Thomas Robitaille

Aberystwyth University, Aberystwyth, Wales, United Kingdom

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Publications (138)413.79 Total impact

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    ABSTRACT: We present Atacama Large Millimeter/submillimeter Array (ALMA) line and continuum observations at 1.2mm with ~0.3" resolution that uncover a Keplerian-like disk around the forming O-type star AFGL 4176. The continuum emission from the disk at 1.21 mm (source mm1) has a deconvolved size of 870+/-110 AU x 330+/-300 AU and arises from a structure ~8 M_sun in mass, calculated assuming a dust temperature of 190 K. The first-moment maps, pixel-to-pixel line modeling, assuming local thermodynamic equilibrium (LTE), and position-velocity diagrams of the CH3CN J=13-12 K-line emission all show a velocity gradient along the major axis of the source, coupled with an increase in velocity at small radii, consistent with Keplerian-like rotation. The LTE line modeling shows that where CH3CN J=13-12 is excited, the temperatures in the disk range from ~70 to at least 300 K and that the H2 column density peaks at 2.8x10^24 cm^-2. In addition, we present Atacama Pathfinder Experiment (APEX) 12CO observations which show a large-scale outflow from AFGL 4176 perpendicular to the major axis of mm1, supporting the disk interpretation. Finally, we present a radiative transfer model of a Keplerian disk surrounding an O7 star, with a disk mass and radius of 12 M_sun and 2000 AU, that reproduces the line and continuum data, further supporting our conclusion that our observations have uncovered a Keplerian disk around an O-type star.
    Preview · Article · Sep 2015
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    ABSTRACT: Submillimetre-luminous galaxies at high-redshift are the most luminous, heavily star-forming galaxies in the Universe, and are characterised by prodigious emission in the far-infrared at 850 microns (S850 > 5 mJy). They reside in halos ~ 10^13Msun, have low gas fractions compared to main sequence disks at a comparable redshift, trace complex environments, and are not easily observable at optical wavelengths. Their physical origin remains unclear. Simulations have been able to form galaxies with the requisite luminosities, but have otherwise been unable to simultaneously match the stellar masses, star formation rates, gas fractions and environments. Here we report a cosmological hydrodynamic galaxy formation simulation that is able to form a submillimetre galaxy which simultaneously satisfies the broad range of observed physical constraints. We find that groups of galaxies residing in massive dark matter halos have rising star formation histories that peak at collective rates ~ 500-1000 Msun/yr at z=2-3, by which time the interstellar medium is sufficiently enriched with metals that the region may be observed as a submillimetre-selected system. The intense star formation rates are fueled in part by a reservoir gas supply enabled by stellar feedback at earlier times, not through major mergers. With a duty cycle of nearly a gigayear, our simulations show that the submillimetre-luminous phase of high-z galaxies is a drawn out one that is associated with significant mass buildup in early Universe proto-clusters, and that many submillimetre-luminous galaxies are actually composed of numerous unresolved components (for which there is some observational evidence).
    Full-text · Article · Sep 2015 · Nature
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    ABSTRACT: This paper presents SunPy (version 0.5), a community-developed Python package for solar physics. Python, a free, cross-platform, general-purpose, high-level programming language, has seen widespread adoption among the scientific community, resulting in the availability of a large number of software packages, from numerical computation ( NumPy , SciPy ) and machine learning ( scikit-learn ) to visualization and plotting ( matplotlib ). SunPy is a data-analysis environment specializing in providing the software necessary to analyse solar and heliospheric data in Python. SunPy is open-source software (BSD licence) and has an open and transparent development workflow that anyone can contribute to. SunPy provides access to solar data through integration with the Virtual Solar Observatory (VSO), the Heliophysics Event Knowledgebase (HEK), and the HELiophysics Integrated Observatory (HELIO) webservices. It currently supports image data from major solar missions (e.g., SDO , SOHO , STEREO , and IRIS ), time-series data from missions such as GOES , SDO /EVE, and PROBA2 /LYRA, and radio spectra from e-Callisto and STEREO /SWAVES. We describe SunPyʼs functionality, provide examples of solar data analysis in SunPy, and show how Python-based solar data-analysis can leverage the many existing tools already available in Python. We discuss the future goals of the project and encourage interested users to become involved in the planning and development of SunPy.
