Solar-System-Like Dwarf Galaxies Orbiting Andromeda –“Point to Something Profound” (Today’s Most Popular)





“It’s a very unusual, unexpected configuration,” says astrophysicist Dr. Julio Navarro, at the University of Victoria, an expert in evolution of galaxies and galaxy clusters and the structure and evolution of their dark matter component. “It’s so unexpected that we don’t know yet what it’s telling us. The fact that it is there at all is pointing us toward something profound. Somehow, they have a plane-like structure similar to a solar system, but with a completely different origin and we don’t know what that origin is,” Navarro said. Understanding how and why the dwarf galaxies form the ring around Andromeda is expected to offer new information on the formation of all galaxies. Twelve of the 13 dwarf galaxies — they range in size from 10 million to 100 million stars — are on one side of the orbital plane, as if they are held by a string being swung from Andromeda.

“This looks like they are all moving together and they all know where to go, like some pre-existing structure has been sucked in by Andromeda,” Navarro said. The image at the top of the page shows a stream of stars from one of Andromeda's satellite galaxies, a dwarf galaxy called Andromeda II, which is less than one percent of the size of the Milky Way detected by researchers at the Niels Bohr Institute at the University of Copenhagen.




The fascinating discovery about dwarf galaxies orbiting the Andromeda suggests that conventional ideas regarding the formation of galaxies like our own Milky Way are missing something fundamental. The string of 13 dwarf galaxies in orbit around the massive galaxy Andromeda are spread across a flat plane more than one million light years wide and only 30,000 light years thick, moving in synchronicity with one another, according to Navarro, one of the co-authors of an article on the phenomenon in the journal Nature. The dwarfs are spread across a distance so vast that they have yet to complete a single orbit.

The behavior of Andromeda's dwarfs is so extreme from the usual chaotic orbits of galaxies around each other that the the researchers believe they have revealed a huge hole in science’s understanding of galaxy formation. Computer models show that the dwarf galaxies should orbit independently, almost randomly. But the structure of the synchronous galaxies orbiting Andromeda is much more like a mature solar system.

In the paper published in the journal Nature, the international team of astronomers described the discovery that almost half of the 30 dwarf galaxies orbiting Andromeda do so in an enormous plane more than a million light years in diameter, but only 30,000 light years thick. The findings defied scientists’ expectation—based on two decades of computer modeling—that satellite galaxies would orbit in independent, seemingly random patterns. Instead, many of these dwarf galaxies seem to share a common orbit, an observation that currently has no explanation.

"Astronomers have been observing Andromeda since Persian astronomers first noted it over a thousand years ago, but it is only in the past decade that we have truly studied it in exquisite detail with the Pan-Andromeda Archaeological Survey," said Geraint Lewis from the University of Sydney's School of Physics. "The Pan-Andromeda Archaeological Survey – cutely called PAndAS – is a large project that ran between 2008 and 2011, using the Canada-France-Hawaii Telescope situated on the Mauna Kea volcano on the Big Island of Hawaii. Now that we're examining the data it collected, it is providing our first panoramic view of our closest large companion in the cosmos," explained Lewis.

"When we looked at the dwarf galaxies surrounding Andromeda, we expected to find them buzzing around randomly, like angry bees around a hive. Instead, we've found that half of Andromeda's satellites are orbiting together in an immense plane, which is more than a million light years in diameter but only 30,000 light years thick. These dwarf galaxies have formed a ring around Andromeda. This was completely unexpected – the chance of this happening randomly is next to nothing. It really is just weird," Lewis added.

Stars in a dwarf galaxy often move around at random, but this is not exactly the case for Andromeda II. In particular we could see that a stream of stars is moving around differently than the rest in a very coherent way. These stars are situated in an almost complete ring and are rotating around the center of the galaxy," says astrophysicist Nicola C. Amorisco, Dark Cosmology Centre, Niels Bohr Institute. Amorisco says this kind of merger between small galaxies is bizarre at this point in the galaxy formation process and was until now an unseen event.

Large galaxies, like Andromeda and our own Milky Way, have long been known to be orbited by an entourage of smaller galaxies. These small galaxies, which are individually anywhere from ten to at least hundreds of thousands of times fainter than their bright hosts, were thought to trace a path around the big galaxy that was independent of every other dwarf galaxy. For several decades, astronomers have used computer models to predict how dwarf galaxies should orbit large galaxies, and every time they found that dwarfs should be scattered randomly over the sky. Never, in these synthetic universes, did they see dwarfs arranged in a plane like that observed around Andromeda.

"Now that we've found that the majority of these dwarf galaxies orbit in a disc around the giant galaxy Andromeda, it looks like there must be something about how these galaxies formed or subsequently evolved that has led them to trace out this peculiar coherent structure," said Lewis. "Dwarf galaxies are the most numerous galaxy type in the universe, so understanding why and how they form this disc around the giant galaxy is expected to shed new light on the formation of galaxies of all masses.




The top left hand image above is a true color photograph (taken with the Canada-France-Hawaii Telescope) of Andromeda galaxy, the closest giant galaxy to our own, and in many ways our Milky Way’s twin sister. Also visible in this photograph are two of its satellites (these are much smaller galaxies, containing up to about a billion stars). The study has measured the distances and velocities of 27 other such dwarf galaxies : their three-dimensional positions are shown with red spheres in the other panels. The top right hand panel depicts how these appear to us as viewed from Earth, while the bottom left panel shows the positions of the satellites as they would be seen from the side. This immense grouping is more than a million light years across and rotates in the sense shown by the arrows.

There have been similar claims of an extensive plane of dwarf galaxies about our own Milky Way Galaxy, with some claiming that the existence of such strange structures points to a failing in our understanding of the fundamental nature of the Universe. "We don't yet know where this is pointing us, but it surely is very exciting," said Dr Rodrigo Ibata, from the Observatoire astronomique de Strasbourg, in France, and lead author on the report.

The Daily Galaxy via University of Victoria, Nature and the University of Sydney



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