A faint “satellite galaxy” 10
billion light years from Earth is the lowest-mass object ever detected at such
a distance, says University of California, Davis,
physics professor Chris Fassnacht, who aided in the satellite’s discovery.
The find, described in Nature, 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, according to
Fassnacht. Astronomers have detected such satellites around our own galaxy, the
Milky Way, and nearby.
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 analyzing 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 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.
“Or, we might find as many satellites as we see in the simulations, and that
will tell us that dark matter has the properties we think it has.”
