Astronomers have argued for years over whether massive
galaxies form from scratch, or by chunking together smaller galaxies.
Lately, evidence is building for the latter theory, and a
new study adds to the growing picture of galaxy formation as a clumpy
affair. Using an array of both ground-based and space telescopes, including
ESO's Very Large Telescope in Chile and the Hubble Space Telescope, a team of astronomers
recently observed groups of huge galaxies in the process of merging, showing
that large, established galaxies can still grow bigger.
"The question was whether or not you could still form
very massive galaxies at relatively recent times through these merging processes,"
said researcher Kim-Vy Tran of the University of Zürich, Switzerland. "We
saw three examples of this happening now."
Tran and colleagues observed the meshing galaxies in a group
called Sg1120-12 about 4 billion light-years away. The team detailed their
discovery in the August issue of the Astrophysical Journal Letters.
Clumpy growth
The new finding adds to a mounting list of evidence
supporting the theory, called hierarchical formation, that large galaxies come
about in steps as smaller galaxies are pulled together by their mutual
gravitational attraction and blend to form more massive galaxies. In this
version, galaxies would form most of their stars early on as small galaxies,
but accumulate most of their mass later through
mergers.
The competing hypothesis, called monolithic collapse, posits
that giant galaxies form all at once, with the bulk of star formation happening
at the same time as the galaxy gains the bulk of its mass.
"Tran's paper is showing these galaxies in the process
of assembling at a later epoch than when their stars formed," said Romeel Dave,
an astrophysicist at the University of Arizona who was not involved in the
research. "I don't think monolithic collapse is yet dead in everyone's
mind, but I think the majority [of astronomers] have come around in the last
few years, particularly with direct evidence, such as in this paper, that these
galaxies, which obviously have very old stars, show signs of still forming."
Another study, published in 2005 by Pieter van Dokkum of
Yale University, found a large number of established galaxies with old stars
that displayed signs of having recently merged with other galaxies to add on to
their mass. Observational findings such as these, which show that galaxies can
continue to grow long after they have formed most of their stars, confirm the
theoretical predictions of the hierarchical formation model.
"I think the observational results have only come
within the last 5 to 10 years," Dave told SPACE.com. "Theoretically
this has been accepted for a long time."
Questions remain
Though many astronomers agree that hierarchical formation
seems to be occurring, there are still some wrinkles to the theory that need to
be ironed out.
For example, the very most massive galaxies don't seem to be
growing at as high a rate as middle-mass galaxies. When astronomers look at the
brightest galaxies now compared to the brightest galaxies at an earlier time
(by looking farther away researchers can peer back in time
because distant light has taken longer to reach us), they don't seem to have
gained much mass.
"Why aren't the largest galaxies growing in that
way?" Dave said. "I think that's an unsolved problem now."
It suggests to astronomers that there might be an upper
ceiling to how large a galaxy can grow. Perhaps when a galaxy gets to be very
large, its gravity is so strong that it rips up smaller galaxies that pass
nearby before they can join it.
Another question is why, if all galaxies are mash-ups of
smaller ones, many of them don't look it. Beautiful spiral galaxies, for
instance, appear neat and symmetrical, not as though they were formed from
violent collisions of multiple smaller galaxies.
"When we look at merging galaxies, they look like train
wrecks," Tran said. "But maybe they only look like train wrecks for a
relatively short amount of time."
Perhaps there are stabilizing forces, such as the galaxies'
angular momentum and the large halos of dark matter that surround them, that
help galaxies regain their orderly spiral structure after a merger.
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