Milky Way ate another galaxy

 Scientists think they have found bones of a galaxy called Loki, ate by The Milky Way        

An unusual collection of stars may represent the remnants of a dwarf galaxy that the Milky Way devoured about 10 billion years ago. Astronomers have dubbed the ancient galaxy Loki. The finding could change the current understanding of how the Milky Way evolved in the distant past. An unusual collection of stars may represent the remnants of a dwarf galaxy that the Milky Way devoured. The finding could change the current understanding of how the Milky Way evolved in the distant past. The vast Milky Way spans about 100,000 light-years and contains anywhere between 100 billion and 400 billion stars. A light-year is the distance light travels in one year, which is 5.88 trillion miles (9.46 trillion km's).  It grew over time starting about 12 billion years ago by merging with a multitude of dwarf galaxies. But the original size and mass of the Milky Way remain an open question, driving scientists to search for evidence of the galaxies it consumed to determine its history and evolution. To identify those missing puzzle pieces, astronomers have now zeroed in on a cluster of metal-lacking stars detected oddly close to the galactic disk. 

By studying the chemistry of these stars and their motion close to the galactic disk, the researchers found that the stars' home galaxy, nicknamed "Loki," might have merged with our galaxy about 10 billion years ago. Studying these stars could be "very important" in understanding the history of the Milky Way and the universe itself, lead author of the study Federico Sestito, an astrophysicist at the University of Hertfordshire in the U.K, said. Sestito added that Loki may have been among "the very first small galaxies formed in the young universe." The Milky Way's Central Molecular Zone (CMZ) surrounds our galaxy's supermassive black hole and may share characteristics with the dense and chaotic galaxies of the early universe. Massive galaxies are not born whole. They are assembled over billions of years through mergers with smaller galaxies, which are sometimes absorbed. In the early universe, shortly after the Big Bang, matter clumped into clouds of gas which collapsed into the first primitive galaxies. These small systems then fell into one another, merged and gradually built up into the large structures we see today. The astronomers are interested in these stars near the disk, a massive rotating pancake-like region containing much of the Milky Way’s stars, because the first stars in the universe were comprised of hydrogen and helium, which fused heavier elements together in their cores before exploding and unleashing the heavy elements which enriched future generations of stars. Metal-poor stars are often associated with ancient dwarf galaxies, which the Milky Way might have consumed over time to grow to its current massive state, and remnants of these cosmic meals might be hiding deep within the galaxy. The metal-poor composition of such ancient stars close to the galactic disk suggests that the Milky Way once made a rather large meal of another galaxy early in its history, and it could represent a critical, previously overlooked building block of our galaxy.

In the study, astronomers identified 20 old, very-metal-poor stars orbiting unusually close to the galactic disk, the flat, rotating region of the Milky Way where most stars, including the sun, reside, and examined whether a past merger might explain what they were seeing. The very first stars which formed in the universe were made of hydrogen and helium. It was only inside those early stars that hydrogen and helium fused into heavier elements, which astronomers call metals. These stars, when they eventually exploded, enriched the surrounding gas with those metals. Each successive generation of stars was therefore born from material slightly more enriched than the last. As these small galaxies collided and merged, their stars, gas and dark matter became part of the growing young Milky Way. Because of this, computer simulations suggest that stars from the earliest mergers are expected to be found deeper inside the Milky Way today, while stars from galaxies which merged later are more likely to be scattered farther out in the galactic halo, a vast, spherical region that extends beyond the bright disk. However, very few metal-poor stars have been found in the inner regions of the Milky Way to test this idea. So, when the team identified 20 metal-poor stars orbiting close to the galactic disk, they wondered whether the stars could be remnants of an ancient merger.

The Milky Way is suspected to have merged with up to a dozen or more dwarf galaxies over its 12-billion-year history. Astronomers are like the detectives of the universe, searching the cosmos for clues of its origins, and very-metal-poor, or VMP, stars are a powerful tool in that quest, said Dr. Cara Battersby, associate professor of physics at the University of Connecticut, who did not participate in the study. “VMP stars have been around for billions of years, holding within them clues to the formation of the Universe’s earliest generations of stars,” Battersby said. Studying the metal-poor stars’ composition and motion can unlock details about the conditions and dynamics of the early universe, she added. The search for metal-poor stars in the Milky Way has largely centered on the plentiful range of old stars in the galaxy’s stellar halo, so named because it’s a large, round diffuse cloud that surrounds the galactic disk. Some astronomers believe evidence of more ancient mergers could be found deeper inside the Milky Way, such as in its disk. An abundance of young, metal-rich stars, as well as a plethora of dust, crowded within the galactic disk has made it hard to spot metal-poor stars there, said Dr. Federico Sestito.

