The Milky way had a big brother or sister a long time ago – and Andromeda at the

The Andromeda galaxy with satellite galaxies M32 (centre left) and M110 (lower right). A new study suggests that M32 is the remnant of a much larger galaxy consumed by Andromeda about 32 billion years ago. Credit: S. Ozime

The Galaxy had a previously unknown big brother or sister that was torn up by the neighboring Andromeda galaxy long ago, a new study suggests.

Andromeda and the Milky way are the two largest members of the Local Group, a collection of more than 50 galaxies packaged in a dumbbell-shaped region of space about 10 million light-years away. Andromeda was not kind to the non-recurring third-largest member of this family, where the approximately 2 billion years ago, according to new research.

“Astronomers have been studying the Local Group — the Milky way, Andromeda, and their escorts — for so long,” study co-author Eric Bell, a professor of astronomy at the University of Michigan (UM), said in a statement. “It was shocking to realize that the Milky way had a big brother or sister, and we have not known.” [When Galaxies Collide: Photos of Great Galactic Crashes]

Andromeda, also known as M31, is a prolific cannibal; the large spiral galaxy is thought to have fragmented in hundreds of smaller relatives over the centuries. The number and complexity of these mergers makes it difficult to tease out the details of a particular — but Bell and the study’s lead author Richard D’Souza, post-doctoral researcher at the UM, were able to do that.

With the help of computer simulations, the duo found that most of the stars in the faint outer reaches of Andromeda’s “halo” — the approximately spherical region around the galaxy’s disk — came from a single smashup.

“It was a ‘Eureka’ moment,” D’Souza said in the same statement. “We realized that we can use this information of Andromeda’s outer stellar halo to infer the properties of the largest of this torn galaxies.”

Further modelling work could they be the date of the merger to approximately 2 billion years ago, and the reconstruction of a number of basic data from which long-dead galaxy. M32p, as the researchers call it, was probably at least 20 times larger than the galaxy, that the Milky way has ever merged with the new results.

And M32p is apparently not completely gone. D’Souza and Bell think that an odd satellite galaxy of Andromeda called the M32 is the lost galaxy’s corpse — the bones that remain after the big, nasty spiral munched from M32p meat.

“The M32 is a weirdo,” Bell said. “It looks like a compact example of an old, elliptical galaxy, it actually has a lot of young stars. It is one of the most compact galaxies in the universe. There is not another galaxy.

The timing of the merger fits. Another research team independently decided earlier this year that Andromeda probably underwent a major merger, and a concurrent wave of star formation, between 1.8 billion and 3 billion years ago.

The new study, which was published today (July 23) in the journal Nature, Astronomy, scientists must better understand the evolution and the consequences of galaxy mergers, D’Souza and Bell said.

For example, it has long been assumed that there is a massive crash destroying the disks of spiral galaxies, the rotation of these beautiful objects instead of boring elliptical galaxies. But Andromeda preservation of the spiral disk, which suggests that the conventional wisdom does not always hold.

As dramatic as the Andromeda-M32p collision is likely, something much bigger is on the horizon. About 4 billion years from now, the Milky way and Andromeda will come together in an epic crash that will shake up the Local Group. The merger will give an impulse to some pretty impressive star formation fireworks in the Earth night sky, if there is anyone still around to see.

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