    Full-text · Article · Jul 2015 · Computational Science & Discovery
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    Barbara Ercolano · Christine Koepferl · James Owen · Thomas Robitaille
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    ABSTRACT: By means of radiative transfer simulation, we study the evolution of the far-infrared colours of protoplanetary discs undergoing inside-out dispersal, often referred to as transition discs. We show that a brightening of the mid- and far-infrared emission from these objects is a natural consequence of the removal of the inner disc. Our results can fully explain recent observations of transition discs in the Chamaleon and Lupus star-forming regions from the Herschel Gould Belt Survey, which shows a higher median for the 70 μm (Herschel PACS 1) band of known transition objects compared with primordial discs. Our theoretical results hence support the suggestion that the 70 μm band may be a powerful diagnostic for the identification of transition discs from photometry data, provided that the inner hole is larger than tens of au, depending on spectral type. Furthermore, we show that a comparison of photometry in the K, 12 μm and 70 μm bands to model tracks can provide a rough, but quick estimate of the inner hole size of these objects, provided their inclination is below ∼85° and the inner hole size is again larger than tens of au.
    Full-text · Article · Jul 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: This paper presents SunPy (version 0.5), a community-developed Python package for solar physics. Python, a free, cross-platform, general-purpose, high-level programming language, has seen widespread adoption among the scientific community, resulting in the availability of a large number of software packages, from numerical computation (NumPy, SciPy) and machine learning (scikit-learn) to visualisation and plotting (matplotlib). SunPy is a data-analysis environment specialising in providing the software necessary to analyse solar and heliospheric data in Python. SunPy is open-source software (BSD licence) and has an open and transparent development workflow that anyone can contribute to. SunPy provides access to solar data through integration with the Virtual Solar Observatory (VSO), the Heliophysics Event Knowledgebase (HEK), and the HELiophysics Integrated Observatory (HELIO) webservices. It currently supports image data from major solar missions (e.g., SDO, SOHO, STEREO, and IRIS), time-series data from missions such as GOES, SDO/EVE, and PROBA2/LYRA, and radio spectra from e-Callisto and STEREO/SWAVES. We describe SunPy's functionality, provide examples of solar data analysis in SunPy, and show how Python-based solar data-analysis can leverage the many existing tools already available in Python. We discuss the future goals of the project and encourage interested users to become involved in the planning and development of SunPy.
    Full-text · Article · May 2015
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    ABSTRACT: The Flexible Image Transport System (FITS) standard has been a great boon to astronomy, allowing observatories, scientists and the public to exchange astronomical information easily. The FITS standard, however, is showing its age. Developed in the late 1970s, the FITS authors made a number of implementation choices that, while common at the time, are now seen to limit its utility with modern data. The authors of the FITS standard could not anticipate the challenges which we are facing today in astronomical computing. Difficulties we now face include, but are not limited to, addressing the need to handle an expanded range of specialized data product types (data models), being more conducive to the networked exchange and storage of data, handling very large datasets, and capturing significantly more complex metadata and data relationships. There are members of the community today who find some or all of these limitations unworkable, and have decided to move ahead with storing data in other formats. If this fragmentation continues, we risk abandoning the advantages of broad interoperability, and ready archivability, that the FITS format provides for astronomy. In this paper we detail some selected important problems which exist within the FITS standard today. These problems may provide insight into deeper underlying issues which reside in the format and we provide a discussion of some lessons learned. It is not our intention here to prescribe specific remedies to these issues; rather, it is to call attention of the FITS and greater astronomical computing communities to these problems in the hope that it will spur action to address them.
