatic hydrocarbons (PAHs) may become important for gas heating in the central regions. We compare the observed flux of the FIR cooling lines and total IR emission with the predicted flux from a PDR model to determine the gas density, surface temperature and the strength of the incident far-ultraviolet (FUV) radiation field, $G_{0}$. Resolving details on physical scales of ~0.6 kpc, a pixel-by-pixel analysis reveals that the majority of the PDRs in NGC 891's disc have hydrogen densities of 1 < log ($n$/cm$^{-3}$) < 3.5 experiencing an incident FUV radiation field with strengths of 1.7 < log $G_0$ < 3. Although these values we derive for most of the disc are consistent with the gas properties found in PDRs in the spiral arms and inter-arm regions of M51, observed radial trends in $n$ and $G_0$ are shown to be sensitive to varying optical thickness in the lines, demonstrating the importance of accurately accounting for optical depth effects when interpreting observations of high inclination systems. With an empirical relationship between the MIPS 24 $\mu$m and [NII] 205 $\mu$m emission, we estimate an enhancement of the FUV radiation field strength in the far north-eastern side of the disc.
    Full-text · Article · Oct 2014 · Astronomy and Astrophysics
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    ABSTRACT: With the newly available SPIRE images at 250 and 500 micron from Herschel Space Observatory, we study quantitative correlations over a sub-kpc scale among three distinct emission components in the interstellar medium of the nearby spiral galaxy M 81 (NGC 3031): (a) $I_{8}$ or $I_{24}$, the surface brightness of the mid-infrared emission observed in the Spitzer IRAC 8 or MIPS 24 micron band, with $I_8$ and $I_{24}$ being dominated by the emissions from Polycyclic Aromatic Hydrocarbons (PAHs) and very small grains (VSGs) of dust, respectively; (b) $I_{500}$, that of the cold dust continuum emission in the Herschel SPIRE 500 micron band, dominated by the emission from large dust grains heated by evolved stars, and (c) $I_{{\rm H}\alpha}$, a nominal surface brightness of the H$\alpha$ line emission, from gas ionized by newly formed massive stars. The results from our correlation study, free from any assumption on or modeling of dust emissivity law or dust temperatures, present solid evidence for significant heating of PAHs and VSGs by evolved stars. In the case of M 81, about 67% (48%) of the 8 micron (24 micron) emission derives its heating from evolved stars, with the remainder attributed to radiation heating associated with ionizing stars.
    Preview · Article · Oct 2014 · The Astrophysical Journal
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    ABSTRACT: Physical conditions of the interstellar medium in galaxies are closely linked to the ambient radiation field and the heating of dust grains. In order to characterize dust properties in galaxies over a wide range of physical conditions, we present here the radial surface brightness profiles of the entire sample of 61 galaxies from Key Insights into Nearby Galaxies: Far-Infrared Survey with Herschel (KINGFISH). The main goal of our work is the characterization of the grain emissivities, dust temperatures, and interstellar radiation fields responsible for heating the dust. After fitting the dust and stellar radial profiles with exponential functions, we fit the far-infrared spectral energy distribution (SED) in each annular region with single-temperature modified black bodies using both variable (MBBV) and fixed (MBBF) emissivity indices beta, as well as with physically motivated dust models. Results show that while most SED parameters decrease with radius, the emissivity index beta also decreases with radius in some galaxies, but in others is increasing, or rising in the inner regions and falling in the outer ones. Despite the fixed grain emissivity (average beta~ 2.1) of the physically-motivated models, they are well able to accommodate flat spectral slopes with beta<= 1. We find that flatter slopes (beta<= 1.5) are associated with cooler temperatures, contrary to what would be expected from the usual Tdust-beta degeneracy. This trend is related to variations in Umin since beta and Umin are very closely linked over the entire range in Umin sampled by the KINGFISH galaxies: low Umin is associated with flat beta<=1. Both these results strongly suggest that the low apparent \beta values (flat slopes) in MBBV fits are caused by temperature mixing along the line-of-sight, rather than by intrinsic variations in grain properties. Abstract truncated for arXiv.
