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A New Galaxy & Mysterious Dark Matter
Washington: Astronomers have discovered a "dwarf" galaxy some 10
billion light-years away which they believe is consisting mainly of
mysterious "dark matter".
The dwarf galaxy, only the second ever to be discovered, is incredibly
distant and extremely small. It orbits as a satellite of a larger galaxy.
Though telescopes can't spot the dwarf galaxy, scientists detected its
presence through the tiny distortions its gravity causes to light that
passes it by.
It weighs about 190 million times the mass of the sun , a seemingly
hefty sum, although typical galaxies pack in the mass of tens of
billions of suns, SPACE.com reported.
Scientists think dark matter, which may be made of some exotic
particle that doesn't reflect light, makes up about 98 per cent of all
matter in the universe. Yet it has never been detected directly.
Discovering dark objects like this tiny, distant galaxy could help
researchers understand better what dark matter is and how it affects
regular matter around it, they said.
The finding, detailed in the journal Nature, suggest that the fact so few
dwarf galaxies can be seen in our own cosmic neighborhood may be
explained by them being mainly composed of dark matter. "This is the
lowest mass galaxy that we've seen at this distance by far," said study
co-author Matthew Auger of the University of California, Santa
Barbara.
Even farther away is another galaxy whose light passes by the dark
dwarf and its host on its way to Earth. As this light travels through
space, it is bent by the gravity of the intervening galaxy and its
satellite, causing a distorted and curved picture called an "Einstein
ring". This process is called gravitational lensing.
Though most of this warping is caused by the larger gravitational tug
of the host galaxy, researchers used a computer model to detect a
slight excess of warping caused by the tiny addition of the dwarf
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satellite's gravity.
The astronomers also took help of high-resolution images taken by the
Keck Observatory in Hawaii, using a system called adaptive optics to
compensate for atmospheric blurring. "The image quality in the data
we used here is somewhat better than the quality you can get from
the Hubble Space Telescope," Auger said.
Dwarf galaxies aren't a rarity in the cosmos; even our Milky Way has
them. In fact, the newfound galaxy is about the same size as our own
galaxy's Sagittarius dwarf satellite.
The discovery could help astronomers find similar objects and confirm
or reject theories about the structure of the cosmos.
Theory predicts that galaxies should be surrounded by halos of
smaller, satellite blobs of mass, said University of California, Davis,
physics professor Chris Fassnacht, who aided in the satellite’s
discovery.
But they had not previously detected the predicted satellites of more
distant galaxies. Because most of the mass of galaxies is made up, not
of stars, but of “dark matter,” which does not absorb or emit light,
these distant objects may be very faint or even completely dark.
The team looked for faint or dark satellites of distant galaxies using a
method called gravitational lensing. Using the Keck II telescope at the
W.M. Keck Observatory on Mauna Kea, Hawaii, with “adaptive optics,”
they found two galaxies aligned with each other, as viewed from
Earth.
The nearer object’s gravitational field deflects the light from the more
distant object as the light passes through or near the other object’s
gravitational field, creating a distorted image as if passed through a
lens. By analysing these distorted images, the researchers could
determine if there were any satellite galaxies clustered around the
“lens” galaxy.
Adaptive optics make constant, tiny adjustments to the telescope
mirror to compensate for the effects of the Earth’s atmosphere. As a
result, the Keck telescopes can achieve higher resolution than the
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Hubble Space Telescope.
The technique can now be applied to many more galaxies, Fassnacht
said. “As we collect more objects, we can do more precise tests of our
simulations and make predictions about the structure of the universe,”
he said.
First author Simona Vegetti, a postdoctoral researcher at the
Massachusetts Institute of Technology, said: “Now we have one dark
satellite, but suppose that we don’t find enough of them — then we
will have to change the properties of dark matter.
"For the first time we're getting information about something with a
mass that's comparable to some of the smaller Milky Way satellites but
outside of the local universe," said co-author David Lagattuta of the
University of California. "Add in the fact that it's something like six or
seven billion light years away, it's really true that we've never been
able to see something like this before!"
The Milky Way's satellites are also little understood, as they are hard
to observe, and theory predicts many more of them than have been
discovered. Scientists hope that finding more dwarf galaxies around
distant hosts may help shed light on the problem.
The finding was described in a paper published online in the journal
Nature.