December 14, 1999
Survey Reveals Massive Galaxies
By JAMES GLANZ
sing a technique akin to overlaying
thousands of faint X-ray images to create one sharp picture, astronomers
have discovered that typical galaxies
may be twice as large and contain twice
as much mass as suggested by previous
measurements. The new observations,
which have emerged from a five-year
census of the heavens called the
Sloan Digital Sky Survey, indicate that an
average galaxy extends invisibly for
well over a million light-years into
space and weighs the equivalent of at
least five trillion Suns.
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Stephen Kent/Fermilab
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In a strip of sky about one degree long in the constellation Cetus, the Sloan survey found some 10,000 objects, including a bright blue galaxy.
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By comparison, the glittering disk of
stars in a typical galaxy like the Milky
Way, where Earth is located, stretches
only about 50,000 light-years from center to edge and contains about 100 billion
stars. The finding raises the possibility
that the Milky Way and its nearest
galactic neighbor -- Andromeda, more
than two million light-years away --
actually brush against each other in the
remote darkness of space.
The observations probed not the
bright disk but the mysterious cloud of
nonluminous or "dark" matter, called a
halo, that surrounds the disk. Although
the dark matter is invisible and scientists have been unable to determine its
precise nature, they know of its existence because of the gravitational pull it
exerts on ordinary matter -- the stars
and gas in galaxy disks.
The Sloan measurements relied on
another effect of a halo's gravity: like a
lens or a clear marble, it bends light
rays that pass through it. One consequence is that extremely distant background galaxies, as seen through a foreground galaxy's halo, appear distorted.
Because the effect is so minute, however, the Sloan astronomers observed it in
about 30,000 foreground galaxies and
added the data together.
Then, working backward to deduce
what kind of halo would produce those
distortions, the astronomers found that
the dark matter trails deep into space,
even farther than earlier studies by
other methods had suggested.
"The result from the Sloan analysis is that the halos of galaxies are
enormous," said Dr. Tereasa Brainerd, an astronomer at Boston University, whose earlier work helped
develop the technique, "perhaps extending 10 to 20 times the size of the
visible regions of the galaxies."
Dr. Vera C. Rubin, the astronomer
at the Carnegie Institution of Washington who discovered that galaxies
must contain dark matter, added
that the Sloan observations had taken a "conceptually brilliant" idea,
whose application had been disappointing in the past, and turned it into
a practical technique.
"For years, when I would talk
about dark matter, I had to explain
why it hadn't been seen that way,"
Dr. Rubin said, adding that the technique was "observationally very difficult."
The group's report on the finding,
which has been submitted to the Astronomical Journal, is also likely to
be seen as an early sign that the
unusual $80 million project will help
change the way astronomy is done.
The report has been posted on the
Los Alamos National Laboratory's
preprint server (xxx.lanl.gov/abs
/astro-ph/9912119).
The Sloan survey is an ambitious
effort to collect and categorize hundreds of millions of objects over a
vast swath of the northern sky. To
complete the project -- which involves nine institutions in the United
States, Germany and Japan -- scientists and engineers have constructed
a special telescope, the largest electronic camera ever built, and other
apparatus at Apache Point, N.M.
Rather than picking out interesting objects and studying them one by
one, the Sloan telescope sweeps
across the sky, scooping up high-quality data on everything in its
view.
Later, any astronomer who
wishes can obtain data on particular
objects and then analyze them.
"We've gotten into a mode in our
country, and pretty much worldwide,
of using a telescope for a night or
three nights, looking at an object and
going home," said Dr. Tony Tyson,
the astronomer at Bell Laboratories,
Lucent Technologies who first proposed the idea of using the lensing of
background galaxies to determine
the size of halos.
With its entirely different philosophy, Dr. Tyson said, the Sloan survey
"is going to come up with some fantastic answers by the time the experiment is complete."
Indeed, the galaxy results have
emerged from just 3 percent of the
data that the survey is expected to
compile over five years. And that is
not the only discovery to pop out of
the data, which is eventually expected to yield a tremendous haul of
celestial oddities, a detailed "field
guide" to all the varieties of ordinary
objects in the heavens, and a three-dimensional map of a million galaxies in the Milky Way's corner of the
cosmos.
Dr. Michael Turner, a cosmologist
at the University of Chicago and the
Fermi National Accelerator Laboratory, who is the Sloan survey's
spokesman, said: "It's kind of like a
wall of water coming at you. When
you feel the spray, you won't have
much time to catch your breath before the wall of water comes." He
added that with the first results,
"we're feeling the spray."
The results on galaxy halos give
some insight into the multifarious
ways astronomers will use the coming tidal wave of Sloan data.
In the more than two decades
since Dr. Rubin discovered the galactic dark matter from studying the
rotation patterns of spiral galaxies,
astrophysicists and cosmologists
have learned indirectly, through
studies of pristine traces of the Big
Bang in distant space and other
methods, that at least 90 percent of
the matter in the universe is dark.
