The largest galaxies in the Universe aren't beautiful spirals like our Milky Way; they are enormous egg-shaped structures known as giant elliptical galaxies. We don't know how they formed, but observations of very distant and bright galaxies revealed information about the formation of smaller elliptical galaxies. The giants remained mysterious.
Where one galaxy is insufficient, two may do instead. A new set of observations caught two bright elliptical galaxies right before the act of merging into one that would have a combined mass large enough to make the equivalent of 400 billion Suns. Hai Fu and colleagues determined that these galaxies collided more than 10 billion years ago and that the merger was driving extremely rapid star formation, at least ten times the rate seen in ordinary galaxies. Based on these observations, the researchers concluded that such collisions could be responsible for the birth of the largest galaxies, allowing for most of them to finish forming by 9.5 billion years ago.
Nearby elliptical galaxies contain virtually no young stars and are poor in the raw ingredients of star formation—gas and dust. However, based on observation, those stars must have formed fairly rapidly as a group about 10 billion years ago. Such aggressive star formation would pump a lot of light out, leading to extremely bright galaxies.
Astronomers identified a class of objects known as submillimeter bright galaxies (SBGs) from surveys of the distant Universe that appear to serve as the progenitors of at least some elliptical galaxies. However, SBGs weren't massive enough to explain the formation of the largest ellipticals, such as those seen at the centers of galaxy clusters. These immense objects must have formed fairly early on in the Universe's history, because they are quiet now; there is no star formation or any of the signs of mergers in their recent past.
However, astronomers also know that large galaxies can eat smaller galaxies or form through the merger of two lesser bodies. Catching this process in the act is another matter, which makes the newly published study interesting.
Researchers first identified the colliding galaxies as an extremely bright object in the Herschel Multi-tiered Extragalactic Survey (HerMES), which was given the label of HXMM01. (HerMES used the infrared Herschel Space Telescope, which has now exhausted its coolant supply.) Follow-up observations in radio and optical light by the authors of the present study determined that HXMM01 was actually two SBGs, barely separated in the sky from our perspective. Measurement of the galaxies' spectra made it clear that they were at the same distance from Earth rather than coincidentally appearing near each other (like stars in a constellation).
The researchers determined that HXMM01 is nearly 4 times as luminous as typical SBGs at the same distance, indicating extraordinarily rapid star formation. Based on models of galaxies, they estimated that the combination was forming somewhere between 1,600 and 2,400 solar masses of new stars each year. (One solar mass is the mass of the Sun, but most stars are smaller than that.) Taking that into account, the astronomers predicted that the two SBGs merged to make a single giant elliptical galaxy with stars equivalent in mass to 400 billion Suns.
These results could explain the origin of some of the biggest galaxies in the Universe, filling in the gap left by observations of SBGs. Larger surveys of galaxies at these great distances should be able to confirm this hypothesis by providing more observations of mergers when the Universe was less than 4 billion years old.
Nature, 2013. DOI: 10.1038/nature12184 (About DOIs).
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