    Full-text · Article · Feb 2015 · Astronomy and Computing
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    ABSTRACT: In contrast to most other galaxies, star-formation rates in the Milky Way can be estimated directly from Young Stellar Objects (YSOs). In the Central Molecular Zone (CMZ) the star-formation rate calculated from the number of YSOs with 24 microns emission is up to order of magnitude higher than the value estimated from methods based on diffuse emission (such as free-free emission). Whether this effect is real or whether it indicates problems with either or both star formation rate measures is not currently known. In this paper, we investigate whether estimates based on YSOs could be heavily contaminated by more evolved objects such as main-sequence stars. We present radiative transfer models of YSOs and of main-sequence stars in a constant ambient medium which show that the main-sequence objects can indeed mimic YSOs at 24 microns. However, we show that in some cases the main-sequence models can be marginally resolved at 24 microns, whereas the YSO models are always unresolved. Based on the fraction of resolved MIPS 24 microns sources in the sample of YSOs previously used to compute the star formation rate, we estimate the fraction of misclassified "YSOs" to be at least 63%, which suggests that the star-formation rate previously determined from YSOs is likely to be at least a factor of three too high.
    Full-text · Article · Nov 2014 · The Astrophysical Journal
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    ABSTRACT: The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and lifecycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21 cm, CO, and Halpha observations. In the diffuse atomic ISM, we derive the gas-to-dust ratio as the slope of the dust-gas relation and find gas-to-dust ratios of 380+250-130 in the LMC, and 1200+1600-420 in the SMC, not including helium. The atomic-to-molecular transition is located at dust surface densities of 0.05 Mo pc-2 in the LMC and 0.03 Mo pc-2 in the SMC, corresponding to AV ~ 0.4 and 0.2, respectively. We investigate the range of CO-to-H2 conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on XCO to be 6x1020 cm-2 K-1 km-1 s in the LMC (Z=0.5Zo) at 15 pc resolution, and 4x 1021 cm-2 K-1 km-1 s in the SMC (Z=0.2Zo) at 45 pc resolution. In the LMC, the slope of the dust-gas relation in the dense ISM is lower than in the diffuse ISM by a factor ~2, even after accounting for the effects of CO-dark H2 in the translucent envelopes of molecular clouds. Coagulation of dust grains and the subsequent dust emissivity increase in molecular clouds, and/or accretion of gas-phase metals onto dust grains, and the subsequent dust abundance (dust-to-gas ratio) increase in molecular clouds could explain the observations. In the SMC, variations in the dust-gas slope caused by coagulation or accretion are degenerate with the effects of CO-dark H2. Within the expected 5--20 times Galactic XCO range, the dust-gas slope can be either constant or decrease by a factor of several across ISM phases. Further modeling and observations are required to break the degeneracy between dust grain coagulation, accretion, and CO-dark H2.
    Full-text · Article · Nov 2014 · The Astrophysical Journal
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    ABSTRACT: The very long and thin infrared dark cloud "Nessie" is even longer than had been previously claimed, and an analysis of its Galactic location suggests that it lies directly in the Milky Way's mid-plane, tracing out a highly elongated bone-like feature within the prominent Scutum-Centaurus spiral arm. Re-analysis of mid-infrared imagery from the Spitzer Space Telescope shows that this IRDC is at least 2, and possibly as many as 5 times longer than had originally been claimed by Nessie's discoverers (Jackson et al. 2010); its aspect ratio is therefore at least 300:1, and possibly as large as 800:1. A careful accounting for both the Sun's offset from the Galactic plane ($\sim 25$ pc) and the Galactic center's offset from the $(l^{II},b^{II})=(0,0)$ position shows that the latitude of the true Galactic mid-plane at the 3.1 kpc distance to the Scutum-Centaurus Arm is not $b=0$, but instead closer to $b=-0.4$, which is the latitude of Nessie to within a few pc. An analysis of the radial velocities of low-density (CO) and high-density (${\rm NH}_3$) gas associated with the Nessie dust feature suggests that Nessie runs along the Scutum-Centaurus Arm in position-position-velocity space, which means it likely forms a dense `spine' of the arm in real space as well. The Scutum-Centaurus arm is the closest major spiral arm to the Sun toward the inner Galaxy, and, at the longitude of Nessie, it is almost perpendicular to our line of sight, making Nessie the easiest feature to see as a shadow elongated along the Galactic Plane from our location. Future high-resolution dust mapping and molecular line observations of the harder-to-find Galactic "bones" should allow us to exploit the Sun's position above the plane to gain a (very foreshortened) view "from above" of the Milky Way's structure.
