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Signs of a 'fossil galaxy' are spotted lurking in the heart of the Milky Way

Signs of a 'fossil galaxy' are spotted lurking in the heart of the Milky Way

Saturday 21st November 2020

Our very own Milky Way galaxy had a dramatic childhood. Astronomers have unveiled a new chapter in its memoir with the discovery of a likely "fossil galaxy" hidden near its heart.

The proposed fossil galaxy is named Heracles for the Greek hero. It probably tangled with the Milky Way around 10 billion years ago, back when our galaxy was a baby.

"Stars originally belonging to Heracles account for roughly one-third of the mass of the entire Milky Way halo today -- meaning that this newly-discovered ancient collision must have been a major event in the history of our galaxy," the Sloan Digital Sky Survey (SDSS) said in a statement Thursday. The SDSS was involved in the research.

A research team led by Liverpool John Moores University (LJMU) graduate student Danny Horta published a paper on Heracles this week in the Monthly Notices of the Royal Astronomical Society journal. 

To find Heracles, the team spotted stars that didn't match the Milky Way's. "These stars are so different that they could only have come from another galaxy. By studying them in detail, we could trace out the precise location and history of this fossil galaxy," said Horta.

We've seen evidence of dramatic galactic mergers in the Milky Way's deep past. Recent studies have looked into a time when our galaxy gobbled up a dwarf galaxy called Gaia-Enceladus. 

Heracles has been particularly elusive since signs of its existence are obscured by interstellar dust clouds. The research team used the SDSS Apache Point Observatory Galactic Evolution Experiment (APOGEE) to peer through this mess using near-infrared light. "To find a fossil galaxy like this one, we had to look at the detailed chemical makeup and motions of tens of thousands of stars," said study co-author and LJMU astrophysicist Ricardo Schiavon.

The Milky Way may not be done with its galaxy-colliding ways. A Milky Way-Andromeda galaxy crash is lurking billions of years into the future. It's tough being a galaxy. Sometimes you're the Milky Way, eating them up. Sometimes you're Heracles, the one getting swallowed. 

An ultramassive monster galaxy dating back to the early days of the universe lived fast and died young, astronomers say in a new study.

The galaxy, MXM-2599, existed 12 billion years ago. It rapidly formed a bunch of stars and died, according to data and measurements astronomers took using the W. M. Keck Observatory's Multi-Object Spectrograph for Infrared Exploration. The observatory is near the summit of the dormant Mauna Kea volcano in Hawaii.

"Even before the universe was 2 billion years old, MXM-2599 had already formed a mass of more than 300 billion suns, making it an ultra massive galaxy," said Benjamin Forrest, lead study author and a postdoctoral researcher in the University of California, Riverside's Department of Physics and Astronomy.

"More remarkably, we show that MXM-2599 formed most of its stars in a huge frenzy when the universe was less than 1 billion years old, and then became inactive by the time the universe was only 1.8 billion years old."

The possible evolution of an unusual monster galaxy is shown from left to right, from its beginning as a massive galaxy bursting with star formation, transitioning to a dead galaxy and perhaps its eventual fate: becoming a bright cluster galaxy.

During its heyday, the galaxy was able to create more than 1,000 solar masses of stars in a year. That's an incredibly high rate of star formation when contrasted with our own galaxy, the Milky Way, and its one new star per year.

"MXM-2599 may be a descendant of a population of high star-forming, dusty galaxies in the very early universe that new infrared telescopes have recently discovered," said Danilo Marchesini, study co-author and an associate professor of astronomy at Tufts University in Massachusetts.

But astronomers are unsure of how the giant galaxy evolved. It's unusual when compared to other known galaxies in size. And based on their models, MXM-2599 should still be forming stars.

"What makes MXM-2599 so interesting, unusual, and surprising is that it is no longer forming stars, perhaps because it stopped getting fuel or its black hole began to turn on," said Gillian Wilson, a professor of physics and astronomy at the University of California, Riverside. Forrest works in her lab.

"Our results call for changes in how models turn off star formation in early galaxies. We have caught MXM-2599 in its inactive phase."

The Galaxy can't lose mass, it's just not forming stars anymore, so astronomers wonder what will become of it.

"As time goes by, could it gravitationally attract nearby star-forming galaxies and become a bright city of galaxies?" Wilson hypothesized.

The astronomers have been granted more time at the observatory to continue studying the strange galaxy and hopefully discover answers to their questions.

"Perhaps during the following 11.7 billion years of cosmic history, MXM-2599 will become the central member of one of the brightest and most massive clusters of galaxies in the local universe," said Michael Cooper, study co-author at the University of California, Irvine.

"Alternatively, it could continue to exist in isolation. Or we could have a scenario that lies between these two outcomes."