No Merger Needed: A Rotating Ring of Gas Creates A Hyperluminous Galaxy

Some galaxies experience rapid star formation hundreds or even thousands of times greater than the Milky Way. Astronomers think that mergers are behind these special galaxies, which were more abundant in the earlier Universe. But new results suggest no mergers are needed.

These galaxies are called Hyper Luminous Infrared Galaxies (HyLIRGs), and they emit most of their energy in the infrared. New research examined a HyLIRG that’s 10,000 times brighter than the Milky Way in infrared. Instead of a chaotic merger, they found an organized rotating ring of gas that they say is responsible for the galaxy’s abundant star formation.

Their results are in a paper in Nature Astronomy titled “Detailed study of a rare hyperluminous rotating disk in an Einstein ring 10 billion years ago.” The lead author is Daizhong Liu, a Research Professor at Purple Mountain Observatory near Nanjing, China.

HyLIRGs are the rarest type of starburst galaxy, and they’re the most extreme type. They’re found only in the distant, ancient Universe. The galaxy is named PJ0116-24 and has a redshift of z=2.125. That redshift value means the light we’re seeing was emitted about 10.5 billion years ago, and the distant galaxy is now about 16 billion light-years away. At that distance, astronomers had to use gravitational lensing to look at the galaxy. That not only magnified the galaxy, it created an Einstein Ring.

This image is a VLT MUSE image of PJ0116-24 distorted into an Einstein Ring by a gravitational lens. The foreground lens is not removed in this image. Image Credit: Liu et al. 2024.

The researchers used a pair of telescopes to observe the galaxy. The Very Large Telescope traced the warm gas with its Enhanced Resolution Imager and Spectrograph (ERIS) instrument, and the Atacama Large Millimetre/submillimetre Array traced the cold gas. By combining the observations from both, the astronomers found an organized ring of rotating gas. If a merger had occurred and triggered the galaxy’s abundant star formation, an organized structure like this wouldn’t have been present. Instead, the galaxy’s morphology would be much more chaotic.

The authors write, “A widely accepted scenario is that HyLIRGs are the distant higher-luminosity tail of the local ultra-luminous IR galaxies with extreme starburst activities triggered by major mergers.” Another possibility is that these galaxies are very young and are experiencing their maximum star formation rates associated with youth. The problem is that astronomers haven’t observed enough of them to be certain exactly what’s going on.

This galaxy was identified by the Planck All-Sky Survey to Analyze Gravitationally-lensed Extreme Starbursts project (PASSAGES), which found about 20 HyLIRGs. PJ0116-24 is the brightest one found in the southern sky.

This image from the research shows how the gravitational lensing created an Einstein Ring. It’s a distorted but still scientifically revealing image of the distant HyLIRG PJ0116-24. The gravitational lensing creates two images of the galaxy, with two AGN, labelled A1 and A2. The foreground lens has been removed from the image. Blue to green colours show stars, and red shows the cold gas out of which more stars form. (Note that the Einstein Ring is an artifact of gravitational lensing and is not the gaseous ring that the researchers found. That ring is revealed in velocity maps.) Image Credit: Liu et al. 2024.

The authors write, “We found PJ0116-24 to be highly rotationally supported with a richer gaseous substructure than other known HyLIRGs. Our results imply that PJ0116-24 is an intrinsically massive and rare starburst disk probably undergoing secular evolution.” Its star formation rate (SFR) is 1,490 solar masses yr^-1.

Simulations predict that the maximum SFR is greater than or equal to 1,000?solar masses yr^-1. If these observations are correct, then they show that a galaxy can reach its maximum SFR even if it is alone and hasn’t been involved in a merger.

“Unlike almost all other extreme HyLIRGs, which are major mergers, PJ0116-24 does not obviously have massive companions or disturbed kinematics as evidence for major mergers,” the authors explain in their paper.

These velocity maps clearly show a coherent rotating gaseous ring structure in PJ0116-24. If the galaxy’s rapid SFR were because of a merger, no such orderly structure would be present. Image Credit: Liu et al. 2024.

The galaxy also shows much higher metallicity than others in the early Universe. “These diagnostics indicate solar to supersolar metallicity,” the authors write. “This is much higher than in non-starburst galaxies at the same redshifts.”

Amit Vishwas is a postdoc at the Cornell Center for Astrophysics and Planetary Sciences. He’s a co-author of this paper and a previous paper in 2023 that used the JWST to observe another galaxy at an earlier epoch with similar gas conditions and metallicity. PJ0116-24 is about five times more massive and luminous than that one. Vishwas says both of these galaxies are helping astronomers build a better picture of how galaxies evolve.

“In both cases, gravitational lensing helped us zoom in to study the details of the interstellar medium of these galaxies,” Vishwas said in a press release. “I believe these new observations are helping us build an argument for the way galaxies evolve and build up – efficiently converting gas to stars in rapid growth spurts separated by long periods of relative calm.”

“The robust confirmation of PJ0116-24 as the most rotationally supported HyLIRG from this work is key evidence suggesting that secular evolution, that is, without recent major mergers, can be responsible for maximal star formation in the Universe,” the authors conclude in their work.

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