Observations from several radio telescopes reveal that, when two galaxies merge, their progeny often have gaseous disks — a hypothesis that before now didn’t have solid observational evidence.
When it comes to merging galaxies, a crash doesn’t always create a mess. For a long time, astronomers thought it did. Conventional wisdom (based on computer simulations from the 1970s) said that when two big disk-shaped galaxies merged, they’d create a big elliptical, a fairly featureless spheroid of stars.
But about a decade ago, new-and-improved simulations by several teams suggested that, if the disk galaxies have a lot of gas, the object their merger creates will also be a disk galaxy, with spiral arms or maybe even a bar in the center.
The reason is angular momentum. In a galaxy’s disk, stars and gas rotate together around the galactic center. When the disk galaxies merge and their material mixes, the stars can steal rotational energy from the gas via gravitational interactions, begetting a bulbous shape — but gas doesn’t steal from other gas. Instead, gas from both galaxies “shares” rotational energy. When there aren’t enough stars to steal the gas’s angular momentum, the material will inevitably settle down and create a disk in the newly formed galaxy.
Junko Ueda (National Astronomical Observatory of Japan) and colleagues have now confirmed this prediction with observations. The team looked at carbon monoxide (CO) emission from 37 merger-created galaxies, using both new and archival data from a cadre of facilities. CO is a standard “tracer” compound that astronomers use to track the presence of cold molecular gas, from which stars form. Of the 37 galaxies, the team easily detected CO in 30.
Of those 30 gassy galaxies, 24 showed signs of disk rotation; the others seem to be too clumpy or complicated. Eighteen have hints of a ring or bar structure.
However, that doesn’t mean that all 24 are settling down to be grandiose spiral galaxies. The astronomers also compared the size of the disks to how far out the star-dominated bulge extended. They found that 13 of the 24 have disks that are smaller than the stellar component, meaning that the rotating disk hides inside the larger stellar bulge and that these galaxies will become ellipticals. Previous observations have revealed compact, molecular gas disks in ellipticals, confirming such structures exist.
The remaining 11 of 24 have big gas disks, meaning they’re on their way to forming pancake-shaped galaxies, either spirals or so-called lenticular (“lens-shaped”) galaxies.
In short, roughly half (46%) of the galaxies created by mergers are disk galaxies. The team can’t actually confirm that the galaxies that created the 37 they observed were also disk-shaped — but given that the long-gone progenitors had a whole lot of gas and the right rotational properties to create gas disks, it’s not crazy to connect the result with predictions for disks’ mergers.
Regardless, the observations confirm that, indeed, disks are a common byproduct of galaxy mergers. That makes sense: big spirals are the most common type of galaxy in today’s universe, and so the fact that mergers can create them helps explain why.
If you'd like a visual guide to this process, NAOJ has created the video below to help. This artist’s impression shows two disk-shaped galaxies merging to create a new disk galaxy. The video then changes to connect the disk’s rotation with what the radio-wavelength observations reveal. Gas that is moving away from us is marked red; blue means the gas is approaching. The combination of the radio contours and the toward-and-away movement indicate a gas disk that is rotating about the galaxy’s center. (Be patient if the movie takes a bit to load.)
Reference: J. Ueda et a. “Cold Molecular Gas in Merger Remnants. I. Formation of Molecular Gas Disks.” Astrophysical Journal Supplement Series, September 2014.
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