M81 (NGC 3031) as seen in the infrared

Research Interests

Although I have collaborated on a broad range of projects ranging from the Solar System to the high redshift universe, my research has mainly focused on studying interstellar dust and star formation in nearby galaxies. I have divided my research into a few key areas, which are discussed below.

Far-infrared and submillimeter dust emission

Throughout my career, I have worked on measuring the far-infrared and submillimeter spectral energy distributions (the power emitted as a function of wavelength) of dust in nearby galaxies. However, the emphasis of my work has changed over time.

My more recent work with the Herschel Space Observatory involved identifying which stars are heating the interstellar dust by comparing the infrared colors to the light from the old and young stars within the galaxies. Using this novel approach, I was able to demonstrate that Herschel was able to detect a cooler dust component heated by the evolved stellar population that is difficult to detect by other infrared telescopes.

In my earlier work with the Infrared Space Observatory, the Spitzer Space Telescope, and the James Clerk Maxwell Telescope, I focused on searching for "submillimeter excess emission", anomalous emission at >500 microns that was higher than what was expected from any dust emission models. I found this emission in the spiral galaxy NGC 4631, which has been one of the very few spiral galaxies where this emission has been detected. I also identified unusually strong submillimeter emission from the nucleus of the Sombrero Galaxy that has no counterpart at any other wavelength.

Star formation in nearby galaxies

As interstellar dust is primarily found around star forming regions, I often shift my research from studying the dust to studying the star formation.

My most recent work has involved using Atacama Large Millimeter/submillimeter Array (ALMA) to detect millimeter hydrogen line emission (also called recombination line emission) from photoionized gas within nearby starburst galaxies. This has been technically difficult for many telescopes, but ALMA has been able to do this very effectively. In my first results on NGC 253, I argued that the star formation rate measurements from ALMA should be more accurate and more trustworthy than measurements from other millimeter and radio telescopes, and I also demonstrated that the dust obscuration in NGC 253 was much worse than what was inferred from the near-infrared data. I am continuing to study this topic using ALMA observations of other nearby galaxies.

Earlier in my career, I used dust emission to illustrate some variations in star formation among spiral galaxies. I found that galaxies with smaller bulges have higher specific star formation rates (ratios of young stellar populations to total stellar populations) than galaxies with larger bulges and that the star forming regions were distributed more broadly and more asymmetrically throughout the galaxy's discs in galaxies with smaller bulges. I also did some limited spectroscopic analyses of galaxies and demonstrated using near-infrared spectra that star formation in the nuclei of nearby galaxies is episodic.

Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are large molecules containing multiple hexagonal carbon rings. They are found mixed with dust in the interstellar medium, and they emit strongly in the mid-infrared between 6 and 13 microns. While many astronomers have suggested that the PAHs are linked with star formation, my research has demonstrated that the PAH emission does not trace star formation well but that it does appear strongly associated with the larger dust grains emitting at far-infrared wavelengths. My work on understanding how the PAHs are associated with the dust and how the PAHs are excited is ongoing.

Infrared and submillimeter data processing

I take a hands-on approach to data processing, and I often prefer to process images myself rather than use other people's data. I have always held positions where one of my principle roles has been infrared and submillimeter data processing, and so I have either acquired or developed a lot of data processing software and also developed a lot of experience and skills in working with data. This data processing work in itself has led to a couple of scientific publications, including a paper on Spitzer far-infrared data I processed to supplement data from the Herschel Space Telescope for several nearby galaxy surveys and the paper describing the derivation of the flux calibration for the SPIRE photometer on Herschel.

Dynamics from galaxy merger simulations

As a master's degree project very early on in my career, I worked with Joshua Barnes on analyzing the motions of stars in simulations of galaxy mergers. The analysis demonstrated that it was possible to produce several dynamical effects that astronomers had observed in other galaxies, including counterrotation (where some stars orbit in a direction opposite to most other stars within a galaxy) and orthogonal rotation (where stars in a galaxy orbit around the longest axis of the galaxy). The cumulative results of this work became my first science publication. While I am not actively pursuing this work at this time, my work with recombination line emission from ALMA has touched on these topics, so I may find myself revisiting the subject in the future.

Additional Information

Additional information on my professional work is available from the following pages: