Astronomers Spot a Disk Orbiting a Star in Another Galaxy For The First Time Ever

Signs of an extended disc of gas and dust circling a far-off star have been observed by astronomers.

An artist's impression of the newly discovered disk

This is not unusual in any way. This phase of a star's and its planetary system's development is typical. The fact that this discovery is the first to be observed around a star outside of our own galaxy is what makes it so amazing.

The feature was observed in a dwarf galaxy located approximately 179,000 light-years away from the Milky Way, called the Large Magellanic Cloud. Furthermore, even though it would seem obvious to assume that star formation processes are universal, we have never been able to observe the whims of these processes outside of our own galaxy.

Astronomer Anna McLeod of Durham University in the UK says, "I could not believe that we had detected the first extragalactic accretion disc when I first saw evidence for a rotating structure in the ALMA data; it was a special moment."

"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."

Dense clusters in clouds of molecular gas and dust that linger in interstellar space give birth to stars. A clump that becomes too dense to support itself collapses due to gravity; as it spins, more material from the surrounding cloud is drawn in. However, instead of falling onto the protostar in any old fashion, this material forms a disc around the equator of the star and descends onto it in a more regulated, steady stream, akin to water running down a drain.

MUSE and ALMA images revealing the disk in the system HH 1177

The disc that is left over after the star has finished forming remains there and coalesces to form all the other components of a planetary system, including the planets, comets, asteroids, and meteors. Because of this, the planets in the Solar System essentially orbit the Sun in a flat plane. Like the sentient mould that developed on the Sun's breakfast leftovers, we are also like that.

Numerous such discs in varying stages of development have been imaged by the powerful radio telescope Atacama Large Millimeter/submillimeter Array (ALMA); some have obvious gaps that are thought to be cleared by planets clumping together as they orbit. However, even with a powerful telescope, things become more difficult to resolve the farther away they are.

Upon discovering indications of a jet in the HH 1177 system, data from the Multi Unit Spectroscopic Explorer (MUSE) instrument on the Very Large Telescope prompted McLeod and her associates to begin their search for an extragalactic stellar disc.

These are also a telltale sign of star formation: part of the material whirling around the forming star is carried away to the poles by the star's magnetic field, where it is released into space as a strong jet.

The location and orientation of the jets and disk identified in HH 1177.

Using ALMA to search for rotational signals, the researchers hoped to detect the disc in the dusty centre of star formation. The way light wavelengths get shorter when the source is pushed towards us and longer when it is pulled away is one way to observe this.

The speed at which the gas emitted the light is travelling towards or away from us determines the frequency of light, according to astronomer Jonathan Henshaw of Liverpool John Moores University in the United Kingdom. "This is precisely the same phenomenon that occurs when the pitch of an ambulance siren changes as it passes you and the frequency of the sound goes from higher to lower."

It's interesting to note that this rotation was clearly visible in the ALMA data. The team's analysis showed that the star is massive and extremely young, still receiving food from the disc surrounding it. This is rather typical. However, the HH 1177 disc differs from the protostellar discs in the Milky Way in that it is visible at optical wavelengths.

According to the researchers, this is related to the Large Magellanic Cloud's interstellar environment. Because there is a significant decrease in dust, the HH 1177 star is not as covered in material as young, massive Milky Way stars typically are.

Because of this, the discovery is significant for understanding not only how stars form in various environments, but also the constraints that these environments may have on star formation in general.

"We are in an era of rapid technological advancement when it comes to astronomical facilities," says McLeod. "Being able to study how stars form at such incredible distances and in a different galaxy is very exciting."

The research has been published in Nature.

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