According to a newly-released paper, the Andromeda galaxy may have committed fratricide on a cosmic scale some 2 billion years ago, by ripping apart an enormous galaxy, and cannibalizing much of the body. The victim was likely the third largest member of the local group of galaxies, behind its sibling galaxy, the Milky Way, and its aggressor, the 220,000 light-year-wide Andromeda spiral galaxy.
Massive galaxies such as our own Milky Way have a violent history of drawing in and ripping apart smaller galaxies unfortunate enough to have been captured in their gravitational well. These dramatic events are an important part of the galactic evolutionary process, fueling star birth on a massive scale and, depending on the relative masses of the interacting bodies, dramatically altering the dynamics and shape of the victor.
New research carried out by University of Michigan postdoctoral researcher Richard D'Souza, and professor of astronomy Eric F. Bell, has revealed that the serial killer galaxy has serious form when it comes to taking down big game.
The scientists used advanced computer simulations to identify a merger event that could account for a number of Andromeda's observed features, including a near invisible halo of stars surrounding M31, a separate stream of stars, and the presence of the compact elliptical satellite galaxy, M32.
The simulations reveal that all of these features could be explained by a collision with a single large progenitor galaxy, referred to as M32p, which boasted a mass roughly 20 times that of any galaxy thought to have been encountered by the Milky Way.
"Astronomers have been studying the Local Group – the Milky Way, Andromeda and their companions – for so long," comments professor Bell. "It was shocking to realize that the Milky Way had a large sibling, and we never knew about it."
The simulations show that as M32p interacted with the Andromeda galaxy, that it was stripped of vast quantities of material. The majority of the stolen mass went into forming M31's faint stellar halo, and the more concentrated, elongated structure of the giant stellar stream. Meanwhile, the core of the previously gargantuan M32p survived to orbit Andromeda as the satellite galaxy astronomers now refer to as M32.
As M32p was being disrupted by the Andromeda galaxy, it would likely have experienced a sudden burst of star formation, triggered by an influx of material to the galactic center. This would explain why present-day M32 plays host to a high population of relatively youthful stellar bodies, while having the outward appearance of an old elliptical galaxy.
The team was able to place a timeframe for the merger by dating the starburst event that created the young stars in M32. According to the paper, the two galaxies began to interact with one another roughly 5 billion years ago, with the chaotic merger coming to an end roughly 3 billion years later.
The paper also suggests that the event was responsible for adding to the overall thickness of M31's disk, and prompting a galaxy-wide burst of star formation that accounted for roughly a fifth of the stars created in the colossal structure.
The sheer size of the proposed galaxy may require a rethink to current theories regarding how galaxies evolve in the wake of large scale mergers. It had previously been thought that a classic spiral galaxy, such as our own Milky Way, would lose its well-defined disk after colliding with a large neighbor, resulting in an elliptical galaxy.
Astronomers know from present day observations that M31 has maintained its well-defined disk structure. Therefore, if the study is correct, and M31 did indeed merge with the then massive galaxy M32p, the findings would suggest that massive mergers do not always have a dramatic effect on the structure of a galaxy.
The method used to piece together Andromeda's dramatic past can now be applied to other galaxies, allowing scientists to gain a greater understanding of large galactic mergers, and how they influence the evolution of some of the most massive structures to populate the universe.
As for the Milky Way? Well, you had a sibling, and you'll have a chance to avenge its cosmic murder in about 4 billion years when you meet the Andromeda galaxy face to face.
A paper detailing the findings has been published in the journal Nature Astronomy.
Source: University of Michigan
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