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By Josh Simon, with excerpts taken from an interview by George
Musser for Scientific
American in January, 2008
Listen as Dr.
Simon describes the history of dark matter research.
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| Image: Ursa Major I is one of the recently
discovered dwarfs from the Sloan
Digital Sky Survey (SDSS). Dont be worried if you
cant see anything in the picture of Ursa Major I thats
the point. Image courtesy of David W. Hogg, Michael Blanton,
and the SDSS Collaboration. |
One of the most important astronomical discoveries in recent
decades is that galaxies are much more massive than would be
expected from simply looking at them. It is relatively straightforward
to count all the stars in a galaxy, add up their masses, and
include a little bit extra to account for the gas and dust.
But this total mass is nowhere close to being enough to explain
how fast the stars and gas within galaxies are moving! Apparently,
galaxies also contain a whole lot of additional material that
we cant see --- dark matter.
While we know that dark matter feels the force of gravity,
its nature is a mystery to physicists and astronomers. Dark
matter is probably made of a new kind of subatomic particle,
but even though there are many ideas about what those particles
could be, we have not yet succeeded in actually identifying
a dark matter particle or detecting one in a laboratory. Whatever
it is, there is about 6 times as much dark matter in the universe
as ordinary atoms and molecules. A wide variety of evidence
indicates that dark matter is the major building block of the
largest structures in the universe: galaxies and giant clusters
of galaxies.
When dark matter was first discovered, astronomers considered
several different types of this new material, which they classified
according to how fast the individual dark matter particles
zoom around us. Heavy, slow-moving particles are called cold
dark matter, while very light particles moving at close to
the speed of light are referred to as hot dark matter. One
of the main differences between the two is that the slow speed
of the cold dark matter particles allows galaxies to clump
together more easily. Rapidly moving hot dark matter particles,
on the other hand, act something like little electric mixers
in preventing clumps of matter from accumulating. The effects
that these different theoretical types of dark matter would
have on the universe are still under active investigation.
The cold dark matter theory, which agrees much better with
observations than hot dark matter, predicts that at least 100,
and possibly as many as 500, dwarf galaxies should be in our
local neighborhood. Until 2005, however, only 11 dwarf galaxies
near the Milky Way had been found. My research seeks to investigate
this serious disagreement, known as the missing satellite
problem, in detail to help prove or disprove the theory.
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| Image: More than 1,500 individual images
have been assembled into this large mosaic of the Large
Magellanic Cloud, one of the largest and brightest of
our local dwarf galaxies. Courtesy of C. Smith, S. Points,
the MCELS Team and NOAO/AURA/NSF. |
To provide general context for this dwarf galaxy research,
the Milky Way galaxy is tens of thousands of light years across,
with a mass of approximately a trillion times the mass of the
Sun. Dwarf galaxies in general are about one tenth the size
of the Milky Way and smaller. The dwarf galaxies we know about
tend to range from one millionth to one tenth the size of the
Milky Way. They orbit the Galaxy in elliptical or circular
patterns, just as planets orbit around the Sun. Two examples
of the brightest and most massive of our local dwarf galaxies
are the Large and Small Magellanic Clouds.
There have been many suggestions regarding potential causes
for the discrepancy between the theory and current observations.
One possibility could be that dark matter does not even exist,
and instead our understanding of gravity is the problem, though
most astronomers today agree that dark matter is real. A second
possibility is that there are indeed hundreds of dwarf galaxies
orbiting the Milky Way but nobody has managed to find them
yet. The third possibility is that hundreds of low mass clumps
of dark matter really are out there as the theory predicts,
but for some reason most of them happen not to form any stars,
so we cant see them. Rather, they would be completely invisible
clouds of dark matter, and obviously it would be very difficult
to detect such objects --- or even prove that they exist. The
general theoretical thinking leans toward this third possibility.
Over the last five years, the Sloan
Digital Sky Survey has observed about one fifth of the
sky and discovered a new population of very faint groups of
stars that look like tiny dwarf galaxies. The reason that Sloan
was able to find these objects is that it employed a bigger
telescope and a better camera than previous sky surveys, so
it could see much fainter stars. Starting in 2005, a number
of teams of researchers led by Beth Willman of Harvard University
and Dan Zucker and Vasily Belokurov of the University of Cambridge
used this method to find 13 new candidate dwarf galaxies near
the Milky Way. This series of discoveries was remarkable because
over the entire history of astronomy before Sloan there were
only 11 known dwarf galaxies orbiting our Galaxy! It was obvious
that these newly found objects could dramatically impact the
missing satellite problem, so we knew it was important to determine
whether the new objects were really dwarf galaxies. To start
this investigation, my collaborator Marla Geha and I traveled
out to Hawaii to study these dwarf candidates with the Keck
telescopes and the DEep Imaging Multi-Object Spectrograph (DEIMOS).
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| Photo: DEIMOS in the shadow of the tertiary
mirror casing of the Keck II telescope. What sets DEIMOS
apart from other spectrographs on big telescopes is its
very large field of view it can observe an area about
1/12 as large as the full moon all at once. Credit: Rick
Peterson/WMKO. |
DEIMOS was
built in 2002 for the DEEP Extragalactic Imaging Probe 2 (DEEP2)
survey, and was a collaboration between astronomers at the
University of California and other institutions. W. M. Keck
Observatory support astronomer Greg Wirth describes what sets
DEIMOS apart from spectrographs on other big telescopes.
