Honors, Awards and Grants

  • 2014-2016
    JSPS International Postdoctoral Fellow
    JSPS Grants-in-Aid for Scientific Research (¥2.2 billion)
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    Project description
  • 2004-2010
    The MacCracken Fellowship
    (New York University)
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  • Spring 2008
    Margaret and Herman Sokol Research Award
    (New York University)
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  • January 2007
    The Chambliss Astronomy Achievement Student Award
    (Honorable Mention)
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Main Collaborators

David A. Doe

Postdoctoral fellow

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James Doe

Postdoctoral fellow

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Great collaborators!

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Research Projects

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    Title of Preject

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    Title of Preject

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    Title of Preject

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    Title of Preject

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    Title of Preject

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    Title of Preject

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Disentangling the ISM phases of the dwarf galaxy NGC 4214 using [CII] SOFIA/GREAT observations

K. Fahrion, D. Cormier, F. Bigiel, N. P. Abel, P. Cigan, et al.
Refereed Astronomy & Astrophysics
DOI:10.1051/0004-6361/201629341

Abstract

The [CII] 158 um fine structure line is one of the dominant cooling lines in the interstellar medium (ISM) and is an important tracer of star formation. Recent velocity-resolved studies with Herschel/HIFI and SOFIA/GREAT showed that the [CII] line can constrain the properties of the ISM phases in star-forming regions. The [CII] line as a tracer of star formation is particularly important in low-metallicity environments where CO emission is weak because of the presence of large amounts of CO-dark gas. The nearby irregular dwarf galaxy NGC 4214 offers an excellent opportunity to study an actively star-forming ISM at low metallicity. We analyzed the spectrally resolved [CII] line profiles in three distinct regions at different evolutionary stages of NGC 4214 with respect to ancillary HI and CO data in order to study the origin of the [CII] line. We used SOFIA/GREAT [CII] 158 um observations, HI data from THINGS, and CO(2-1) data from HERACLES to decompose the spectrally resolved [CII] line profiles into components associated with neutral atomic and molecular gas. We use this decomposition to infer gas masses traced by [CII] under different ISM conditions. Averaged over all regions, we associate about 46% of the [CII] emission with the HI emission. However, we can assign only around 9% of the total [CII] emission to the cold neutral medium (CNM). We found that about 79% of the total molecular hydrogen mass is not traced by CO emission. On average, the fraction of CO-dark gas dominates the molecular gas mass budget. The fraction seems to depend on the evolutionary stage of the regions: it is highest in the region covering a super star cluster in NGC 4214, while it is lower in a more compact, more metal-rich region.

Radiative and mechanical feedback into the molecular gas in the Large Magellanic Cloud-I. N159W

M-Y Lee, SC Madden, Vianney Lebouteiller, Antoine Gusdorf, Benjamin Godard, Ronin Wu, et al.
Refereed Astronomy & Astrophysics, Volume 596, A85 (December 2016)
DOI:10.1051/0004-6361/201628098

Abstract

We present Herschel SPIRE Fourier Transform Spectrometer (FTS) observations of N159W, an active star-forming region in the Large Magellanic Cloud (LMC). In our observations, a number of far-infrared cooling lines, including carbon monoxide (CO) J = 4 → 3 to J = 12 → 11, [CI] 609 μm and 370 μm, and [NII] 205 μm, are clearly detected. With an aim of investigating the physical conditions and excitation processes of molecular gas, we first construct CO spectral line energy distributions (SLEDs) on ~10 pc scales by combining the FTS CO transitions with ground-based low-J CO data and analyze the observed CO SLEDs using non-LTE (local thermodynamic equilibrium) radiative transfer models. We find that the CO-traced molecular gas in N159W is warm (kinetic temperature of 153–754 K) and moderately dense (H2 number density of (1.1−4.5) × 103 cm-3). To assess the impact of the energetic processes in the interstellar medium on the physical conditions of the CO-emitting gas, we then compare the observed CO line intensities with the models of photodissociation regions (PDRs) and shocks. We first constrain the properties of PDRs by modeling Herschel observations of [OI] 145 μm, [CII] 158 μm, and [CI] 370 μm fine-structure lines and find that the constrained PDR components emit very weak CO emission. X-rays and cosmic-rays are also found to provide a negligible contribution to theCO emission, essentially ruling out ionizing sources (ultraviolet photons, X-rays, and cosmic-rays) as the dominant heating source for CO in N159W. On the other hand, mechanical heating by low-velocity C-type shocks with ~10 km s-1 appears sufficient enough to reproduce the observed warm CO.

