COLUMBUS, Ohio -- Two Ohio State University researchers have found a
way to detect dark matter in our galaxy using 35-millimeter cameras and
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."
Contact: Andrew Gould, (614) 292-1892; Gould.firstname.lastname@example.org
Written by Pam Frost, (614) 292-9475; Frost.email@example.com
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