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The
Sloan Digital Sky Survey (SDSS-II) announced in
May discoveries of two new faint companion galaxies
to the Milky Way.
The first was found in the
direction of the constellation Canes Venatici (the
Hunting Dog) by SDSS-II researcher Daniel Zucker at
Cambridge University (UK). His colleague Vasily
Belokurov discovered the second in the constellation
Bootes (the Herdsman).
"I was poring over the survey's map of distant stars
in the Northern Galactic sky — what we call a Field of
Streams — and noticed an overdensity in Canes Venatici,"
Zucker explained. "Looking further, it proved to be a
previously unknown dwarf galaxy. Its about 640,000 light
years (200 kiloparsecs) from the Sun. This makes it one
of the most remote of the Milky Way's companion
galaxies."
Zucker emailed Belokurov with the news, and, just as
discoveries often build upon one another, Belokurov
excitedly emailed back a few hours later with the
discovery of a new, even fainter dwarf galaxy. The new
galaxy in Bootes, which Belokurov called Boo, shows a
distorted structure that suggests it is being disrupted
by the Milky Way's gravitational tides. "Something
really bashed Boo about," said Belokurov.
Although the dwarf galaxies are in our own cosmic
backyard, they are hard to discover because they are so
dim. In fact, the new galaxy in Bootes is the faintest
galaxy so far discovered, with a total luminosity of
only about 10,000 Suns. But because of its distance
(190,000 light years) it appears almost invisible to
most telescopes. The previous dimness record holder was
discovered last year in Ursa Major using SDSS-II
data.
New galactic neighbors are exciting in their own
right, but the stakes in searches for ultra-faint dwarfs
are especially high because of a long-standing conflict
between theory and observations. The leading theory of
galaxy formation predicts that hundreds of clumps of
"cold dark matter" should be orbiting the Milky Way,
each one massive enough in principle to host a visible
dwarf galaxy. But only about ten dwarf companions have
been found to date.
One possibility is that the galaxies in the smaller
dark matter clumps are too faint to have appeared in
previous searches, but might be detectable in deep
surveys like SDSS-II.
"It's like panning for gold. Our view of the sky is
enormous, and we're looking for very small clumps of
stars," explained Cambridge University astronomer Wyn
Evans, a member of the SDSS-II research team.
Added collaborator Mark Wilkinson: "Finding and
studying these small galaxies is really important. From
their structure and their motions, we can learn about
the properties of dark matter, as well as measure the
mass and the gravity field of the Milky Way".
The new discoveries are part of the SEGUE project
(Sloan Extension for Galactic Understanding and
Exploration), one of the three component surveys of
SDSS-II. SEGUE will probe the structure and stellar
make-up of the Milky Way Galaxy in unprecedented
detail.
"I'm confident there are more dwarf galaxies out
there and SEGUE will find them, said Heidi Newberg of
Rensselaer Polytechnic Institute, co-chair of SEGUE.
Text & Image
credit:
SDSS
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The most unique feature in
a new image taken with the NASA/ESA Hubble Space
Telescope is a group of five quasar images produced by a
process called gravitational lensing, in which the
gravitational field of a massive object - in this case,
a cluster of galaxies - bends and amplifies light from
an object - in this case, a quasar – farther behind it.