A milestone toward understanding PDR properties in the extreme environment of LMC-30 Doradus

M Chevance, SC Madden, V Lebouteiller, B Godard, D Cormier, et al.
Refereed Astronomy & Astrophysics, Volumn 590, A36 (June 2016)
DOI:10.1051/0004-6361/201527735

Abstract

More complete knowledge of galaxy evolution requires understanding the process of star formation and interaction between the interstellar radiation field and the interstellar medium in galactic environments traversing a wide range of physical parameter space. Here we focus on the impact of massive star formation on the surrounding low metallicity ISM in 30 Doradus in the Large Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the early universe, results in less ultra-violet shielding for the formation of the molecular gas necessary for star formation to proceed. The half-solar metallicity gas in this region is strongly irradiated by the super star cluster R136, making it an ideal laboratory to study the structure of the ISM in an extreme environment. Our spatially resolved study investigates the gas heating and cooling mechanisms, particularly in the photo-dissociation regions where the chemistry and thermal balance are regulated by far-ultraviolet photons (6 eV< h\nu <13.6 eV).

We present Herschel observations of far-infrared fine-structure lines obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines from Herschel and Spitzer observations with ground-based CO data to provide diagnostics on the properties and the structure of the gas by modeling it with the Meudon PDR code. We derive the spatial distribution of the radiation field, the pressure, the size, and the filling factor of the photodissociated gas and molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR modeling. Assuming a plane-parallel geometry and a uniform medium, we find a total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to 3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three dimensional structure of the gas. (Abridged)

The spatially resolved correlation between [NII] 205 μm line emission and the 24 μm continuum in nearby galaxies

TM Hughes, M Baes, MRP Schirm, TJ Parkin, Ronin Wu, et al.
Refereed Astronomy & Astrophysics, Volume 587, A45 (March 2016)
DOI:10.1051/0004-6361/201527644

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.

AKARI NIR spectroscopy of interstellar ices

Takashi Onaka, Tamami I Mori, Itsuki Sakon, Fumihiko Usui, Ronin Wu, Takashi Shimonishi
Conference Astronomy in Focus, Volume 11, A29A (August 2015)
DOI:10.1017/S1743921316003161

Abstract

The Infrared Camera (IRC) onboard AKARI has a near-infrared (2--5μm) spectroscopic capability with high sensitivity that allows us to study the major ice components in various objects. In particular, H2O and CO2 ice absorption features have been detected towards nearby galaxies, including several young stellar objects (YSOs) in the Large Magellanic Cloud (LMC), as well as a number of HII region-PDR complexes for the first time by IRC spectroscopy. While observations in the LMC show a high ratio (~0.34) of the CO2 to H2O ice column densities, the ratios in Galactic HII-region-PDR complexes are in the range of 0.1--0.2, being compatible with those found in Galactic massive YSOs in previous studies. The good correlation supports concurrent formation of the two ice species on the grain surface and the higher ratio in the low-metallicity LMC suggests possible environmental effects in the formation process.

AKARI observations of dust processing in merger galaxies: NGC2782 and NGC7727

Takashi Onaka, Tomohiko Nakamura, Itsuki Sakon, Ryou Ohsawa, Tamami Mori, Ronin Wu, Hidehiro Kaneda
Conference IAU General Assembly, Meeting #29, id.2254465 (August 2015)