    Full-text · Article · Sep 2014 · Astronomy and Astrophysics
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    ABSTRACT: We present new images of Arp 220 from the Atacama Large Millimeter/submillimeter Array with the highest combination of frequency (691 GHz) and resolution (0.''36 × 0.''20) ever obtained for this prototypical ultraluminous infrared galaxy. The western nucleus is revealed to contain warm (200 K) dust that is optically thick (τ434 μm = 5.3), while the eastern nucleus is cooler (80 K) and somewhat less opaque (τ434 μm = 1.7). We derive full width at half-maximum diameters of 76 × ≤ 70 pc and 123 × 79 pc for the western and eastern nucleus, respectively. The two nuclei combined account for ( (calibration) (systematic))% of the total infrared luminosity of Arp 220. The luminosity surface density of the western nucleus ( in units of L ☉ kpc–2) appears sufficiently high to require the presence of an active galactic nucleus (AGN) or a "hot starburst," although the exact value depends sensitively on the brightness distribution adopted for the source. Although the role of any central AGN remains open, the inferred mean gas column densities of (0.6-1.8) × 1025 cm–2 mean that any AGN in Arp 220 must be Compton-thick.
    Full-text · Article · Jun 2014 · The Astrophysical Journal Letters
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    G. Natale · K. Foyle · C. D. Wilson · N. Kuno
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    ABSTRACT: We present a multiwavelength study of a sample of far-infrared (FIR) sources detected on the Herschel broad--band maps of the nearby galaxy M33. We perform source photometry on the FIR maps as well as mid-infrared (MIR), H$\alpha$, far-ultraviolet and integrated HI and CO line emission maps. By fitting MIR/FIR dust emission spectra, the source dust masses, temperatures and luminosities are inferred. The sources are classified based on their H$\alpha$ morphology (substructured versus not-substructured) and on whether they have a significant CO detection ($S/N>$3$\sigma$). We find that the sources have dust masses in the range 10$^2$-10$^4$~M$_\odot$ and that they present significant differences in their inferred dust/star formation/gas parameters depending on their H$\alpha$ morphology and CO detection classification. The results suggests differences in the evolutionary states or in the number of embedded HII regions between the subsamples. The source background--subtracted dust emission seems to be predominantly powered by local star formation, as indicated by a strong correlation between the dust luminosity and the dust-corrected H$\alpha$ luminosity and the fact that the extrapolated young stellar luminosity is high enough to account for the observed dust emission. Finally, we do not find a strong correlation between the dust-corrected H$\alpha$ luminosity and the dust mass of the sources, consistent with previous results on the breakdown of simple scaling relations at sub-kpc scales. However, the scatter in the relation is significantly reduced by correcting the H$\alpha$ luminosity for the age of the young stellar populations in the star--forming regions.
    Preview · Article · Apr 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We search for variations in the disk of Centaurus A of the emission from atomic fine structure lines using Herschel PACS and SPIRE spectroscopy. In particular, we observe the [C II](158 μm), [N II](122 and 205 μm), [O I](63 and 145 μm), and [O III](88 μm) lines, which all play an important role in cooling the gas in photo-ionized and photodissociation regions (PDRs). We determine that the ([C II]+[O I]63)/F TIR line ratio, a proxy for the heating efficiency of the gas, shows no significant radial trend across the observed region, in contrast to observations of other nearby galaxies. We determine that 10%-20% of the observed [C II] emission originates in ionized gas. Comparison between our observations and a PDR model shows that the strength of the far-ultraviolet radiation field, G 0, varies between 101.75 and 102.75 and the hydrogen nucleus density varies between 102.75 and 103.75 cm–3, with no significant radial trend in either property. In the context of the emission line properties of the grand-design spiral galaxy M51 and the elliptical galaxy NGC 4125, the gas in Cen A appears more characteristic of that in typical disk galaxies rather than elliptical galaxies.
