The late Carl Sagan got lots of praise for his brilliance in explaining
science to the masses, especially on his PBS TV series Cosmos back in
the 1980s. But he also got teased unmercifully for the super-geeky way
he said �billions� � and given the subject matter, he said it a lot. The
universe is billions of years old, and billions of light-years across,
and contains a hundred billion galaxies, each containing a hundred
billion stars�and Sagan got his teeth into every �billion� he could find.
Which is why he would be at something of a loss talking about a new
report in The Astrophysical Journal describing the least massive galaxy
every found. Known as Segue 2, it contains just 1,000 or so stars, and
while this puny object, like most galaxies, is somewhat bulked up with
invisible dark matter, that only adds another 100,000 stars� worth.
There�s not a billion to be found, unless you start weighing the stars
and dark matter in pounds. �Finding a galaxy as tiny as Segue 2,� says
co-author James Bullock, of the University of California, Irvine, �is
like discovering an elephant smaller than a mouse.�
But it�s also very different. A mouse-sized elephant is the last thing
you�d ever go looking for; a galaxy this tiny, by contrast, is something
astronomers have long sought. The reason: it�s firmly established by now
that most of the mass in the universe comes in the form of dark matter,
not stars. The leading candidate for what makes up the dark matter is
some sort of still-undiscovered subatomic particle. And computer models
suggest that while these particles should coalesce into gigantic blobs
or haloes that surround normal galaxies, they should also form into
thousands of much smaller clumps, buzzing around galactic fringes.
Those smaller clumps would have their own tiny retinue of stars as well
� and in 2006, astronomers finally found an example. Called Segue 1 (it
was discovered by the Sloan Extension for Galactic Understanding and
Exploration, or SEGUE, an offshoot of the Sloan Digital Sky Survey) it
had a mere 300 or so stars, but its dark matter component was equivalent
to about 600,000 stars � small, but not as small as what astronomers
were hoping to find.
Then, in 2009, the SEGUE survey snagged a true pipsqueak: Segue 2 has
three times the star count as its brother, but only one-sixth the dark
matter � right in the ballpark of what theory says should be there.
�It�s quite encouraging,� says Kirby.
Maybe, but it�s reasonable to wonder why you�d give a tiny thing like
this a grandiose name such as �galaxy.� Why not just call it a star
cluster? The answer, says lead author Evan Kirby, also at Irvine: a
galaxy is defined as a system of stars that can enrich its own chemical
composition. What that means is that the earliest stars were made almost
entirely of hydrogen and helium. Those atoms were forged into heavier
elements in the stars� cores, and eventually blown out into space at
high velocity in supernova explosions. The heavier elements, known as
�metals,� then became incorporated into the next generation of stars (in
astronomy jargon, iron and aluminum are metals, but so are oxygen and
carbon).
In a star cluster like the Pleiades, there isn�t enough gravity to keep
those metals from flying away; in a galaxy, there is � and that gravity
can come from either stars or dark matter, doesn�t matter which. Kirby
and his team used the powerful Keck II telescope, in Hawaii, to measure
the metal content of the stars in Segue 2, and found that they vary a
lot. That�s evidence that some stars are younger, some older, and that
the metals have stayed put. Puny or not, Segue 2 is a galaxy. It�s also
likely not to be the only one of its kind. �We think there are lots
more,� says Kirby.
Unfortunately, those junior members of the galactic corps might not be
easy to find. Spotting very dim collections of stars on the fringe of
the Milky Way isn�t incredibly hard, but probing their light for
evidence of metal composition, and measuring their orbital speeds as an
indirect method of weighing their dark matter, is. The Keck II armed
with an instrument called the DEIMOS spectrograph � among the world�s
most powerful systems for doing this kind of work � can just manage it.
At this point, says Bullock, there�s no set of instruments, either in
existence or on the drawing board, that could do a lot better. �Right
now,� he says, �we�re thinking of what it would take.� Once the
astronomers come up with something though, they�re betting that the
little galaxies will be out there waiting.
--
"OK you cunts, let's see what you can do now" -Hit Girl
http://www.youtube.com/watch?v=CjO7kBqTFqo