Michigan: The Milky Way's Long-Lost Sibling Finally Found?

[bystander]

The Milky Way's Long-Lost Sibling Finally Found
University of Michigan, Ann Arbor | 2018 Jul 23

In this image, the Andromeda galaxy shreds the large galaxy M32p,
which eventually resulted in M32 and a giant halo of stars.
Credit: Richard D’Souza. Image of M31 courtesy of Wei-Hao Wang.
Image of stellar halo of M31 courtesy of AAS/IOP.

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Scientists at the University of Michigan have deduced that the Andromeda galaxy, our closest large galactic neighbor, shredded and cannibalized a massive galaxy two billion years ago.

Even though it was mostly shredded, this massive galaxy left behind a rich trail of evidence: an almost invisible halo of stars larger than the Andromeda galaxy itself, an elusive stream of stars and a separate enigmatic compact galaxy, M32. Discovering and studying this decimated galaxy will help astronomers understand how disk galaxies like the Milky Way evolve and survive large mergers.

This disrupted galaxy, named M32p, was the third-largest member of the Local Group of galaxies, after the Milky Way and Andromeda galaxies. Using computer models, Richard D’Souza and Eric Bell ... were able to piece together this evidence, revealing this long-lost sibling of the Milky Way. ...

Using new computer simulations, the scientists were able to understand that even though many companion galaxies were consumed by Andromeda, most of the stars in the Andromeda’s outer faint halo were mostly contributed by shredding a single large galaxy. ...

The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor - Richard D’Souza, Eric F. Bell

• Nature Astronomy (online 23 Jul 2018) DOI: 10.1038/s41550-018-0533-x
  arXiv.org > astro-ph > arXiv:1807.08819 > 23 Jul 2018

[BDanielMayfield]

More from the article:

This work might also solve a long-standing mystery: the formation of Andromeda’s enigmatic M32 satellite galaxy, the scientists say. They suggest that the compact and dense M32 is the surviving center of the Milky Way’s long-lost sibling, like the indestructible pit of a plum.

“M32 is a weirdo,” Bell said. “While it looks like a compact example of an old, elliptical galaxy, it actually has lots of young stars. It’s one of the most compact galaxies in the universe. There isn’t another galaxy like it."

....

Their study may alter the traditional understanding of how galaxies evolve, the researchers say. They realized that the Andromeda’s disk survived an impact with a massive galaxy, which would question the common wisdom that such large interactions would destroy disks and form an elliptical galaxy.

The timing of the merger may also explain the thickening of the disk of the Andromeda galaxy as well as a burst of star formation two billion years ago, a finding which was independently reached by French researchers earlier this year.

“The Andromeda Galaxy, with a spectacular burst of star formation, would have looked so different 2 billion years ago,” Bell said. “When I was at graduate school, I was told that understanding how the Andromeda Galaxy and its satellite galaxy M32 formed would go a long way towards unraveling the mysteries of galaxy formation.”

Good news! This means that when the Milky Way and Andromeda start merging in ~ 4BY the end result several billion years later could well be a spectacularly large spiral instead of just an ordinary elliptical.

I can hardly wait to see what happens!

Bruce

[Ann]

Click to view full size image

Compact dwarf galaxy M60-UCD1 near giant galaxy M60.
Credit: NASA / Space Telescope Science Institute / European Space Agency.

Very interesting! It is obvious that M32 is the "naked core" of a much larger galaxy. There are really very few galaxies like M32, so it is clear that something dramatic must have happened to it.

The only other galaxy I know of that is slightly similar to M32 is M60-UCD1, a tiny, extremely compact companion of the giant elliptical galaxy, M60.

M32 appears to be large as a naked galactic core goes. Its diameter is some 6.5 thousand light-years, according to Wikipedia, compared with only 300 light-years for extremely tiny and compact M60-UCD1. We may also compare M32 with Omega Centauri, the largest globular cluster of the Milky Way, which is believed to be the naked core of a dwarf galaxy that was shredded by the Milky Way. The radius of Omega Centauri is only about 86 light-years, and its mass is only about 4 million solar masses.

And there is another similarity between M32 and many globular clusters of the Milky Way. M32 follows an orbit that makes it plunge right through the disk of M31, in the same way that many globular clusters plunge right through the disk of our own galaxy. But the fact that Andromeda is regularly being pummeled by a body as massive as M32 must have had dramatic effects on Andromeda, too.

Click to view full size image

The disk of the Andromeda galaxy has a somewhat ring-like structure, rather than a typical spiral shape. The rings could be the remnants of continuous "plunges" of M32 right through the disk of M31, in the same way as a pebble thrown into a pond will cause rings to spread across the surface of the pond.

NASA Jet Propulsion Laboratory wrote:

The Spitzer multiband imaging photometer's 24-micron detector recorded 11,000 separate snapshots to create this new comprehensive picture. Asymmetrical features are seen in the prominent ring of star formation. The ring appears to be split into two pieces, forming the hole to the lower right. These features may have been caused by interactions with satellite galaxies around Andromeda as they plunge through its disk.

Ann