Observing an Extragalactic Star's Disc: A Remarkable Cosmic Achievement

An artist's depiction presents the HH 1177 system, which can be found in our neighbouring galaxy, the Large Magellanic Cloud. The brightly glowing object at the centre is a youthful and massive star, which is simultaneously collecting matter from a dusty disc and expelling it in robust jets. A team of astronomers, armed with the Atacama Large Millimeter/submillimeter Array (ALMA), a partnership that ESO is a part of, managed to discover traces of this disc by observing its rotation. The discovery marks the first time that the disc around a young star — identical to those leading to planetary formation in the Milky Way — has been spotted in a foreign galaxy. Credit: ESO/M. Kornmesser.

Help from a Rice astronomer leads to the discovery of an accretion disk encompassing a young star located outside the Milky Way galaxy.

Proof of a rotating disc of material orbiting a large young star in a nearby galaxy has been discovered by astronomers for the first time. Assistant Professor of physics and astronomy at Rice University, Megan Reiter, was part of the research team responsible for announcing the discovery, as documented in a study published in Nature.

"This discovery provides powerful evidence that high-mass stars that are significantly larger than the Sun form in the same manner as their lower-mass counterparts," said Reiter, addressing a long-standing question.

The discovery occurred in the Large Magellanic Cloud, a galaxy neighbouring the Milky Way. The detecting of the disc-wearing star was facilitated by the signature attribute of a star in formation known as a protostellar jet.

Observations with ESO’s Very Large Telescope (VLT) and the Atacama Large Millimeter/submillimeter Array (ALMA), of which ESO is a partner, has enabled the sighting of a disc around a young star in a different galaxy. While the original discovery was facilitated by the Multi Unit Spectroscopic Explorer (MUSE) on the VLT, additional data was gathered by ALMA about the rotating disc around the star. Credit: ESO/ALMA (ESO/NAOJ/NRAO)/A. McLeod et al.

"The disk forms as the cloud of matter surroundings the star collapses," Reiter explained. "The star is fed by the disc with material, which the star will in turn expel 1-10% of it as large bipolar jets. Being a part of the star's accretion process, these jets provide a record of the star's history and formation."

The jet's initial sighting was facilitated by the Multi Unit Spectroscopic Explorer instrument which is stationed on the European Southern Observatory’s Very Large Telescope.

A real-life photo of the young star system HH 1177 was captured in the Large Magellanic Cloud, a close neighbour of the Milky Way. The image was captured by the Multi Unit Spectroscopic Explorer (MUSE) on ESO's Very Large Telescope (VLT), showing the jets being propelled from the star. Later, astronomers used the Atacama Large Millimeter/submillimeter Array (ALMA) to discover a disc encircling the young star. An artist's assumption of the system including the jets and disc is illustrated in the right panel. Credit: ESO/A. McLeod et al./M. Kornmesser.

"Once we spotted the jet, the next natural question was whether a disc surrounds that star," Reiter said.

The hypothesis was validated by the data collected about the young star and its neighbourhood by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

"Detecting a disk around a star of high mass poses a challenge due to its relatively short lifespan," as explained by Reiter. A low-mass star, like the Sun with an approximate lifespan of 10 billion years, would have a disc for only between 3-10 million years during its formation.

This breath-taking region within the Large Magellanic Cloud (LMC), teeming with newly formed entities, was captured by the Multi Unit Spectroscopic Explorer instrument of ESO’s Very Large Telescope. The relatively minor amount of dust in the LMC together with the acuteness of MUSE's vision enabled the intricate details of the region to be visible in visible light. Credit: ESO, A McLeod et al.

In the Milky Way, at least, the dust swirling around heavy stars tends to obscure the star's surroundings, intimidating any attempts of observing a disc in formation. Thankfully, the Large Magellanic Cloud vastly improves visibility due to its star-forming matter being different.

“It’s arguably more exciting to discover a disc in this neighboring galaxy as opposed to our own, because the conditions there are closer to what we think things were like earlier in the universe,” Reiter said. “It’s like we’re getting a window into how stars formed earlier on in the evolution of the universe.”

Megan Reiter is an assistant professor of physics and astronomy at Rice University. Credit: Brandon Martin/Rice University

Anna McLeod, an associate professor at Durham University in the U.K. and lead author of the study, said that upon seeing evidence for a rotating structure in the ALMA data, she and her team could scarcely believe they had detected the first extragalactic accretion disc.

“It was a special moment,” McLeod said. “We know discs are vital to forming stars and planets in our galaxy, and here for the first time we’re seeing direct evidence for this in another galaxy.”