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Release No. STScI-PRC08-38
Hubble Resolves Puzzle About Loner Starburst Galaxy
NASA, ESA, the Hubble Heritage Team (STScI/AURA), and A. Aloisi (STScI/ESA)
Astronomers have long puzzled over why a small, nearby, isolated galaxy is pumping out
new stars faster than any galaxy in our local neighborhood.
Now NASA's Hubble Space Telescope has helped astronomers solve the mystery of the
loner starburst galaxy, called NGC 1569, by showing that it is one and a half times farther
away than astronomers thought.
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.
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
"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. Put the puzzle pieces in the right place, as for NGC
1569, and the picture makes much more sense."
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 lead 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.
The Hubble study observed both the galaxy's cluttered core and its sparsely populated outer
fringes. The sharpness of Hubble's Advanced Camera pinpointed individual red giants,
which led to a precise distance to the galaxy. Astronomers measured the galaxy's distance
at nearly 11 million light-years away, about 4 million light-years farther than the old distance.
"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.
The science team for the NGC 1569 observations consists of Alessandra Aloisi and Marco Sirianni
(STScI/ESA), Aaron Grocholski, Jennifer Mack, and Roeland van der Marel (STScI), Luca Angeretti,
Donatella Romano, and Monica Tosi (INAF-OAB), and Francesca Annibali, Laura Greggio, and Enrico
The Hubble Space Telescope is a project of international cooperation between NASA and
the European Space Agency (ESA) and is managed by NASA's Goddard Space Flight
Center (GSFC) in Greenbelt, Md. The Space Telescope Science Institute (STScI) conducts
Hubble science operations. The institute is operated for NASA by the Association of
Universities for Research in Astronomy, Inc., Washington, D.C.
STScI is an International Year of Astronomy 2009 (IYA 2009) program partner.