Astronomers using the National Science Foundation’s Green Bank Telescope
(GBT) in West Virginia, along with data from other large radio
telescopes, have discovered that our nearest galactic neighbors, the
dwarf spheroidal galaxies, are devoid of star-forming gas, and that our
Milky Way Galaxy is to blame.
These new radio observations,
which are the highest sensitivity of their kind ever undertaken, reveal
that within a well-defined boundary around our Galaxy, dwarf galaxies
are completely devoid of hydrogen gas; beyond this point, dwarf galaxies
are teeming with star-forming material.
The Milky Way Galaxy is
actually the largest member of a compact clutch of galaxies that are
bound together by gravity. Swarming around our home Galaxy is a
menagerie of smaller dwarf galaxies, the smallest of which are the
relatively nearby dwarf spheroidals, which may be the leftover building
blocks of galaxy formation. Further out are a number of similarly sized
and slightly misshaped dwarf irregular galaxies, which are not
gravitationally bound to the Milky Way and may be relative newcomers to
our galactic neighborhood.
“Astronomers wondered if, after
billions of years of interaction, the nearby dwarf spheroidal galaxies
have all the same star-forming ‘stuff’ that we find in more distant
dwarf galaxies,” said astronomer Kristine Spekkens, assistant professor
at the Royal Military College of Canada and lead author on a paper
published in the Astrophysical Journal Letters.
Previous
studies have shown that the more distant dwarf irregular galaxies have
large reservoirs of neutral hydrogen gas, the fuel for star formation.
These past observations, however, were not sensitive enough to rule out
the presence of this gas in the smallest dwarf spheroidal galaxies.
By
bringing to bear the combined power of the GBT (the world’s largest
fully steerable radio telescope) and other giant telescopes from around
the world, Spekkens and her team were able to probe the dwarf galaxies
that have been swarming around the Milky Way for billions of years for
tiny amounts of atomic hydrogen.
“What we found is that there is
a clear break, a point near our home Galaxy where dwarf galaxies are
completely devoid of any traces of neutral atomic hydrogen,” noted
Spekkens. Beyond this point, which extends approximately 1,000
light-years from the edge of the Milky Way’s star-filled disk to a point
that is thought to coincide with the edge of its dark matter
distribution, dwarf spheroidals become vanishingly rare while their
gas-rich, dwarf irregular counterparts flourish.
There are many
ways that larger, mature galaxies can lose their star-forming material,
but this is mostly tied to furious star formation or powerful jets of
material driven by supermassive black holes. The dwarf galaxies that
orbit the Milky Way contain neither of these energetic processes. They
are, however, susceptible to the broader influences of the Milky Way,
which itself resides within an extended, diffuse halo of hot hydrogen
plasma.
The researchers believe that, up to a certain distance
from the galactic disk, this halo is dense enough to affect the
composition of dwarf galaxies. Within this “danger zone,” the pressure
created by the million-mile-per-hour orbital velocities of the dwarf
spheroidals can actually strip away any detectable traces of neutral
hydrogen. The Milky Way thus shuts down star formation in its smallest
neighbors.
"These observations therefore reveal a great deal
about size of the hot halo and about how companions orbit the Milky
Way," concludes Spekkens.
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The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.
Contact:
Charles E. Blue, Public Information Officer
(434) 296-0314
Email: cblue@nrao.edu