Abstract

Dust grains are the major reservoir of heavy elements and play significant roles in the thermal balance and chemistry in the interstellar medium. Where dust grains are formed and how they evolve in the ISM are one of the key issues for the understanding of the material evolution in the Universe. Although theoretical studies have been made, very little is so far known observationally about the lifecycle of dust grains in the ISM and that associated with Galactic scale events. The lifecycle of very small carbonaceous grains that contain polycyclic aromatic hydrocarbons (PAHs) or PAH-like atomic groups are of particular interest because they emit distinct band emission in the near- to mid-infrared region and they are thought to be most vulnerable to environmental conditions. PAHs may be formed in carbon-rich stars, while recent AKARI observations suggest that they may be formed by fragmentation of large carbonaceous grains in shocks in a supernova remnant or a galactic wind (Onaka et al. 2010, A&A, 514, 15; Seok et al. 2012, ApJ, 744, 160).Here we report results of AKARI observations of two mergers. NGC2782 (Arp 215) and NGC7727 (Arp 222). NGC2782 is a merger of 200Myr old. It shows a very long western tail of HI gas by a tidal interaction and the eastern tail that consists mainly of stellar components without an appreciable amount of gas and is thought to be a relic of the colliding low-mass galaxy whose gas component has been stripped off Smith 1994, AJ, 107, 1695. We found significant emission at the 7 μm band of the IRC onboard AKARI, which must come from PAH 6.2 and 7.7 μm bands, in the eastern tail. Based on dust model fitting, we found a low abundance of ~10nm size dust despite of the presence of PAHs, suggesting that PAHs may be formed from fragmentation of ~10nm carbonaceous dust grains. NGC7727 is a 1.2Gyr old merger and shows a SED similar to the NGC2782 tail in the northern tail of the merger event product, suggesting also the formation of PAHs from fragmentation. The observations of both galaxies indicate that PAHs can survive in violent events. We discuss these results in relation to the PAH formation and destruction.

AKARI and Spinning Dust: A look at microwave dust emission via the Infrared

Bell, Aaron Christopher; Onaka, Takashi; Wu, Ronin; Doi, Yasuo
Conference IAU General Assembly, Meeting #29, id.2257267 (August 2015)

Abstract

Rapidly spinning dust particles having a permanent electric dipole moment have been shown to be a likely carrier of the anomalous microwave emission (AME), a continuous excess of microwave flux in the 10 to 90 GHz range. Small grains, possibly polycyclic aromatic hydrocarbons (PAHs), are a leading suspect. Due to the overlap frequency overlap with the CMB, the AME is requiring cosmologists to consider the ISM with more care. ISM astronomers are also needing to consider the contribution of cosmological radiation to large-scale dust investigations. We present data from AKARI/Infrared Camera (IRC) due to the effective PAH band coverage of its 9 um survey to investigate PAH emission within 98 AME candidate regions identified by Planck Collaboration et al. (2014). We supplement AKARI data with the four Infrared Astronomical Satellite (IRAS) all-sky maps and complement with the Planck High Frequency Instrument (HFI) bands at 857 and 545GHz to constrain the full dust SED. We sample analyse the SEDs of all 98 regions. We utilize all 7 AKARI photometric bands, as well as the 4 IRAS bands and 2 HFI. We carry out a modified blackbody fitting, and estimate the optical depth of thermal dust at 250 um, and compare this to AME parameters. We also show plots of each band's average intensity for all 98 regions vs. AME parameters. We find a positive trend between the optical depth and AME. In the band-by-band comparison the AKARI 9 um intensity shows a weaker trend with AME. In general, the MIR correlates less strongly with AME than the FIR. The optical depth vs. AME trend improves slightly when looking only at significant AME regions. Scaling the IR intensities by the ISRF strength G0 does not improve the correlations. We cannot offer strong support of a spinning dust model. The results highlight the need for full dust SED modelling, and for a better understanding of the role that magnetic dipole emission from dust grains could play in producing the AME.

Spatially resolved physical conditions of molecular gas: a zoom-in from circumnuclear region of M83 to Carina nebula.

