COLUMBUS, Ohio -- Two Ohio State University researchers have found a
way to detect dark matter in our galaxy using 35-millimeter cameras and
low-power telescopes.
This technique may enable astronomers to find the missing mass in our galaxy
without using expensive, high-power telescopes and satellites.
Andrew Gould and Darren DePoy, associate professors of astronomy, plan to
modify and install 35-millimeter cameras on 1-meter telescopes -- those
which carry light-gathering mirrors that measure approximately 1 meter across.
Astronomers have often abandoned 1-meter telescopes for ones with larger
mirrors and more powerful magnification.
"For a few years now there's been a feeling in the astronomical community
that the era of the 1-meter telescope is over, that there's no use for them,
and they should all be closed down," said Gould. "But I think
that if you're creative in how you use 1-meter telescopes you can still
do state-of-the-art work."
Gould studies gravitational lensing -- what happens when a massive dark
object in space, like a planet, dim star, or black hole, crosses in front
of a luminous source star in the background. The object's strong gravitational
pull bends the light rays from the star and magnifies them like a lens.
Here on Earth, we see the star get brighter as the lens crosses in front
of it, and then fade as the lens gets farther away.
Gravitational lensing is one of the few ways astronomers may detect the
presence of dark, massive objects in our galaxy. If many such objects exist,
they could account for the missing mass of the universe. The dark matter,
which may account for up to 99 percent of the mass of the universe, has
so far eluded detection by the most powerful instruments such as the Hubble
Space Telescope.
Gould and DePoy were inspired by the recent successes of gravitational lensing
experiments which used small 1-meter telescopes. These experiments require
a long observing time, so obtaining a lot of telescope time is more important
to the astronomers than having a big telescope mirror.
To find missing mass in our galaxy, Gould and DePoy adopted a photographic
technique that astronomers use to pick out individual stars in distant galaxies:
They take a series of pictures of a galaxy and then digitally subtract later
images from earlier images to see which stars got brighter.
But Gould and DePoy wanted to look for gravitational lenses near the central
bulge region of our galaxy, an area much too large to cover with a single
photograph from a conventional telescope.
"We decided to take the small-telescope idea to the extreme,"
said DePoy. "And see what we could accomplish with a small camera --
not too different from a 35-millimeter camera that you may have at home.
In astronomy we tend to take little pictures of a specific area of the sky
with lots of detail. To photograph the entire galactic bulge, we'd have
to take hundreds of pictures and combine them," said DePoy.
"But if you stepped out in your back yard at night and looked in the
direction of the bulge, you could take a picture that would cover a much
wider angle of the sky," DePoy continued. "Your picture of the
bulge would then be similar to a picture that astronomers would take of
a nearby galaxy."
The camera that Gould and DePoy want to build will hold a lens that costs
about $1,000. The most expensive part of the camera will be a charge-coupled
device (CCD), a computer chip that records pictures digitally as a grid
of pixels, or picture elements. CCDs detect stars very accurately, and will
help make individual stars in the bulge more noticeable. The CCD will probably
cost about $25,000.
"So we'll just take a single 35-millimeter picture of the bulge and
analyze it the same way we'd analyze a picture of a neighboring galaxy.
This will allow us to detect a new type of gravitation lensing event, what
we call an extreme event, in which a faint star is magnified a few hundred
times. These extreme events can give us information that is otherwise very
hard to get," DePoy said.
Once astronomers identify a lens object and measure its size, they can calculate
how much mass it has. One way to perform these measurements is to launch
a satellite, and compare the size of the lens object to the distance from
the earth to the satellite. That would work, but would cost a great deal
of money.
A few years ago, two researchers at the University of Chicago proposed that
the same measurement could be made without a satellite, just by comparing
observations from two different places on Earth.
"Most astronomers, including me, just broke out laughing when we heard
that, because a typical lens event covers tens of millions of miles, and
two observers on Earth would be too close together for an adequate comparison,"
said Gould. "But I later realized that its not how far apart observers
are compared to the size of the lens that's important, its how far apart
they are compared to the distance between the lens and the source star."
So this technique should work for lenses that pass close to Earth's line
of sight to a source star. Such events would still be difficult to detect,
but they are possible.
"No one's attempted to find these objects before," said Gould,
"but it can be done."
Gould and DePoy are currently working with a world-wide network of observers
who monitor ongoing microlensing events using dedicated 1-meter telescopes
on 3 continents. After a group of observers finds an event, other groups
perform follow-up observations about once per hour. Gould projects that
the teams will detect about thirty lens events per year. They should be
able to detect somewhat small dark objects.
"We're searching for planets that may be orbiting around the lens stars,"
DePoy said, "If any of those lenses have planets, then the lensing
event would have a small blip of light caused by the planet. It wouldn't
last very long compared to the whole lensing event, because planets are
much less massive than the lensing stars. We are observing from around the
world in order to get as close as possible to 24-hour coverage so that we
don't miss any of those blips."
"I think we'll be able to find relatively small planets," said
Gould. "People have found big planets -- Jupiter-size planets -- with
other techniques. But people only dream about finding Neptune-size planets.
I think we can do that with gravitational lensing."
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Contact: Andrew Gould, (614) 292-1892; Gould.34@osu.edu
Written by Pam Frost, (614) 292-9475; Frost.18@osu.edu