J.D. Smith on the Evolution of Galaxies Through Cosmic Time
Fast Moving Front Commentary, July 2010
Article: The mid-infrared spectrum of star-forming galaxies: Global properties of polycyclic aromatic hydrocarbon emission
Authors: Smith, JDT;Draine, BT;Dale,
DA;Moustakas, J;Kennicutt, RC;Helou, G;Armus, L;Roussel,
H;Sheth, K;Bendo, GJ;Buckalew, BA;Calzetti, D;Engelbracht,
CW;Gordon, KD;Hollenbach, DJ;Li, A;Malhotra, S;Murphy,
J.D. Smith talks with ScienceWatch.com and answers a few questions about this month's Fast Moving Fronts paper in the field of Space Science.
Why do you think your paper is highly cited?
This paper calibrates the form and behavior of PAH emission in a local, broadly selected sample of galaxies, and provides tools for making use of these powerful vibrational emission bands to study the dust content, heavy element abundance, star formation, and nuclear activity from accretion onto supermassive black holes in galaxies.
These results have been widely applied to a range of galaxy samples from the earliest epochs of the universe to recent times.
Does it describe a new discovery, methodology, or synthesis of knowledge?
Though the presence of PAHs in galaxies, including our own, was hypothesized over 30 years ago, and has enjoyed a surge of recent interest thanks to the capabilities of the Spitzer Space Telescope, utilizing these unusual emission bands as tools for studying galaxies has been hampered by the lack of a model to extract the information they contain.
We have developed a new, simple, physical model describing PAH emission, which makes it possible to use the PAH bands as primary galaxy diagnostics.
Would you summarize the significance of your paper in layman’s terms?
"As future ground and space long wavelength observatories greatly improve our ability to track the form and behavior of this "missing half" of the energy output of galaxies..."
Galaxies emit radiation "directly," in the form of the aggregate starlight from their billions of constituent stars, as well as lines of emission from their cool and hot gas. They also emit "indirectly"—dust in galaxies absorbs starlight, heats up, and "reprocesses" this energy to long wavelengths, in the infrared part of the spectrum.
A unique type of dust grain consisting of small rings of hydrocarbons is found almost universally in galaxies. Polycyclic Aromatic Hydrocarbons, or PAHs for short, emit strongly in the mid-infrared and are responsible for up to 10% of the energy output of a galaxy.
This paper provides a missing "road map" for the use of these PAH emission bands to study galaxies and galaxy evolution.
How did you become involved in this research, and how would you describe the particular challenges, setbacks, and successes that you've encountered along the way?
In graduate school, I became involved with a space satellite mission which would, after its launch in 2003, be named the Spitzer Space Telescope. After graduate school, I joined the Spitzer Infrared Nearby Galaxies Survey (SINGS), and became fascinated by the untapped potential of PAH emission in galaxies, both as a key to understanding the life cycle of dust in galaxies, as well as a tool for following the processes which shape the changing form and content of galaxies throughout the full history of the universe.
The PAH bands, though bright, are very challenging to exploit, since they are profoundly blended with each other and with other emission components of galaxy emission. Our success came primarily in developing a simple but widely applicable model for PAH emission, and calibrating the behavior of the PAH emission bands in a robust local sample.
Where do you see your research leading in the future?
As future ground and space long wavelength observatories greatly improve our ability to track the form and behavior of this "missing half" of the energy output of galaxies—in particular at the earliest epochs of formation in the universe—we will gain powerful new tools in our arsenal for studying the physical contents and remarkable evolution of galaxies through cosmic time.
Do you foresee any social or political implications for your research?
Although distant galaxies, far too remote to ever be visited or contacted by humankind, may seem entirely disconnected from any more terrestrial concerns, knowledge of the origins of the universe and the galaxies it contains can have a real impact, in particular in capturing the imaginations of future scientists, and putting into a broader context the challenges and opportunities we face here on planet Earth.
Assistant Professor of Physics & Astronomy
University of Toledo
Toledo, OH, USA
KEYWORDS: GALAXIES, ACTIVE GALAXIES, ISM, INFRARED, TECHNIQUES, SPECTROSCOPIC, SPITZER SPACE TELESCOPE, DIFFUSE INTERSTELLAR MEDIUM, ACTIVE GALACTIC NUCLEI, DEEP FIELD SOUTH, NEARBY GALAXIES, MU-M, SILICATE EMISSION, PAH EMISSION.