The team observed existing catalog of metal-poor stars using a powerful spectrograph at the Canada-France-Hawaii Telescope, which revealed their chemical abundances. Using precise positional data from the Gaia space telescope, they calculated the stars' distances and how they orbit in our galaxy. Sestito said that "a mixture of information from the chemistry and the orbits of these stars" nudged them to examine the stars' origin. Rather than drifting through the halo of the galaxy where ancient, metal-poor stars have been mostly observed, these stars were tracing paths close to the Milky Way's disk within just 6,500 light-years from the sun. "Usually, stars in the disk are metal-rich and younger, like the sun," he said, "while our stars [in the study] are old and very metal-poor (like in dwarf galaxies)." Additionally, some of these stars were found moving in the same direction as the Milky Way's rotation, while others traveled in the opposite direction. But these two groups did not show any difference in their chemical abundances. Explaining how a single in falling galaxy could leave stars moving in opposite directions was also challenging. The answer came from computer simulations of galaxy formation. If the merger happened early enough, when the young Milky Way was still lightweight and had not yet settled into a spinning disk, the in falling galaxy would have had enough freedom to scatter its stars in all directions.

"The early merging history of a large galaxy might be very chaotic, with various smaller systems merging together and dispersing their stars with many different orbits," Sestito explained. This scenario could produce both prograde and retrograde orbits, placing the merger event around 3 billion years after the Big Bang. As a result, the simulations showed that a single dwarf galaxy swallowed by the young Milky Way more than 10 billion years ago, could have scattered its stars into exactly the orbital pattern observed today. The models also helped estimate the total mass of this galaxy to be around 1.4 billion solar masses. The exact age of the stars is hard to pin down, but their chemical composition suggests they are older than 10 billion years, Sestito said, and all of them are located roughly 7,000 light-years from our solar system. The stars also have similar chemical compositions, suggesting they all came from the same metal-poor dwarf galaxy. Gaia's mapping shows how 40,000 stars, all located within 326 light-years of the solar system, will move in the next 400,000 years. Eleven of the stars were in a prograde orbit, or moving in the same direction of the galactic disk, while nine were on a retrograde orbit, or moving in the opposite direction, possible remnants of a dwarf galaxy gobbled up by the Milky Way. The study authors believe the accreted, or hijacked, stars, simply remained as part of our galaxy, getting knocked around and ending up in different orbital patterns, Battersby said.

“If the Loki scenario is correct, then a system merged with our galaxy could deposit its stars into both prograde and in the opposite direction,” Sestito said. “This can be allowed only if the merger event happened when our Milky Way was still infant/smaller and its gravitational potential was weaker than nowadays. Cosmological simulations suggest that this could have happened no later than 3 or 4 billion years from the Big Bang.” Dr. Hans-Walter Rix, director of the department of galaxies and cosmology at the Max Planck Institute for Astronomy in Germany, said what was most impressive about the study was “how they use the detailed chemical element abundances as a fingerprint to identify a common birth origin of these stars in a now-shredded satellite galaxy, even though some of the stars are going the right way around and some the wrong way.” “Similarly, our accreted stars gave us some hard time in understanding their origin,” Sestito said. “At first it was not easy to reconcile the fact that an accreted system can disperse its stars in both prograde and opposite orbits.” Another explanation for the stars could be that they stem from more than one merger event with the Milky Way, he said. Because researchers are still in the early stages of exploring the chemical signatures of the lowest-metallicity stars in the Milky Way disk, it remains plausible that these stars belong to a subgroup of stars or substructure within the Milky Way, Chiti noted. "I'm looking forward to what future work mapping the chemistry of large samples of very metal-poor stars in the Milky Way disk may show," he said. To confirm the nature of Loki, the team would need to observe its stars and other non-Loki targets with the same telescope setup to better understand the differences between this system and other parts of the Milky Way halo.

The Milky Way has grown through galactic cannibalism, or when a large galaxy eats a small galaxy and uses immense gravitational force to absorb its stars and gas. The leftover shreds of such meals enable astronomers to assemble the galaxy’s “eating history,” said Dr. Alexander Ji, assistant professor in the department of astronomy and astrophysics at the University of Chicago. There are lots of little mergers happening all the time, but the really big meals can change the growth history of the Milky Way,” Ji said. One such transformative event occurred as the Milky Way merged with the Gaia-Sausage-Enceladus galaxy between 8 billion and 10 billion years ago. “We think it helped ‘reset’ the Milky Way from its early turbulent phase to the more stable growing disk that it has today,” Ji said. The new study suggests that the Milky Way merging with the Loki galaxy was almost on the scale of the Gaia-Sausage-Enceladus event. But the evidence is largely hidden because Loki’s remnants are hard to find near the Milky Way’s disk, Ji added. “If this is real, it would indicate that we are missing a major part of our Milky Way’s formation history, and we might need to revisit our current picture to see the impact of such an event,” Ji said. Ji doubts Loki is a previously unknown galaxy, given that possible discoveries of merger events often turn out to be extensions of known systems, but he noted the study authors included appropriate caveats in their work. “It’s an interesting new possibility worth pursuing, and I expect there will be people looking to test whether Loki is real with larger datasets,” Ji said. With upcoming advanced spectroscopic facilities, astronomers will be able to observe hundreds of stars with available high-quality data on their trajectories and chemical abundances. The hidden systems in the inner regions of the galaxy could hold clues to the primitive galaxies of the young universe, though detecting them in the crowded disk would be challenging in our universe.