A galaxy's gothic tale
Posted: Tuesday, October 16, 2007 8:50 PM by Alan Boyle
Oscar Wilde’s only published novel, "The Picture of Dorian Gray," is a gothic tale about a man who stays young while his portrait ages. Now scientists have found a Wildean plot twist in the Hubble Space Telescope's image of the galaxy I Zwicky 18.
The galaxy looks like a youngster, but when astronomers checked its cosmic clocks, they found that it was much older and farther away than they originally thought. As a result, they've dubbed it the "Dorian Gray" galaxy. Fortunately, the galaxy's portrait ends up much nicer-looking than the character in Wilde’s book.
The galaxy's youthful appearance was first noticed four decades ago as the result of observations at the Palomar Observatory. Astronomers thought that I Zwicky 18 (named after discoverer Fritz Zwicky, who's famous for his role in the dark-matter mystery) looked a lot like the dwarf irregular galaxies they saw way back in the early universe. That was seen as a good thing, because they hoped little Zwicky could thus serve as a close-up, easy-to-study example of a baby galaxy.
A. Aloisi / NASA / ESA / STScI |
Astronomers pored over this imagery of the
galaxy I Zwicky 18, focusing on three Cepheid
variable stars highlighted by red circles. Click on
the image for a larger version.
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The new findings from a team led by Alessandra Aloisi of the Space Telescope Science Institute and the European Space Agency, published in the Oct. 1 issue of The Astrophysical Journal Letters, spoiled that fantasy.
The astronomers focused on a standard type of star known as a Cepheid variable, which can be analyzed to yield information about distances and the composition of the stars. In the past, Cepheid variables have been used to calculate the universe's age and rate of expansion.
The astronomers looked at three Cepheid variables in I Zwicky 18 - and determined that the galaxy didn't just start forming stars in the past 500 million years or so, but has been pumping out stars for at least a billion years and perhaps for as long as 10 billion years. They also reported that the galaxy was 59 million light-years away, almost 10 million light-years farther than previously thought.
The Cepheid variable data were confirmed by measuring the observed brightness of the brightest red stars older than 1 billion years. Based on their observations, Aloisi and her colleagues concluded that I Zwicky 18 wasn't a youngster at all, but formed at about the same time as most other galaxies.
This galaxy is still an odd sort, just like Dorian Gray: It has formed far fewer stars than your typical galaxy, and its stellar production rate is still below par. Astronomers don't know why that is - leaving a big mystery yet to be solved in a later chapter.
"Although the galaxy is not as youthful as was once believed, it is certainly developmentally challenged and unique in the nearby universe," Aloisi said in a news release from the Space Telescope Science Institute. So you can expect astronomers to be pondering pictures of the Dorian Gray galaxy for years to come.
Other members of Aloisi's team include Francesca Annibali, Jennifer Mack, and Roeland van der Marel of the Space Telescope Science Institute; Marco Sirianni of the Space Telescope Science Institute and the European Space Agency; Abhijit Saha of the National Optical Astronomy Observatories; and Gisella Clementini, Rodrigo Contreras, Giuliana Fiorentino, Marcella Marconi, Ilaria Musella, and Monica Tosi of the Italian National Astrophysics Institutes in Bologna and Naples. For still more information, visit the European Space Agency's Hubble Web site or the Italian National Astrophysics Institute.