    Full-text · Article · Jul 2014 · The Astrophysical Journal
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    ABSTRACT: The Magellanic Bridge is the nearest low-metallicity, tidally stripped environment, offering a unique high-resolution view of physical conditions in merging and forming galaxies. In this paper we present analysis of candidate massive young stellar objects (YSOs), i.e., {\it in situ, current} massive star formation (MSF) in the Bridge using {\it Spitzer} mid-IR and complementary optical and near-IR photometry. While we definitely find YSOs in the Bridge, the most massive are $\sim10 M_\odot$, $\ll45 M_\odot$ found in the Large Magellanic Cloud (LMC). The intensity of MSF in the Bridge also appears decreasing, as the most massive YSOs are less massive than those formed in the past. To investigate environmental effects on MSF, we have compared properties of massive YSOs in the Bridge to those in the LMC. First, YSOs in the Bridge are apparently less embedded than in the LMC: 81% of Bridge YSOs show optical counterparts, compared to only 56% of LMC sources with the same range of mass, circumstellar dust mass, and line-of-sight extinction. Circumstellar envelopes are evidently more porous or clumpy in the Bridge's low-metallicity environment. Second, we have used whole samples of YSOs in the LMC and the Bridge to estimate the probability of finding YSOs at a given \hi\ column density, N(HI). We found that the LMC has $\sim3\times$ higher probability than the Bridge for N(HI) $>10\times10^{20}$ cm$^{-2}$, but the trend reverses at lower N(HI). Investigating whether this lower efficiency relative to HI is due to less efficient molecular cloud formation, or less efficient cloud collapse, or both, will require sensitive molecular gas observations.
    Full-text · Article · Mar 2014 · The Astrophysical Journal
  • Chris Beaumont · Thomas Robitaille · Michelle Borkin
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    ABSTRACT: Glue, written in Python, links visualizations of scientific datasets across many files, allowing for interactive, linked statistical graphics of multiple files. It supports many file formats including common image formats (jpg, tiff, png), ASCII tables, astronomical image and table formats (FITS, VOT, IPAC), and HDF5. Custom data loaders can also be easily added. Glue is highly scriptable and extendable.
    No preview · Article · Jan 2014
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    ABSTRACT: The Astropy Project is a community effort to develop an open source Python package of common data structures and routines for use by other, more specialized astronomy software in Python in order to foster software interoperability in the astronomical community. The project encompasses Astropy's ”core” and ”affiliated" packages that adopt Astropy’s coding, testing and documentation standards. By doing so we aim to improve interoperability with other Python packages in astronomy, and help a broader community implement more Pythonic solutions to astronomy computing problems while minimizing duplication of effort. The project provides a template for other projects that use Astropy to reuse much of Astropy’s development framework without reinventing the wheel. Here we present an overview of the key features of the core package (existing and upcoming), current and planned affiliated packages, and how we manage a large open source project with a diverse community of contributors.
    No preview · Article · Jan 2014
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    ABSTRACT: In order to investigate whether the feedback produced by photo-ionisation has an important effect on the geometry of the circumstellar dust and gas around forming massive stars, we have observed the luminous southern embedded star AFGL 4176 in transitions of NH3 and the hydrogen recombination line H68α. We present our preliminary results, which show a compact H ii region embedded in a parsec-scale (radius ̃ 0.7 pc) rotating envelope/torus. In addition, the H ii region is found to be offset from the centre of the envelope, and the velocity gradient in the ionised gas is not aligned with the rotation axis of the envelope, suggesting complex dynamics and multiplicity.
    No preview · Article · Nov 2013
  • Thomas P. Robitaille · Barbara A. Whitney
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    ABSTRACT: In 2006, we made available a set of model spectral energy distributions (SEDs) for young stellar objects (YSOs) that has since been used by the community to model thousands of sources in nearby and more distant star forming regions in the Milky-Way and Magellanic clouds. We describe a new and much larger set of model SEDs that addresses many issues with the original models, such as the coverage of parameter space, the dependence on stellar evolutionary tracks, and is well suited to modelling long-wavelength observations such as those from Herschel. The new models were computed with Hyperion, a new Monte-Carlo radiative transfer code. We present the design of the new set of models and discuss improvements compared to the original models.