    Preview · Article · Apr 2014 · The Astrophysical Journal
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    ABSTRACT: We investigate the connection between dust and gas in the nearby edge-on spiral galaxy NGC 891. High resolution Herschel PACS and SPIRE 70, 100, 160, 250, 350, and 500 $\mu$m images are combined with JCMT SCUBA 850 $\mu$m observations to trace the far-infrared/submillimetre spectral energy distribution (SED). Maps of the HI 21 cm line and CO(J=3-2) emission trace the atomic and molecular hydrogen gas, respectively. We fit one-component modified blackbody models to the integrated SED, finding a global dust mass of 8.5$\times$10$^{7}$ M$_{\odot}$ and an average temperature of 23$\pm$2 K. We also fit the pixel-by-pixel SEDs to produce maps of the dust mass and temperature. The dust mass distribution correlates with the total stellar population as traced by the 3.6 $\mu$m emission. The derived dust temperature, which ranges from approximately 17 to 24 K, is found to correlate with the 24 $\mu$m emission. Allowing the dust emissivity index to vary, we find an average value of $\beta$ = 1.9$\pm$0.3. We confirm an inverse relation between the dust emissivity spectral index and dust temperature, but do not observe any variation of this relationship with vertical height from the mid-plane of the disk. A comparison of the dust properties with the gaseous components of the ISM reveals strong spatial correlations between the surface mass densities of dust and the molecular hydrogen and total gas surface densities. Observed asymmetries in the dust temperature, and the H$_{2}$-to-dust and total gas-to-dust ratios hint that an enhancement in the star formation rate may be the result of larger quantities of molecular gas available to fuel star formation in the NE compared to the SW. Whilst the asymmetry likely arises from dust obscuration due to the geometry of the line-of-sight projection of the spiral arms, we cannot exclude an enhancement in the star formation rate in the NE side of the disk.
    Full-text · Article · Feb 2014 · Astronomy and Astrophysics
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    ABSTRACT: We model the infrared to submillimetre spectral energy distribution of 11 nearby galaxies of the Key Insights on Nearby Galaxies: A Far-Infrared Survey with Herschel sample using Spitzer and Herschel data and compare model extrapolations at 870 μm (using different fitting techniques) with Large APEX BOlometer CAmera (LABOCA) 870 μm observations. We investigate how the differences between predictions and observations vary with model assumptions or environment. At global scales, we find that modified blackbody models using realistic cold emissivity indices (βc = 2 or 1.5) are able to reproduce the 870 μm observed emission within the uncertainties for most of the sample. Low values (βc < 1.3) would be required in NGC 0337, NGC 1512 and NGC 7793. At local scales, we observe a systematic 870 μm excess when using βc = 2.0. The βc = 1.5 or the Draine & Li (2007) models can reconcile predictions with observations in part of the discs. Some of the remaining ‘excesses’ occur towards the centres and can be partly or fully accounted for by non-dust contributions such as CO(3–2) or, to a lesser extent, free–free or synchrotron emission. In three non-barred galaxies, the remaining excesses rather occur in the disc outskirts. This could be a sign of a flattening of the submm slope (and decrease of the effective emissivity index) with radius in these objects.
    Full-text · Article · Jan 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We investigate the far infrared spectrum of NGC 1266, a S0 galaxy that contains a massive reservoir of highly excited molecular gas. Using the SPIRE-FTS, we detect the $^{12}$CO ladder up to J=(13-12), [C I] and [N II] lines, and also strong water lines more characteristic of UltraLuminous IR Galaxies (ULIRGs). The 12CO line emission is modeled with a combination of a low-velocity C-shock and a PDR. Shocks are required to produce the H2O and most of the high-J 12CO emission. Despite having an infrared luminosity thirty times less than a typical ULIRG, the spectral characteristics and physical conditions of the ISM of NGC 1266 closely resemble those of ULIRGs, which often harbor strong shocks and large-scale outflows.
