|
Using
its near-infrared vision to peer 9 billion years back in time, NASA’s
Hubble Space Telescope has uncovered an extraordinary population of
young dwarf galaxies brimming with star formation. While dwarf galaxies
are the most common type of galaxy in the universe, the rapid star-birth
observed in these newly found examples may force astronomers to
reassess their understanding of the ways in which galaxies form.
The
galaxies are a hundred times less massive, on average, than the Milky
Way, yet churn out stars at such a furious pace that their stellar
content would double in just 10 million years. By comparison, the Milky
Way would take a thousand times longer to double its star population.
The
universe is estimated to be 13.7 billion years old, and these newly
discovered galaxies are extreme even for the young universe—when most
galaxies were forming stars at higher rates than they are today.
Astronomers using Hubble’s instruments could spot the galaxies because
the radiation from young, hot stars has caused the oxygen in the gas
surrounding them to light up like a bright neon sign.
“The
galaxies have been there all along, but up until recently astronomers
have been able only to survey tiny patches of sky at the sensitivities
necessary to detect them,” said Arjen van der Wel of the Max Planck
Institute for Astronomy in Heidelberg, Germany, lead author of a paper
on the results being published online on Nov. 14 in The Astrophysical
Journal. “We weren’t looking specifically for these galaxies, but they
stood out because of their unusual colors.”
The
observations were part of the Cosmic Assembly Near-infrared Deep
Extragalactic Legacy Survey (CANDELS), an ambitious three-year study to
analyze the most distant galaxies in the universe. CANDELS is the first
census of dwarf galaxies at such an early epoch
.
“In
addition to the images, Hubble has captured spectra that show us the
oxygen in a handful of galaxies and confirmed their extreme star-forming
nature,” said co-author Amber Straughn at NASA’s Goddard Space Flight
Center in Greenbelt, Md. “Spectra are like fingerprints. They tell us
the galaxies’ chemical composition.”
The
resulting observations are somewhat at odds with recent detailed
studies of the dwarf galaxies that are orbiting as satellites of the
Milky Way.
“Those
studies suggest that star formation was a relatively slow process,
stretching out over billions of years,” explained Harry Ferguson of the
Space Telescope Science Institute (STScI) in Baltimore, Md., co-leader
of the CANDELS survey. “The CANDELS finding that there were galaxies of
roughly the same size forming stars at very rapid rates at early times
is forcing us to re-examine what we thought we knew about dwarf galaxy
evolution.”
The
CANDELS team uncovered the 69 young dwarf galaxies in near-infrared
images taken with Hubble’s Wide Field Camera 3 and Advanced Camera for
Surveys.
The
observations suggest that the newly discovered galaxies were very
common 9 billion years ago. However, it is a mystery why the newly found
dwarf galaxies were making batches of stars at such a high rate.
Computer simulations show star formation in small galaxies may be
episodic. Gas cools and collapses to form stars. The stars then reheat
the gas and blow it away, as in supernova explosions. After some time,
the gas cools and collapses again, producing a new burst of star
formation, continuing the cycle.
“While
these theoretical predictions may provide hints to explain the star
formation in these newly discovered galaxies, the observed bursts are
much more intense than what the simulations can reproduce,” van der Wel
said.
The
James Webb Space Telescope, an infrared observatory scheduled to launch
later this decade, will be able to probe these faint galaxies at an
even earlier era to see the glow of their stars, reveal their chemical
composition, and offer better details on their formation.
The
Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. Goddard manages the
telescope. STScI conducts Hubble science operations and is operated for
NASA by the Association of Universities for Research in Astronomy Inc.
in Washington.