According to Wirth, DEIMOS is the most powerful spectrograph
on the worlds leading telescope. Three things make this instrument
special. First, it has a wide field of view --- over twice
as large as our other spectrographs at Keck --- which allows
it to observe more objects at once. Second, instead of observing
objects one by one, DEIMOS uses special inserts called slitmasks
to view the light from hundreds of objects at one time, multiplying
the power of the telescope enormously and producing more scientific
results per night. Third, DEIMOS has an image stabilization
system which allows it to take spectra that are sharper than
its competitors and thus measure velocities more precisely,
which are crucial for the science problems were addressing
at Keck.
A three night observing run revealed some exciting news when
we observed stars in eight newly discovered dwarf galaxy candidates.
We obtained spectra of over 800 stars in total, which we used
to measure with high accuracy the velocities with which the
stars are moving.
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| Image: Map showing the distribution of
dwarf galaxies orbiting the Milky Way. The newly discovered
dwarfs are all on the north side of the Galaxy because
that is the only part of the sky that Sloan observed.
Credit: Marla Geha. |
These measurements enabled us to determine the velocity dispersion,
or the range of velocities, that is present in each of these
dwarf galaxies. The typical velocity dispersion we found is
about 5 kilometers per second, which means that if you pick
a random star in any of the galaxies, there is a good chance
that it will have a velocity within 5 km/s of the overall average
velocity. This allows us to measure the mass of the galaxies,
in the same way that we use the velocities of planets moving
around the Sun to figure out the Suns mass. Prior to this
study we didnt know whether these systems were actually dwarf
galaxies or globular clusters. But their velocity dispersions
tell us there must be large quantities of dark matter along
with the stars in these galaxies. If they had been globular
clusters, with only stars and no dark matter, the velocity
dispersions would have been perhaps a few tenths of a km/s,
a factor of ten or more lower than what we measured.
Our observations therefore confirmed not only that these newly
discovered objects really are galaxies but also that they contain
at least one hundred times as much dark matter as stars. Integrated
over the entire universe there is about six times as much dark
matter as normal matter, and normal galaxies like the Milky
Way have a ratio of ten to twenty to one of dark matter over
normal matter. So these tiny dwarf galaxies turned out to be
the most dark matter-dominated galaxies that have ever been
found.
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| Photo: Dr. Marla Geha, co-author of the
study and Assistant Professor in the Astronomy
Department at Yale University. |
By measuring the masses of these galaxies we were also able
to study them in the context of the missing satellite problem.
Because we could go down to a specific mass, we now know how
many dwarf galaxies have at least 1 million or 10 million times
the mass of the Sun, and that is exactly the quantity that
is predicted by the cold dark matter theory. Then we could
compare our observations on a one to one basis with the theory.
What we found is that even though we doubled the number of
dwarf galaxies, the total number is still smaller than what
is predicted by the theory. The initial conclusion was that
the missing satellite problem remains; there just do not seem
to be enough satellite galaxies orbiting the Milky Way.
Based on these results, the only way that the cold dark matter
theory can be correct is if there are hundreds of clouds of
nearly pure dark matter surrounding our Galaxy, but we cant
see them because they have no stars in them and dont give
off any light. We therefore started looking at various scenarios
proposed by theorists to explain why these low mass dark matter
clumps were not able to form any stars. There were several
ideas that we tested. The most important of these possibilities
requires a review of the evolution of the universe since the
Big Bang.
Initially, after the Big Bang everything was ionized because
the universe was extremely hot. As it expanded and cooled,
after about 300,000 years protons and electrons combined into
neutral atoms like Hydrogen (H) and Helium (He). Over the next
epoch large clouds of H and He came together and began to collapse
into stars and galaxies. At some point in this process the
first stars (and perhaps also massive black holes) turned on,
releasing a huge amount of ultraviolet radiation and ending
the so-called Dark Ages. This blast of radiation reionized
the universe, heating up the gas in dwarf galaxies and thereby
preventing it from becoming cool enough to form more stars.
This is our current best guess for why we dont see hundreds
of nearby dwarf galaxies. Our Milky Way has a couple dozen
small dwarf galaxies surrounding it, each with some stars but
mostly containing dark matter. A much larger number of completely
dark clouds of dark matter are probably orbiting the Galaxy.
Now the challenge will be to get definitive evidence to confirm
this picture, which could come from actually identifying some
of these dark, starless, galaxies. But obviously, if dark galaxies
do not have any stars in them, they are going to be pretty
hard to find!
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| Photo: With the power of the Keck telescopes
behind his research, Josh Simon is confident that the
next few years will continue to be an exciting time for
studying the Milky Ways dwarf galaxies. Credit: S. Anderson/WMKO |
Determining whether there really are hundreds of additional
clouds of dark matter around the Milky Way is an extremely
difficult problem for which we still do not have a definite
solution. One possibility is that even though these clouds
do not contain any stars, they could have some hydrogen gas
in them that would be visible with radio waves. Alternatively,
if there are hundreds of these objects orbiting around the
Milky Way some of them will hit the disk of the Galaxy and
might produce observable effects on the matter in the Milky
Way. Another possibility is that there are some dwarf galaxies,
most notably the Sagittarius dwarf, that are being tidally
disrupted by the Milky Way. This dwarf is being torn apart
into enormous streams of stars that wrap around our Galaxy.
Again, if there are large numbers of small clouds of dark matter
zooming around, some of the dark halos should run into the
tidal streams and could distort the path that those streams
are following.
Finally, one very intriguing idea is that it is possible that
dark matter is able to annihilate with itself --- whenever
you put matter and anti matter together they emit a flood of
gamma rays. Whatever particle makes up dark matter may be able
to do exactly the same thing, thus one of the exciting possibilities
in the next few years will be to use the newly completed gamma
ray telescopes to look for gamma rays coming from apparently
blank regions of the sky. These gamma rays might allow us to
see otherwise invisible clumps of dark matter. 
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