Kevin Schawinski on Observational Evidence for AGN Feedback in Early-Type Galaxies
Fast Moving Front Commentary, November 2010
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Article: Observational evidence for AGN feedback in early-type galaxies
Authors: Schawinski, K;Thomas, D;Sarzi,
M;Maraston, C;Kaviraj, S;Joo, SJ;Yi, SK;Silk, J |
Kevin Schawinski 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?
I think people are interested in two aspects:
The science—we show how early-type galaxies shut down their star formation and feed their black holes on an empirical, quantitative timeline.
The methods—we develop a new way to quantitatively age-date galaxies and use visual inspection of galaxy images to find and understand "unusual" galaxies—blue early-type galaxies with both star formation and nuclear activity—that turned out to be a key stage in galaxy formation.
Does it describe a new discovery, methodology, or synthesis of knowledge?
The paper has aspects of all three: first, we systematically described the population of low-mass blue early-type galaxies with ongoing star formation, black hole growth, or both, that previously had only been reported as "odd" objects. We discovered not just that there were far more of them than previously thought, but that their type of activity followed a very clear sequence from star formation via a black hole growth phase to quiescence.
Second, we had to further develop two tools, visual inspection and recovering of star formation histories from photometry and spectroscopy, to accurately age-date each galaxy and therefore put it on a timeline. The visual inspection part directly led to the Galaxy Zoo project, while the star formation history recovery introduced a new way of combining broad-band photometric imaging data with the stellar absorption lines in the spectra of galaxies.
"We realized early on in the analysis that led to this paper that visual classification of galaxy images, as opposed to automated classification algorithms, could lead to exciting new insights."
Finally, we used the age-dating to present a quantitative scenario of how these low-mass early-type galaxies shut down their star formation, feed their black holes and become more like their passive counterparts. Some versions of this scenario had been predicted by simulations of galaxy formation; what we show is that 1) this kind of process happens and 2) some of the timescales involved, especially the long delay between star formation and the peak of black hole accretion, are not well reproduced in those simulations.
Would you summarize the significance of your paper in layman's terms?
The paper shows in detail how low-mass galaxies undergo a major burst of star formation (most likely triggered by the merger of two galaxies) followed by a very rapid shutdown of star formation. During this process, the central supermassive black hole begins feeding with a peak of highly efficient growth some 500 million to 1 billion years after the burst of star formation. The remnant galaxy no longer forms stars and becomes quiescent, joining the majority of "red and dead" early-type galaxies.
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?
The two hardest parts were definitely classifying nearly 50,000 galaxies by eye in a single week, and developing and refining a new method for recovering the recent star formation histories of galaxies using both photometric and spectroscopic information.
Where do you see your research leading in the future?
Our results show how low mass early-type galaxies are transformed in the present-day universe. However, we know that the formation epoch for the most massive early-type galaxies—those containing the bulk of the stars in the universe—lies very far back in the early universe. This early childhood era of our universe is very difficult to observe since more distant galaxies are fainter and require bigger telescopes.
Also cosmological redshift moves all the crucial spectral features into the infrared which is much more challenging to work in because the sky glows in the infrared and many parts of the spectrum are simply blocked. Current instruments are just not sufficiently sensitive to make similar observations but the next generation reaching 8-10m class telescopes in the very near future might take us there. It's an exciting time for observational astrophysics!
Do you foresee any social or political implications for your research?
We realized early on in the analysis that led to this paper that visual classification of galaxy images, as opposed to automated classification algorithms, could lead to exciting new insights. It was also clear that classifying all ~1 million galaxies in the Sloan Digital Sky Survey was not feasible, so we turned to help from the general public; thus, this paper was the seed for what eventually became the Galaxy Zoo project (galaxyzoo.org) which has by now engaged over 320,000 members of the public as active astrophysics researchers.
Many of these citizen scientists have found (or re-found) their interest in
science and astronomy in particular though this project, and a whole slew
of successor citizen science projects is now in the works addressing
questions from astrophysics to climate change.
Kevin Schawinski
Einstein Fellow
Yale University
New Haven, CT, USA
KEYWORDS: GALAXIES; ACTIVE; GALAXIES; ELLIPTICAL AND LENTICULAR, CD; EVOLUTION; FORMATION; DIGITAL SKY SURVEY; ACTIVE GALACTIC NUCLEI; COLOR-MAGNITUDE RELATION; OLD STELLAR POPULATIONS; STAR-FORMING GALAXIES; ULTRALUMINOUS INFRARED GALAXIES; DEEP CHANDRA OBSERVATION; ABSORPTION-LINE SPECTRA; SMALL MAGELLANIC CLOUD; ELLIPTIC GALAXIES.