    Full-text · Article · Nov 2013 · The Astrophysical Journal Letters
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    ABSTRACT: We present 12CO J = 3→2 maps of NGC 2976 and NGC 3351 obtained with the James Clerk Maxwell Telescope (JCMT), both early targets of the JCMT Nearby Galaxy Legacy Survey (NGLS). We combine the present observations with 12CO J = 1→0 data and find that the computed 12CO J = 3→2 to 12CO J = 1→0 line ratio (R31) agrees with values measured in other NGLS field galaxies. We compute the MH2 value and find that it is robust against the value of R31 used. Using H i data from The H i Nearby Galaxy Survey, we find a tight correlation between the surface density of H2 and star formation rate density for NGC 3351 when 12CO J = 3→2 data are used. Finally, we compare the 12CO J = 3→2 intensity with the polycyclic aromatic hydrocarbon (PAH) 8 μm surface brightness and find a good correlation in the high surface brightness regions. We extend this study to include all 25 Spitzer Infrared Nearby Galaxies Survey galaxies within the NGLS sample and find a tight correlation at large spatial scales. We suggest that both PAH 8 μm and 12CO J = 3→2 are likely to originate in regions of active star formation.
    Full-text · Article · Nov 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present a ˜1″ (100 pc) resolution CO(3–2) map of the Antennae galaxies obtained with the Submillimeter Array. We find that only < 30% of the GMAs spatially coincides with the optically detected star clusters, suggesting that the bulk of the CO (3–2) emission traces the regions with very recent or near future star formation activity. A high CO (3–2)/(1–0) ratio is seen in both nuclei and the southern complexes in the overlap region. Higher radiation field associated with intense star formation can account for the nucleus of NGC 4038 and the overlap region, but the nuclear region of NGC 4039 show relatively little star formation or AGN activities and cannot easily explained. We show kinematical evidence that the high line ratio in NGC 4039 is possibly caused by gas inflow into the counter-rotating central disk.
    No preview · Article · Oct 2013
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    ABSTRACT: Data from the Herschel Space Observatory have revealed an unusual elliptical galaxy, NGC 4125, which has strong and extended submillimeter emission from cold dust but only very strict upper limits to its CO and HI emission. Depending on the dust emissivity, the total dust mass is 2-5x10^6 Msun. While the neutral gas-to-dust mass ratio is extremely low (< 12-30), including the ionized gas traced by [CII] emission raises this limit to < 39-100. The dust emission follows a similar r^{1/4} profile to the stellar light and the dust to stellar mass ratio is towards the high end of what is found in nearby elliptical galaxies. We suggest that NGC 4125 is currently in an unusual phase where evolved stars produced in a merger-triggered burst of star formation are pumping large amounts of gas and dust into the interstellar medium. In this scenario, the low neutral gas-to-dust mass ratio is explained by the gas being heated to temperatures >= 10^4 K faster than the dust is evaporated. If galaxies like NGC 4125, where the far-infrared emission does not trace neutral gas in the usual manner, are common at higher redshift, this could have significant implications for our understanding of high redshift galaxies and galaxy evolution.
    Full-text · Article · Sep 2013 · The Astrophysical Journal Letters

Publication Stats

4k Citations
444.04 Total Impact Points

Institutions

  • 2001-2015
    • McMaster University
      • Department of Physics and Astronomy
      Hamilton, Ontario, Canada
  • 2014
    • University of Massachusetts Amherst
      • Department of Astronomy
      Amherst Center, Massachusetts, United States
  • 2013
    • Paris Diderot University
      Lutetia Parisorum, Île-de-France, France
    • Imperial College London
      Londinium, England, United Kingdom
  • 2003
    • University of Waterloo
      Ватерлоо, Ontario, Canada
  • 1989-2003
    • California Institute of Technology
      • Department of Astronomy
      Pasadena, CA, United States