Astronomers have argued for years over whether massivegalaxies form from scratch, or by chunking together smaller galaxies.
Lately, evidence is building for the latter theory, and anew study adds to the growing picture of galaxy formation as a clumpyaffair. Using an array of both ground-based and space telescopes, includingESO's Very Large Telescope in Chile and the Hubble Space Telescope, a team of astronomersrecently observed groups of huge galaxies in the process of merging, showingthat large, established galaxies can still grow bigger.
"The question was whether or not you could still formvery massive galaxies at relatively recent times through these merging processes,"said researcher Kim-Vy Tran of the University of Z?rich, Switzerland. "Wesaw three examples of this happening now."
Tran and colleagues observed the meshing galaxies in a groupcalled Sg1120-12 about 4 billion light-years away. The team detailed theirdiscovery in the August issue of the Astrophysical Journal Letters.
The new finding adds to a mounting list of evidencesupporting the theory, called hierarchical formation, that large galaxies comeabout in steps as smaller galaxies are pulled together by their mutualgravitational attraction and blend to form more massive galaxies. In thisversion, galaxies would form most of their stars early on as small galaxies,but accumulate most of their mass later throughmergers.
The competing hypothesis, called monolithic collapse, positsthat giant galaxies form all at once, with the bulk of star formation happeningat the same time as the galaxy gains the bulk of its mass.
"Tran's paper is showing these galaxies in the processof 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 theresearch. "I don?t think monolithic collapse is yet dead in everyone'smind, but I think the majority [of astronomers] have come around in the lastfew years, particularly with direct evidence, such as in this paper, that thesegalaxies, which obviously have very old stars, show signs of still forming."
Another study, published in 2005 by Pieter van Dokkum ofYale University, found a large number of established galaxies with old starsthat displayed signs of having recently merged with other galaxies to add on totheir mass. Observational findings such as these, which show that galaxies cancontinue to grow long after they have formed most of their stars, confirm thetheoretical predictions of the hierarchical formation model.
"I think the observational results have only comewithin the last 5 to 10 years," Dav? told SPACE.com. "Theoreticallythis has been accepted for a long time."
Though many astronomers agree that hierarchical formationseems to be occurring, there are still some wrinkles to the theory that need tobe ironed out.
For example, the very most massive galaxies don't seem to begrowing at as high a rate as middle-mass galaxies. When astronomers look at thebrightest galaxies now compared to the brightest galaxies at an earlier time(by looking farther away researchers can peer back in timebecause distant light has taken longer to reach us), they don't seem to havegained much mass.
"Why aren?t the largest galaxies growing in thatway?" Dav? said. "I think that's an unsolved problem now."
It suggests to astronomers that there might be an upperceiling to how large a galaxy can grow. Perhaps when a galaxy gets to be verylarge, its gravity is so strong that it rips up smaller galaxies that passnearby before they can join it.
Another question is why, if all galaxies are mash-ups ofsmaller ones, many of them don't look it. Beautiful spiral galaxies, forinstance, appear neat and symmetrical, not as though they were formed fromviolent collisions of multiple smaller galaxies.
"When we look at merging galaxies, they look like trainwrecks," Tran said. "But maybe they only look like train wrecks for arelatively 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, thathelp galaxies regain their orderly spiral structure after a merger.
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Clara Moskowitz is a science and space writer who joined the Space.com team in 2008 and served as Assistant Managing Editor from 2011 to 2013. Clara has a bachelor's degree in astronomy and physics from Wesleyan University, and a graduate certificate in science writing from the University of California, Santa Cruz. She covers everything from astronomy to human spaceflight and once aced a NASTAR suborbital spaceflight training program for space missions. Clara is currently Associate Editor of Scientific American. To see her latest project is, follow Clara on Twitter.