Galaxy Formation: A Clumpy Affair
Four groups of merging galaxies located about 4 billion light-years away. Their discovery provides validation for the hierarchical theory of galaxy formation.
Credit: ESO

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 Z?rich, 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 Dav?, 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," Dav? told "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?" Dav? 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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