Some astronomers have suggested
on general grounds that all the dark
matter might be found in galaxy
halos, while others have theorized
that it might be spread out in the
space between galaxies. But turning
up direct evidence has proved elusive. Since galaxy disks are comparatively small, it is difficult to use
their motions as probes of anything
but the inmost portions of the surrounding halos.
Dr. Penny Sackett of the Kapteyn
Astronomical Institute in the Netherlands, who recently published a review on the topic, said that a different kind of observation, involving
small "satellite" galaxies that felt
the tug of a larger galaxy's halo,
could probe its existence farther into
space.
That study, led by Dr. Dennis F.
Zaritsky of the University of Arizona, suggested that halos extend to
at least 600,000 light-years, but could
not rule out the possibility that they
go farther.
"We had a situation where maybe
they're large, and maybe they're not
so large," Dr. Sackett said.
The work that formed the basis for
the new Sloan report, which was led
by Dr. Philippe Fischer and Dr.
Timothy McKay, both of the University of Michigan, sought to probe the
dark matter halos more directly and
deeper into the space around galaxies.
The researchers turned to the effect called gravitational lensing. According to Einstein's theory of relativity, gravity bends light just as an
ordinary glass lens does. The effect
was first seen in 1919, when astronomers saw a slight deflection in the
apparent position of a star over the
limb of the Sun during an eclipse, an
early step in verifying the now-accepted theory.
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Fermilab Visual Media Services
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This spiral galaxy is one of hundreds of millions of celestial structures that the Sloan Digital Sky Survey will record.
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"Now that it's well understood,
instead of using the lensing to test
the theory, we use the lensing to
measure the mass of objects," Dr.
McKay said.
When rays from a distant source of
light pass through a distributed
mass, like a halo, rather than past its
edge, the source gets smeared into
little arcs rather than simply deflected. So the Sloan team searched
their data for relatively nearby galaxies with more distant galaxies behind them, lying along almost the
same line of sight.
The astronomers immediately
faced a colossal technical challenge:
the expected smearing would be so
slight that it would stretch out any
individual background galaxy by
only about half a percent. Because
galaxies are irregularly shaped anyway, the only option was to add up
the tiny smearing in a statistical
sample of many background galaxies shining through many halos.
And it is in exactly such a study
that the Sloan survey excels, since it
collects great numbers of both kinds
of galaxies: those in the foreground
whose halos do the lensing, and those
in the background whose shapes are
slightly smeared. In the early data,
the team found an average of 50
background galaxies whose light
pierced the halo of each of 30,000
foreground galaxies.
"That's where the Sloan wins out
-- it's simply the vast number of
objects in their data set," said Dr.
Brainerd of Boston University.
The results showed that typical
halos extended to at least 1.3 million
light-years into space from the luminous galactic disks, more than double the previous limit.
In one sense, the results only deepen the mystery of the dark matter.
They shed no new light on exactly
what it is, although Dr.
Turner said
that the new findings were consistent
with the dark matter's being a
strange sort of subatomic particle
left over from the explosive birth of
the universe.
But aside from the light the results
should shed on the formation of galaxies and galaxy clusters, they also
let scientists complete a bit of cosmic bookkeeping, Dr. Turner said.
Unrelated studies of elements that
were produced through nuclear reactions in the Big Bang, the explosion in
which the universe is thought to have
been born, combined with observations of large galaxy clusters, have
allowed cosmologists to estimate the
total amount of matter in the universe. Astronomers have had a hard
time finding that much matter in the
heavens, but the larger halos may
mean that all the matter has now
been accounted for. The vast majority of that matter appears to be of the
dark variety, inhabiting the halos.
"We can now balance the books
and say that we know that the dark
matter in the universe is associated
with galaxies," Dr. Turner said. "It's
a bit mind-blowing. The halos of galaxies are really big."
"I'm quite excited about this result," said Dr. Joshua Frieman, a
Sloan collaborator at Fermilab.
"Considering it's based on a few percent of the total amount of data we'll
have, it's a real indicator of what
we'll be able to accomplish in this
area of the survey."
That same statistical power is
what astronomers in other fields of
study will be looking forward to mining as the Sloan survey scans the
heavens, Dr. Rubin said.
In addition to the University of
Chicago and Fermilab, institutions
taking part in the Sloan survey include Princeton, Johns Hopkins, the
Institute for Advanced Study, the
United States Naval Observatory, a
collaboration called the Japan Participation Group, the University of
Washington, the Max Planck Institute for Astronomy in Heidelberg,
Germany, and several institutions involved in only selected parts.
Already, said Dr. Donald G. York,
a collaborator at the University of
Chicago, who is a founder of the
project, discoveries in the Sloan data
have doubled the number of extremely distant quasars, or beacons
near the edge of the visible universe
that burn with the intensity of hundreds of billions of Suns.
"I like to say it took 35 years to
find the first dozen and 6 months to
find the second dozen," Dr. York said
of the discoveries, which have
emerged as the Sloan telescope has
operated only sporadically as part of
a commissioning period.
"It's really a look at the way astronomers of the future will work,"
Dr. Rubin said.
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