There's room in the
universe for thousands of galaxies but that doesn't stop them from running into
each other. New observations support the idea that galaxies are in constant
interaction with each other and that the biggest ones get bigger by engulfing smaller
ones.
These observations confirm
a long-standing theory about how the universe works in
general and sheds light on how things got started in the first place.
"This is the way that
everything in the universe was formed," said Pieter van Dokkum of Yale University. "It's a never-ending story
of things colliding--small things colliding to make big things, big things
colliding to make bigger things. These are the events that shape today's
galaxies."
The theory goes like this:
The universe was a fairly smooth place following the Big Bang, but there were
little bits of substructure material that attracted matter and began to grow.
As time passed, these clumps grew bigger and bigger. Through gravitational
forces, the bigger ones began attracting some of the smaller clumps, collided,
and merged with them, forming what we know as galaxies today.
Although this logic seemed
reasonable to many scientists, they didn't have much evidence that these large
bodies actually interacted with each other. Confusing the matter, the most
massive galaxies also appear to be the oldest, which would have left very
little time since the Big Bang for them to form via mergers.
Now, van Dokkum's multiple deep-field observations--taken with the
National Science Foundation's 4-meter telescopes at Kitt
Peak National Observatory and Cerro Tololo
Inter-American Observatory--display the telltale signs of inter-galactic
interactions.
Van Dokkum
selected 126 nearby red galaxies, chosen because of their massive size, and
began searching for signs of gravitational influence from outside sources, such
as tails, broad fans of trailing stars, or other obvious asymmetries. Although
these features are faint, they turned out to be quite common, showing up in 53
percent of the galaxies.
"It shows that these
galaxies are not in equilibrium, that something is pulling stars out of these
galaxies," van Dokkum told SPACE.com.
"On this scale the only thing we know of that can do this is another
galaxy, and we can actually see that."
These observations also
show that these merges happen fast--which is probably why they were difficult to
spot before now.
"Well, fast is a few
hundred million years. That's fast compared to the age of the universe,"
van Dokkum said. While that doesn't seem very fast,
it's quick enough to account for those old, massive galaxies.
This research, which is
detailed in the Dec. issue of Astrophysical Journal, also helps solve
the long-standing problem for judging a galaxy's age. Previously, astronomers
equated the age of stars with the age of the galaxy, even though sometimes the
galaxies appeared much younger than the stars.
"We have found that,
though their stars are generally old, the galaxies that result from these
mergers are relatively young," van Dokkum said.
Galaxies smashing into one
another sounds like an explosive event. On the contrary, they probably slide
together smoothly, generating little fanfare. Galaxies are mostly empty space,
and the distance between stars is so huge that the probability of stars
colliding is actually very small.
There is still a chance for
a violent explosion though, especially if the central black holes of the merging galaxies collide and merge. "This event could be so
powerful that it could cause ripples in space time," said van Dokkum.
One would think that two
galaxies mixing together would create a hot-bed for star formation. Cosmic gas
is the fuel for star formation, and the idea is that the same tidal forces that
pull these stars away from their galaxies will also compress the gas and lead
to the formation of new stars
But collisions like these
surprisingly spark very little, if any, new star formation. One possible reason
is that the galaxy has already used up all its gas forming the stars already
there.
"Or, at a previous
point the central black hole created so much energy that it pushed the gas out
of these galaxies," van Dokkum said. "That
is what's next on the agenda to figure out."
It doesn't appear that the
Milky Way has a collision rich history, van Dokkum
said, mainly because it has a very large, intact disc that a merge or collision
would have disrupted. But that could change soon-- the Andromeda galaxy M31 lurks just 2.3 million light years
away and is on a crash course for the Milky Way.
"The Milky Way will
collide in the future, in about 4 billion years with the galaxy Andromeda and
that collision will lead to formation of a much bigger galaxy," van Dokkum said. "So we have that to look forward
to."
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