A team of astronomers using the National Science
Foundation’s Robert C. Byrd Green Bank Telescope (GBT) has made the
first conclusive detection of what appear to be the leftover building
blocks of galaxy formation — neutral hydrogen clouds — swarming around
the Andromeda Galaxy, the nearest large spiral galaxy to the Milky Way.

This discovery may help scientists understand the structure and
evolution of the Milky Way and all spiral galaxies. It also may help
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.”

Thilker’s research is published in the Astrophysical Journal Letters.
Other contributors include: Robert Braun of the Netherlands Foundation
for Research in Astronomy; Rene A.M. Walterbos of New Mexico State
University; Edvige Corbelli of the Osservatorio Astrofisico di Arcetri
in Italy; Felix J. Lockman and Ronald Maddalena of the National Radio
Astronomy Observatory in Green Bank, West Virginia; and Edward Murphy of
the University of Virginia.

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.

Previous 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 indeed galactic building blocks.

Astronomers are able to use radio telescopes to detect the
characteristic 21-centimeter radiation emitted naturally by neutral
atomic hydrogen. The great difficulty in analyzing these low-mass
galactic building blocks has been 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. “We know
the Milky Way HVCs are relatively nearby, but precisely how close is
maddeningly tough to determine,” said Thilker.

Past attempts to find missing satellites around external galaxies at
well-known distances have been unsuccessful because of the need for a
very sensitive instrument capable of producing high-fidelity images,
even in the vicinity of a bright source such as the Andromeda Galaxy.

One might consider this 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.

The Andromeda Galaxy was targeted because it is the nearest massive
spiral galaxy. “In some sense, the rich get richer, even in space,”
said Thilker. “All else being equal, one would expect to find more
primordial clouds in the vicinity of a large spiral galaxy than near a
small dwarf galaxy, for instance. This makes Andromeda a good place to
look, especially considering its relative proximity — a mere 2.5
million light-years from Earth.”

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

“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.”

The GBT is the world’s largest fully steerable radio telescope.

The NRAO is a facility of the National Science Foundation, operated
under a cooperative agreement with Associated Universities, Inc.



Additional Contact Information:
David Thilker
(410) 516-3861



This image depicts several long-sought galactic “building blocks” in
orbit of the Andromeda Galaxy (M31), discovered by astronomers using
the Robert C. Byrd Green Bank Telescope (GBT). The newfound hydrogen
clouds are depicted in a shade of orange, while gas that comprises the
massive hydrogen disk of Andromeda is shown at high-resolution in blue.
The “building block” clouds were found as a result of a dedicated search
conducted on the GBT, in which the GBT scanned the vicinity of M31 for
over a day and a half. The image of M31’s much more prominent hydrogen
disk was created using the Westerbork Synthesis Radio Telescope in the
Netherlands taking about two weeks observing time. (credit: R. Braun, E.
Corbelli, R.A.M. Walterbos, D. Thilker)