NASA's Hubble Space Telescope has
helped astronomers solve a longstanding mystery as to why a small, nearby,
isolated galaxy is pumping out new stars faster than any galaxy in our local
neighborhood.
The loner
starburst galaxy, called NGC 1569, is 1.5 times farther away than
astronomers thought, a new study found.
The extra distance places the galaxy
in the middle of a group of about 10 galaxies centered on the spiral galaxy IC
342. Gravitational interactions among the group's galaxies may be compressing
gas in NGC
1569 and igniting the star-birthing frenzy.
"Now the starburst activity
seen in NGC 1569 makes sense, because the galaxy is probably interacting with
other galaxies in the group," said the study's leader, Alessandra Aloisi of the Space Telescope Science Institute in Baltimore, Md.,
and the European Space Agency. "Those interactions are probably fueling
the star birth."
The farther distance not only means
that the galaxy is intrinsically brighter, but also that it is producing stars
two times faster than first thought. The galaxy is forming stars at a rate more
than 100 times higher than in the Milky Way. This high
star-formation rate has been almost continuous for the past 100 million
years.
Puzzle pieces in place
Discovered by William Herschel in
1788, NGC 1569 is home to three of the most massive star clusters ever
discovered in the local universe. Each cluster contains more than a million
stars.
"This is a prime example of the
type of massive starbursts that drive the evolution of galaxies in the distant
and young universe," said team member Roeland
van der Marel of the Space
Telescope Science Institute. "Starburst galaxies can only be studied in
detail in the nearby universe, where they are much rarer."
"Hubble observations of our
galactic neighborhood, including this study, are helping astronomers put
together a complete picture of the galaxies in our
local universe," van der Marel
added. "Put the puzzle pieces in the right place, as for NGC 1569, and the
picture makes much more sense."
Serendipitous discovery
Aloisi and her team actually discovered
the new distance by accident. They were using Hubble's Advanced Camera for
Surveys to hunt in NGC 1569 for the kind of red
giant stars (stars near the ends of their lives) that shine because of
fusion of helium nuclei in their cores. These stars are dimmer than bright red
giants without helium burning, but when detected, they can be used to estimate
the galaxy's age.
"When we found no obvious trace
of them, we suspected that the galaxy was farther away than originally
believed," said Aaron Grocholski of the Space
Telescope Science Institute and the main author on a paper describing the
results. "We could only see the brightest red giant stars, but we were
able to use these stars to recalibrate the galaxy's distance." Bright red
giants are reliable "standard candles" for measuring distance because
they all shine at the same brightness. Once astronomers know a star's true brightness,
they can calculate its distance from earth.
Previous estimates of the galaxy's
distance made with ground-based telescopes were unreliable because they looked
at the galaxy's crowded core and were unable to resolve individual red giant
stars.
"This was a serendipitous
discovery," Aloisi said. "Hubble didn't go
deep enough to see the faintest red giant stars we were hunting for because the
galaxy is farther away than we thought. However, by capturing the entire
population of the brightest red giant stars, we were able to calculate a
precise distance to NGC 1569 and resolve the puzzle about the galaxy's extreme
starburst activity."
The results were published in the
Oct. 20 issue of the Astrophysical Journal Letters.
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