    No preview · Article · Nov 2013
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    ABSTRACT: The Spitzer Space Telescope Legacy Program SAGE-SMC allows global studies of resolved stellar populations in the SMC in a different environment than our Galaxy. Using the SAGE-SMC IRAC (3.6-8.0 μm) and MIPS (24 and 70 μm) catalogs and images combined with near-infrared (JHK s ) and optical (UBVI) data, we identified a population of ~1000 intermediate- to high-mass young stellar objects (YSOs) in the SMC (three times more than previously known). Our method of identifying YSO candidates builds on the method developed for the Large Magellanic Cloud by Whitney et al. with improvements based on what we learned from our subsequent studies and techniques described in the literature. We perform (1) color-magnitude cuts based on five color-magnitude diagrams (CMDs), (2) visual inspection of multi-wavelength images, and (3) spectral energy distribution (SED) fitting with YSO models. For each YSO candidate, we use its photometry to calculate a measure of our confidence that the source is not a non-YSO contaminant, but rather a true YSO, based on the source's location in the color-magnitude space with respect to non-YSOs. We use this CMD score and the SED fitting results to define two classes of sources: high-reliability YSO candidates and possible YSO candidates. We found that, due to polycyclic aromatic hydrocarbon emission, about half of our sources have [3.6]-[4.5] and [4.5]-[5.8] colors not predicted by previous YSO models. The YSO candidates are spatially correlated with gas tracers.
    Preview · Article · Nov 2013 · The Astrophysical Journal
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    ABSTRACT: We present an overview of the HERschel Inventory of The Agents of Galaxy Evolution (HERITAGE) in the Magellanic Clouds project, which is a Herschel Space Observatory open time key program. We mapped the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) at 100, 160, 250, 350, and 500 μm with the Spectral and Photometric Imaging Receiver (SPIRE) and Photodetector Array Camera and Spectrometer (PACS) instruments on board Herschel using the SPIRE/PACS parallel mode. The overriding science goal of HERITAGE is to study the life cycle of matter as traced by dust in the LMC and SMC. The far-infrared and submillimeter emission is an effective tracer of the interstellar medium (ISM) dust, the most deeply embedded young stellar objects (YSOs), and the dust ejected by the most massive stars. We describe in detail the data processing, particularly for the PACS data, which required some custom steps because of the large angular extent of a single observational unit and overall the large amount of data to be processed as an ensemble. We report total global fluxes for the LMC and SMC and demonstrate their agreement with measurements by prior missions. The HERITAGE maps of the LMC and SMC are dominated by the ISM dust emission and bear most resemblance to the tracers of ISM gas rather than the stellar content of the galaxies. We describe the point source extraction processing and the criteria used to establish a catalog for each waveband for the HERITAGE program. The 250 μm band is the most sensitive and the source catalogs for this band have ~25,000 objects for the LMC and ~5500 objects for the SMC. These data enable studies of ISM dust properties, submillimeter excess dust emission, dust-to-gas ratio, Class 0 YSO candidates, dusty massive evolved stars, supernova remnants (including SN1987A), H II regions, and dust evolution in the LMC and SMC. All images and catalogs are delivered to the Herschel Science Center as part of the community support aspects of the project. These HERITAGE images and catalogs provide an excellent basis for future research and follow up with other facilities.
    No preview · Article · Aug 2013 · The Astronomical Journal
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    ABSTRACT: We present the first public version (v0.2) of the open-source and community-developed Python package, Astropy. This package provides core astronomy-related functionality to the community, including support for domain-specific file formats such as Flexible Image Transport System (FITS) files, Virtual Observatory (VO) tables, and common ASCII table formats, unit and physical quantity conversions, physical constants specific to astronomy, celestial coordinate and time transformations, world coordinate system (WCS) support, generalized containers for representing gridded as well as tabular data, and a framework for cosmological transformations and conversions. Significant functionality is under active development, such as a model fitting framework, VO client and server tools, and aperture and point spread function (PSF) photometry tools. The core development team is actively making additions and enhancements to the current code base, and we encourage anyone interested to participate in the development of future Astropy versions.