Ronin Wu, Suzanne Madden, Frédéric Galliano, Christine Wilson, Takashi Onaka, Tomohiko Nakamura
Conference IAU General Assembly, Meeting #29, id.2254494 (August 2015)

Abstract

Since the launch of the Herschel Space Observatory, our understanding about the photo-dissociation regions (PDR) has taken a step forward. In the bandwidth of the Fourier Transform Spectrometer (FTS) of the Spectral and Photometric Imaging REceiver (SPIRE) on board Herschel, ten CO rotational transitions, including J=4-3 to J=13-12, and three fine structure lines, including [CI] 609, [CI] 370, and [NII] 250 micron, are covered. This presentation focuses on the physical conditions of molecular gas probed by the Herschel SPIRE/FTS.Based on the spatially resolved physical parameters derived from the CO spectral line energy distribution (SLED) map and the comparisons with the dust properties and star-formation tracers, I will first present our findings at the circumnuclear region of M83, and then zoom in toward the young open cluster, Trumpler 14, in Carina nebula. I will discuss (1) the potential of using [NII] 250 and [CI] 370 micron as star-formation tracers; (2) the reliability of tracing molecular gas with CO; (3) the excitation mechanisms of warm CO; (4) the possibility of studying stellar feedback by tracing the thermal pressure of intersetllar molecular gas.

Star formation rates from young-star counts and the structure of the ISM across the NGC 346/N66 complex in the SMC

S Hony, DA Gouliermis, F Galliano, M Galametz, D Cormier, et al.
Refereed Monthly Notices of the Royal Astronomical Society, Volume 448, 2, 1847 (April 2015)
DOI:10.1093/mnras/stv107

Abstract

The rate at which interstellar gas is converted into stars, and its dependence on environment, is one of the pillars on which our understanding of the visible Universe is build. We present a comparison of the surface density of young stars (Σ⋆) and dust surface density (Σdust) across NGC 346 (N66) in 115 independent pixels of 6 × 6 pc2. We find a correlation between Σ⋆ and Σdust with a considerable scatter. A power-law fit to the data yields a steep relation with an exponent of 2.6 ± 0.2. We convert Σdust to gas surface density (Σgas) and Σ⋆ to star formation rate (SFR) surface densities (ΣSFR), using simple assumptions for the gas-to-dust mass ratio and the duration of star formation. The derived total SFR (4 ± 1×10−3 M⊙ yr−1) is consistent with SFR estimated from the Hα emission integrated over the Hα nebula. On small scales the ΣSFR derived using Hα systematically underestimates the count-based ΣSFR, by up to a factor of 10. This is due to ionizing photons escaping the area, where the stars are counted. We find that individual 36 pc2 pixels fall systematically above integrated disc galaxies in the Schmidt–Kennicutt diagram by on average a factor of ∼7. The NGC 346 average SFR over a larger area (90 pc radius) lies closer to the relation but remains high by a factor of ∼3. The fraction of the total mass (gas plus young stars) locked in young stars is systematically high (∼10 per cent) within the central 15 pc and systematically lower outside (2 per cent), which we interpret as variations in star formation efficiency. The inner 15 pc is dominated by young stars belonging to a centrally condensed cluster, while the outer parts are dominated by a dispersed population. Therefore, the observed trend could reflect a change of star formation efficiency between clustered and non-clustered star formation.

Currrent Teaching

  • Present 1995

    Preclinical Endodnotics

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  • Present 2003

    SELC 8160 Molar Endodontic Selective

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  • Present 2010

    Endodontics Postdoctoral AEGD Program

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Teaching History

  • 1997 1995

    Preclinical Endodnotics

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  • 2005 2003

    SELC 8160 Molar Endodontic Selective

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  • 2011 2010

    Endodontics Postdoctoral AEGD Program

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  • 2011 2010

    Endodontics Postdoctoral AEGD Program

    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed ultrices ac elit sit amet porttitor. Suspendisse congue, erat vulputate pharetra mollis, est eros fermentum nibh, vitae rhoncus est arcu vitae elit.

  • 2011 2010

    Endodontics Postdoctoral AEGD Program

    Lorem ipsum dolor sit amet, consectetur adipiscing elit. Sed ultrices ac elit sit amet porttitor. Suspendisse congue, erat vulputate pharetra mollis, est eros fermentum nibh, vitae rhoncus est arcu vitae elit.

At My Office

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At My Work

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At My Lab

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