Astronomers
peering out into our cosmic backyard have long understood that the Milky Way's
galactic neighbors only seem similar on the surface. Now a detailed survey from
NASA's Hubble Space Telescope has revealed the diversity of those galaxies as
they evolve over time.
The
ACS Nearby Galaxy Survey Treasury (ANGST) program zeroed in on 14 million stars
in 69 nearby galaxies. Such galaxies sit close enough so that Hubble's sharp
eyes could single out the brightest stars instead of seeing a giant
smear of light, and may help settle raging debates over how galaxies and
their stars form in the first place.
"Instead
of picking and choosing particular galaxies to study, our survey will be
complete by virtue of looking at 'all' the galaxies in the region," said
Julianne Dalcanton, head of the ANGST survey at the University of Washington in
Seattle. "This gives us a multi-color picture of when and where all the
stars in the local universe formed."
A
galactic fossil record
The
survey covered galaxies ranging 6.5 million light-years to 13 million light-years
from Earth. A light-year is the distance light will travel in a year, or about
6 trillion miles (10 trillion kilometers).
Many
nearby galaxies contain stars that are positive relics compared with the younger
stars in more distant galaxies. Astronomers can guess how those younger
stars may evolve based on the older nearby stars.
"Using
the galaxies in the nearby universe as a 'fossil record,' we can compare them
with young galaxies far away," Dalcanton said. "This comparison gives
us a history of star formation and provides a better understanding of the
masses, structures and environments of the galaxies."
The
formation of galaxies remains poorly understood, although evidence is mounting
that massive galaxies grow by clumping together and merging with smaller
brethren. This step-by-step evolution of gradually larger galaxies competes
with the theory that galaxies simply start from scratch.
However,
some observations suggest that galaxies do have the power to expand rapidly.
Astronomers found a star factory 12.3 billion light-years away that churns out 4,000
stars per year, compared with our Milky Way's 10 stars annually. That burst of
star-making activity suggests such a galaxy would only need 50 million years to
grow into one of the largest ever observed.
Stellar
evolution
Many
such large spiral galaxies created most of their stars early on, according to a
separate new study. Also relying on data from the Hubble galactic survey, the
astronomers examined the outer disk of M81 and found that most stars formed
more than 7 billion years ago, when the universe was half its current age. The
supernova deaths of massive stars within such mammoth galaxies rapidly enriched
them with heavy elements such as carbon.
"We
were surprised by how quickly the elements formed and how the subsequent
star-formation rate for the bulk of the stars in M81 changed after that,"
said Benjamin Williams, an astronomer at the University of Washington in
Seattle.
Astronomers
had believed that star populations become increasingly younger further out
within galaxies, but Hubble data indicated that older stars can also lurk in a
galaxy's outer arms. Recent simulation
work has presented another possible explanation — stars may wander
all over spiral galaxies. The survey should provide further data to flesh
out current theories.
"With
this information, we will be able to trace the complete cycle of star formation
in detail," Dalcanton said.
The
galactic survey results were submitted for publication in The Astrophysical
Journal Supplement Series. The research on M81's star-formation has been
submitted to The Astronomical Journal.
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