This radio image shows the leftover building blocks of galaxy formation -- neutral hydrogen clouds -- swarming around the Andromeda Galaxy,
the nearest large spiral galaxy to the Milky Way.nThis discovery helped
scientists understand the structure and evolution of the Milky Way and
all spiral galaxies and helps explain why certain young stars in mature
galaxies are surprisingly bereft of the heavy elements that their
contemporaries contain.
"Giant galaxies, like Andromeda and our own Milky Way, are thought to
form through repeated mergers with smaller galaxies and through the
accretion of vast numbers of even lower mass 'clouds' -- dark objects
that lack stars and even are too small to call galaxies," said David A.
Thilker of the Johns Hopkins University in Baltimore, Maryland.
"Theoretical studies predict that this process of galactic growth
continues today, but astronomers have been unable to detect the expected
low mass 'building blocks' falling into nearby galaxies, until now."
The Milky Way and Andromeda were formed many billions of years ago in
a cosmic neighborhood brimming with galactic raw materials -- among
which hydrogen, helium, and cold dark matter were primary constituents.
By now, most of this raw material has probably been gobbled up by the
two galaxies, but astronomers suspect that some primitive clouds are
still floating free.
Early studies have revealed a number of clouds of neutral atomic
hydrogen that are near the Milky Way but not part of its disk. These
were initially referred to as high-velocity clouds
(HVCs) when they were first discovered because they appeared to move at
velocities difficult to reconcile with Galactic rotation.
Scientists were uncertain if HVCs comprised building blocks of the
Milky Way that had so far escaped capture, or if they traced gas
accelerated to unexpected velocities by energetic processes (multiple
supernovae) within the Milky Way. The discovery of similar clouds bound
to the Andromeda Galaxy strengthens the case that at least some of these
HVCs are galactic building blocks.
Astronomers used radio telescopes to detect the characteristic 21-centimeter radiation
emitted naturally by neutral atomic hydrogen. The great early
difficulty in analyzing these low-mass galactic building blocks was that
their natural radio emission is extremely faint. Even those nearest to
us, clouds orbiting our Galaxy, are hard to study because of serious
distance uncertainties. A task similar to visually distinguishing a
candle placed adjacent to a spotlight. The novel design of the recently
commissioned GBT met these challenges brilliantly, and gave astronomers their first look at the cluttered neighborhood around Andromeda.
What the GBT was able to pin down was a population of 20 discrete
neutral hydrogen clouds, together with an extended filamentary
component, which, the astronomers believe, are both associated with
Andromeda. These objects, seemingly under the gravitational influence of
Andromeda's halo, are thought to be the gaseous clouds of the "missing"
(perhaps dark-matter dominated) satellites and their merger remnants.
They were found within 163,000 light-years of Andromeda.
Favored cosmological models have predicted the existence of these
satellites, and their discovery could account for some of the missing
"cold dark matter" in the Universe. Also, confirmation that these
low-mass objects are ubiquitous around larger galaxies could help solve
the mystery of why certain young stars, known as G-dwarf stars, are
chemically similar to ones that evolved billions of years ago. As
galaxies age, they develop greater concentrations of heavy elements
formed by the nuclear reactions in the cores of stars and in the
cataclysmic explosions of supernovae. These explosions spew heavy
elements out into the galaxy, which then become planets and get taken up
in the next generation of stars.
Spectral and photometric analysis of young stars in the Milky Way and
other galaxies, however, show that there are a certain number of young
stars that are surprisingly bereft of heavy elements, making them
resemble stars that should have formed in the early stages of galactic evolution.
"One way to account for this strange anomaly is to have a fresh
source of raw galactic material from which to form new stars," said
Murphy. "Since high-velocity clouds may be the leftover building blocks
of galaxy formation, they contain nearly pristine concentrations of
hydrogen, mostly free from the heavy metals that seed older galaxies."
Their merger into large galaxies, therefore, could explain how fresh
material is available for the formation of G-dwarf stars.
The Andromeda Galaxy, also known as M31, is one of only a few
galaxies that are visible from Earth with the unaided eye, and is seen
as a faint smudge in the constellation Andromeda. When viewed through a
modest telescope, Andromeda also reveals that it has two prominent
satellite dwarf galaxies, known as M32 and M110. These dwarfs, along
with the clouds studied by Thilker and collaborators, are doomed to
eventually merge with Andromeda. The Milky Way, M33, and the Andromeda
Galaxy plus about 40 dwarf companions, comprise what is known as the "Local Group."
Today, Andromeda is perhaps the most studied galaxy other than the
Milky Way. In fact, many of the things we know about the nature of
galaxies like the Milky Way were learned by studying Andromeda, since
the overall features of our own galaxy are disguised by our internal
vantage point.
"In this case, Andromeda is a good analogue for the Milky Way," said
Murphy. "It clarifies the picture. Living inside the Milky Way is like
trying to determine what your house looks like from the inside, without
stepping outdoors. However, if you look at neighbors' houses, you can
get a feeling for what your own home might look like."
Source: The Daily Galaxy via NRAO
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