 
High
velocity clouds between galaxies identified as remnants of early
universe and seeds of the Milky Way Galaxy
14
Jan 2000
By
Robert Sanders, Public Affairs
BERKELEY--
Several recent observations support a controversial theory that
high velocity clouds of hydrogen gas seen near our galaxy are
left over from the formation of the Local Group of galaxies,
which includes Andromeda and the Milky Way.
If
these puzzling clouds are indeed remnants of the very early
history of the formation of nearby galaxies, they may represent
the earliest structures that formed in the universe some 15
billion years ago and should contain "dark matter."
Dark matter is the missing mass astronomers know must exist
in galaxies but can't see because it doesn't glow.
"These
clouds are the building blocks of the Milky Way," said
Leo Blitz, professor of astronomy and director of the Radio
Astronomy Laboratory at the University of California, Berkeley.
"They are interesting in and of themselves because they
should be remnants of the very early history of the Local Group.
The clouds tie cosmology and the origins of the universe into
the history of our own local galaxies."
Blitz
details the theory and recent observations that support it in
an invited talk on Jan. 14 at the national meeting of the American
Astronomical Society in Atlanta, Georgia. The talk is at 3:40
p.m. in the Centennial I and II rooms of the Hyatt Regency Atlanta.
To see movies that illustrate the theory, link to the team's
web site at http://www.astro.umd.edu/~teuben/hvc/.
Blitz
and four other astronomers from the United States and Holland
proposed three years ago that these enigmatic clouds are buzzing
about the Local Group, colliding and merging with the galaxies
there.
Discovered
by radio astronomers in 1963, the gas clouds are moving at high
velocity through space in orbits that don't conform to the nicely
circular orbits of most other objects in the disk of the Milky
Way. It became evident in 1972 that they are not simply falling
into the Milky Way, either, since some have high velocities
away from us.
Several
hundred of these clouds have been mapped within the margins
of the Local Group, but their distances have been hard to determine.
Composed of atomic hydrogen, they have the mass of a small galaxy
and are approximately 15,000 parsecs (50,000 light years) across.
According
to the team's scenario, these clouds are what remain of several
thousand such clouds that formed in this region early in the
history of the universe and perhaps were the first large structures
to form. Over the eons these clouds have collided and coalesced
into the galaxies we see today. Andromeda and the Milky Way
are still in the process of forming as they gobble up the remaining
clouds.
Today
the Local Group is a cluster of about 30 galaxies bound together
by their own gravity and scattered throughout a region about
1.5 million parsecs across, or about five million light years.
Nearly 98 percent of the total mass within this group is contained
in the two huge spiral galaxies, Andromeda (M31) and the Milky
Way, which contains the Sun and Earth.
No
theory could explain the diverse observations of high velocity
clouds until Blitz and his colleagues - David Spergel of Princeton
University; Peter Teuben of the University of Maryland, College
Park; Dap Hartmann of the Harvard-Smithsonian Center for Astrophysics
in Cambridge, Mass.; and W. Butler Burton of the University
of Leiden, the Netherlands - proposed theirs in 1997. The theory
was built on an analysis of radio observations of these clouds
and computer simulations of the formation of the Local Group.
"Based
on this hypothesis, we made several predictions," Blitz
said. "So far, our theory has passed the tests."
First,
because these clouds would have formed long ago, the abundance
of heavy elements should be much lower than in the galaxies
in the Local Group. Heavy elements like carbon, oxygen and iron
are formed in stars, or as stars explode, so their abundance
was much less before star formation transformed hydrogen and
helium into heavier elements.
A
paper by University of Wisconsin astronomer B. Wakker in late
November in Nature reported just that. The heavy element abundance
in the closest high velocity cloud is one tenth that in the
Milky Way. The nearest cloud is so close to us it stretches
from horizon to horizon, and appears to be accreting onto the
Milky Way, Blitz said.
A
second prediction was that, if the high velocity clouds are
part of the Local Group, they should show little of the fluorescence
(hydrogen alpha emission) seen in relatively nearby clouds of
atomic hydrogen. Recently, a group led by Ben Weiner of the
Carnegie Institution in Pasadena was able to make measurements
of the hydrogen alpha emission and confirm the low expected
levels of emission. The results are being presented separately
at this week's AAS meeting.
Thirdly,
the clouds should have a low pressure and be "big and puffy,"
Blitz said. A team led by Johns Hopkins University astronomer
Ken Sembach recently measured the pressure in one such cloud
and found it "embarrassingly close to what we expected,"
Blitz said.
A
fourth line of evidence also will be reported Jan. 14 at the
AAS meeting by Blitz and graduate student Timothy Robishaw.
Looking at past radio observations of the Local Group, they
discovered evidence of a large cloud of hot, ionized, million-degree
gas around the Milky Way and Andromeda that would be expected
from the collision of high velocity clouds. The gas, from perhaps
a thousand cloud collisions over the history of the Local Group,
could permeate the entire group.
"This
is something we would have expected, but it needs to be confirmed
by direct observation," Blitz said. The detection of emission
from highly ionized oxygen by the FUSE satellite, the subject
of a press conference earlier in the week, will provide crucial
evidence.
The
theory proposed by Blitz and his colleagues solves several conundrums
about the formation of galaxies and stars. Among these is the
apparent lack of sufficient visible gas in many spiral galaxies
like the Milky Way to continue to fuel the currently observed
amount of star formation. Rather than postulate that all the
galaxies we see are in the last third of their lives and have
run out of fuel, the theory suggests instead that high velocity
clouds fall in and continually replenish the gas needed to fuel
star formation.
Another
implication is that these clouds are one of few places where
the stuff of the early universe can be found relatively uncontaminated
by later star formation.
"In
these clouds, we can still study the universe we had 15 billion
years ago," he said.
Astronomers
have seen similar high velocity clouds in more distant groups
of galaxies. The theory implies that they too are left over
from the formation of their group.
Among
the many unanswered questions about high velocity clouds is:
What is going on inside the clouds themselves? Blitz thinks
many of the clouds are associated with dwarf galaxies seen scattered
about the Local Group. Since there are more high velocity clouds
than known dwarf galaxies in the group, the implication is that
there are many more such galaxies than we see today.
The
recent reanalysis of radio data by Blitz and Robishaw turned
up hydrogen clouds - just like the high velocity clouds - associated
with 10 out of 21 dwarf spheroidal galaxies in the Local Group.
The association is one more confirmation that the high velocity
clouds are part of the Local Group, and thus, as Blitz says,
"the last gasp. All other high velocity clouds already
have been gobbled up by Andromeda and the Milky Way."
The
research was supported by the National Science Foundation.
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