Astronomers spot young stars at the edges of the Milky Way, which is usually home to the galaxy's oldest stars
These newborn stars, which probably formed from material from two dwarf galaxies known as the Magellanic Clouds, could unravel new insights into the Milky Way's history
Astronomers have discovered a group of young stars in an unexpected place: the outskirts of the Milky Way, which are usually home to the galaxy's oldest stars. What is more surprising is that the infant stars have an extragalactic origin.
What this implies is that the stars seemingly formed not from material from the Milky Way, but two nearby dwarf galaxies known as Magellanic Clouds, according to researchers from Flatiron Institute, Simons Foundation, and Department of Astrophysical Sciences, Princeton University, among others.
According to experts, those galaxies are on a collision course of our own. The discovery suggests that a stream of gas extending from the galaxies is about half as far from crashing into the Milky Way as previously thought.
The newfound cluster is relatively young at 117-million-years-old and is on the far outskirts of the Milky Way. It is further than any of the known young stars in the Milky Way, which are typically in the disk.
"We have identified a young, metal-poor stellar association in the Galactic halo—named Price-Whelan 1. The age of the cluster is broadly consistent with the time it would have most recently crossed the Galactic midplane, suggesting the possibility that interaction with the Milky Way disk or tidal compression could have triggered this star formation event," say researchers.
"Previous studies have detected young stars in the leading arm and in the periphery of the Large and Small Magellanic Clouds (LMC and SMC). However, this is the first time that an entire young star cluster has been detected so far from the Clouds," add the researchers in their findings.
These stars could reveal new insights into the Milky Way's history, as well as aid future Milky Way modeling efforts, says the research team. They might, for example, tell if the Magellanic Clouds collided with our galaxy in the past.
It also provides an opportunity to study star formation in a unique environment — that is, low gas density, low metallicity, and large velocity — unlike that of the Milky Way disk or any other Local Cluster-forming region.
"This is a puny cluster of stars — less than a few thousand in total — but it has big implications beyond its local area of the Milky Way," says primary discoverer Adrian Price-Whelan, a research fellow at the Flatiron Institute's Center for Computational Astrophysics in New York City.
The findings were presented at the American Astronomical Society meeting in Honolulu on January 8. The researchers previously reported the discovery of Price-Whelan 1 in The Astrophysical Journal, and their subsequent spectroscopic analysis of the stars, also in The Astrophysical Journal.
According to scientists, it is challenging to identify clusters of stars because our galaxy is chock-full of the radiant orbs. Some stars may appear to be close together in the sky but actually, sit at drastically different distances from Earth.
Other stars may temporarily neighbor one another but move on in opposite directions. Hence, determining which stars are clustered together requires many precise measurements over time, say experts
The research team started with the latest data collected by the Gaia spacecraft, which has measured and cataloged the distances and motions of 1.7 billion stars.
They searched the Gaia dataset for very blue stars, which are rare in the universe, and identified clumps of stars moving alongside them. After cross-matching with the removing known clusters, one remained.
The cluster inhabits a region near a river of gas, dubbed the Magellanic Stream, that forms the outmost edge of the Large and Small Magellanic Clouds and reaches toward the Milky Way. Gas in the stream does not contain much metal, unlike gases in the outer reaches of the Milky Way.
According to the researchers, the cluster could have formed as gas from the Magellanic Stream passed through the gases surrounding the Milky Way. They explain this "pass-through created a drag force" that compressed the Magellanic Stream gas.
This drag, along with tidal forces from the Milky Way's gravitational tug, condensed the gas enough to trigger star formation. Over time, the stars zoomed ahead of the surrounding gas and joined the Milky Way.
The discovery "suggests that young stars can form from tidally stripped gas during low-mass mergers and may, therefore, exist throughout the otherwise aging stellar halo,” say the researchers in their findings.
They add, "The serendipitous discovery of this cluster is a reminder that the combined value of the Gaia data with deep, large-area imaging surveys provides a wealth of information about our Galaxy and stellar halo."
The stars also presented researchers with a unique opportunity. Using the current positions and movements of stars in the cluster, researchers predict that the edge of the Magellanic Stream is 90,000 light-years away from the Milky Way. This is roughly half the distance previously predicted.
The updated distance to the Magellanic Stream will improve models of where the Magellanic Clouds have been and where they are headed, say experts.
The improved numbers could even settle a debate over whether the Magellanic Clouds have crossed through the Milky Way before. Finding an answer to that question will help astronomers better understand the history and properties of our galaxy.
"If the Magellanic Stream is closer, especially the leading arm closest to our galaxy, then it's likely to be incorporated into the Milky Way sooner than the current model predicts," says David Nidever, assistant professor of physics at Montana State University in Bozeman.
"Eventually, that gas will turn into new stars in the Milky Way’s disk. Right now, our galaxy is using up gas faster than its being replenished. This extra gas coming in will help us replenish that reservoir and make sure that our galaxy continues to thrive and form new stars," adds Nidever, who led an analysis of the metal content of the 27 brightest stars in the cluster.