    Full-text · Article · Jul 2013 · Astronomy and Astrophysics
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    B. A. Whitney · T. P. Robitaille · J. E. Bjorkman · R. Dong · M. J. Wolff · K. Wood · J. Honor
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    ABSTRACT: We have updated our publicly available dust radiative transfer code (HOCHUNK3D) to include new emission processes and various 3-D geometries appropriate for forming stars. The 3-D geometries include warps and spirals in disks, accretion hotspots on the central star, fractal clumping density enhancements, and misaligned inner disks. Additional axisymmetric (2-D) features include gaps in disks and envelopes, "puffed-up inner rims" in disks, multiple bipolar cavity walls, and iteration of disk vertical structure assuming hydrostatic equilibrium. We include the option for simple power-law envelope geometry, which combined with fractal clumping, and bipolar cavities, can be used to model evolved stars as well as protostars. We include non-thermal emission from PAHs and very small grains, and external illumination from the interstellar radiation field. The grid structure was modified to allow multiple dust species in each cell; based on this, a simple prescription is implemented to model dust stratification. We describe these features in detail, and show example calculations of each. Some of the more interesting results include the following: 1) Outflow cavities may be more clumpy than infalling envelopes. 2) PAH emission in high-mass stars may be a better indicator of evolutionary stage than the broadband SED slope; and related to this, 3) externally illuminated clumps and high-mass stars in optically thin clouds can masquerade as YSOs. 4) Our hydrostatic equilibrium models suggest that dust settling is likely ubiquitous in T Tauri disks, in agreement with previous observations.
    Preview · Article · Jul 2013 · The Astrophysical Journal Supplement Series
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    ABSTRACT: Spitzer surveys of the inner Galactic Plane (GLIMPSE I & II) revealed a promising new diagnostic for identifying actively accreting (proto)stars: extended excess emission in the IRAC 4.5 micron band, believed to trace shocked molecular gas in active protostellar outflows. We will present initial results from our search for extended excess 4.5 micron sources in the outer Galaxy GLIMPSE360 survey, focusing on the area of the survey associated with the Perseus arm.
    No preview · Article · Jun 2013
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    ABSTRACT: The Spitzer SAGE 8-micron image of the LMC is strongly dominated by interstellar emission from PAH molecules, and offers spatial and brightness dynamic range which far exceeds what is available from spectral-line radio observations. We examine the correlation of this emission with other gas tracers, including the radio lines of CO, HI, and HCO+ as observed by the ATCA and Mopra telescopes, and archival UV absorption spectra of HI and H2 from the HST and FUSE. We show preliminary results of radiative transfer simulations using the HYPERION code aimed at identifying regimes and techniques for using 8-micron emission as a high resolution (sub-pc) tracer of gas density.
    No preview · Article · Jun 2013

Publication Stats

3k Citations
413.79 Total Impact Points

Institutions

  • 2015
    • Aberystwyth University
      • Department of Mathematics and Physics
      Aberystwyth, Wales, United Kingdom
  • 2012-2015
    • Max Planck Institute for Astronomy
      Heidelburg, Baden-Württemberg, Germany
    • Leiden University
      • Leiden Observartory
      Leyden, South Holland, Netherlands
  • 2013
    • University of Toledo
      • Department of Physics and Astronomy
      Toledo, Ohio, United States
    • University College London
      • Department of Physics and Astronomy
      Londinium, England, United Kingdom
  • 2009-2012
    • Harvard-Smithsonian Center for Astrophysics
      • Smithsonian Astrophysical Observatory
      Cambridge, Massachusetts, United States
  • 2011
    • The University of Arizona
      • Department of Astronomy
      Tucson, Arizona, United States
    • University of Texas at Austin
      • Department of Astronomy
      Austin, Texas, United States
    • Keele University
      • Department of Physics and Astrophysics
      Newcastle-under-Lyme, England, United Kingdom
    • Pennsylvania State University
      • Department of Astronomy and Astrophysics
      University Park, Maryland, United States
  • 2010
    • Harvard University
      Cambridge, Massachusetts, United States
  • 2004-2009
    • University of St Andrews
      • School of Physics and Astronomy
      Saint Andrews, SCT, United Kingdom
  • 2008
    • University of Wisconsin, Madison
      • Department of Astronomy
      Mississippi, United States
  • 2007-2008
    • Scottish Universities Physics Alliance
      Glasgow, Scotland, United Kingdom