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A galaxy far, farther away

A. Aloisi / STScI / ESA / NASA / AURA
The galaxy NGC 1569 sparkles with the light from millions of newborn stars in an
image from the Hubble Space Telescope. Click on the image for larger versions.

Long ago, astronomers spotted a galaxy far away and wondered why it was giving birth to so many stars. Using the Hubble Space Telescope, they have finally figured out the answer to the puzzle: The starburst galaxy turns out to be farther away than they thought.

Rather than being all by its lonesome, just 7 million light-years away, the starburst galaxy NGC 1569 is stuck in the middle of crowded galactic cluster nearly 11 million light-years away. The resulting gravitational interactions are probably squeezing the galaxy's gas so much that it's been forming stars at a rate more than 100 times faster than our own Milky Way ... for the past 100 million years or so.

"This was the strongest starburst galaxy in the nearby universe," Alessandra Aloisi, an astronomer at the Baltimore-based Space Telescope Science Institute and the European Space Agency, told me today. "It was really puzzling why it was forming stars at such a high rate. It wasn't fitting in with current theories."

Aloisi and some of her colleagues have been studying NGC 1569 for about 20 years, and they enlisted Hubble's cameras to help crack the case. For the key phase of the study, they were allotted 19 full orbits of Hubble's time, which translated into almost 19 hours' worth of observations. "I can tell you it's very hard to get such a big proposal approved," Aloisi said.

At first, the astronomers tried looking for a special kind of red giant star that fuses helium in its core to produce power. These stars are relatively dim, but if you can spot them, you can use them to estimate a galaxy's age.

"When we found no obvious trace of them, we suspected that the galaxy was farther away than originally believed," Aaron Grocholski, a colleague of Aloisi's at the institute and the lead author of a paper on the galaxy, said today in a news release. "We could only see the brightest red giant stars, but we were able to use these stars to recalibrate the galaxy's distance."

Such stars can serve as "standard candles" to correlate brightness with distance. But it takes a lot of telescope time to get the precise measurements that are required. Ground-based telescopes produced the earlier, closer estimates for the galaxy's distance - but only Hubble was able to resolve the individual red giant stars in NGC 1569 and determine that the galaxy was much more distant.

"Nobody before had thought that galaxy was much farther away," Aloisi said.

She and her colleagues had wondered what sort of mechanism could produce such a high rate of starbirth in an isolated galaxy. But the new distance estimate put the galaxy in close proximity to about 10 other galaxies, and it is well-known that the gravitational interactions between galaxies could be a powerful engine for star formation.

 "Now that we know the galaxy is in a cluster, that makes much more sense," Aloisi said.

The astronomers' observations were made in 1999 using Hubble's Wide Field and Planetary Camera 2, and in 2006 and 2007 with the Advanced Camera for Surveys. Results were published in the Oct. 20 issue of the Astrophysical Journal Letters. In addition to Aloisi and Grocholski, the co-authors included Marco Sirianni of ESA and the Space Telescope Science Institute, or STScI; Jennifer Mack and Roeland van der Marel of STScI; Luca Angeretti, Donatella Romano and Monica Tosi of Italy's Astronomical Observatory of Bologna; and Francesca Annibali, Laura Greggio and Enrico Held of the Astronomical Observatory of Padua.

Correction for 11:19 p.m. ET: Ugh, I typed "billion" instead of "million" in the original version of this item. The galaxy isn't that far away. The fix has been made, so sorry about this stupid kind of mistake (which has happened before). Thanks to all who pointed out the error of my ways.

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