Ancient Cosmic Collision Marked Dwarf Galaxy

The Andromeda Galaxy is revealed here in this telescopic view, along with two of its most prominent small satellite galaxies: M32, lower center, and M110, to the upper right. Credit: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF

Astronomers report evidence of an ancient merger between two “satellite” galaxies, ones that circled our own galactic big brother, the Andromeda Galaxy.

Even larger than our own Milky Way Galaxy, Andromeda is located some 2.5 million light-years away. According to the study released by the journal Nature, evidence of an ancient galactic collision there comes from a stream of stars found in one of its outlying dwarf galaxies. Named Andromeda II, the dwarf galaxy resides some 600,000 light-years from its parent galaxy’s center.

The finding adds to accumulating evidence of mergers between galaxies, large and small, playing a fundamental role in their construction. The finding points to smashups of satellite galaxies agglomerating into larger bodies. Stellar streams similar to the ones described in the study are seen on the outskirts of larger galaxies such as the Milky Way and Andromeda.

Andromeda is one of the premier deep-sky targets for backyard sky-watchers. A great spiral galaxy, it is home to nearly a trillion stars (at least two times more than the Milky Way). A swarm of more than 20 much-smaller galaxies orbit it, including Andromeda II.

In the study, researchers led by Nicola Amorisco of Denmark’s University of Copenhagen note the unusual movement of one particular group of aged stars nestled within the small, round confines of Andromeda II.

“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,” said Amorisco in a statement.

“These stars are situated in an almost complete ring and are rotating around the center of the galaxy.”

What is exciting for astronomers is the fact that Andromeda II, which weighs less than one percent of our own Milky Way Galaxy’s weight, is the lightest known example of the merging of galaxies. That means the dwarf galaxy may give them a glimpse into how even the tiniest members of the galactic family form and evolve.

See for Yourself

The Andromeda Galaxy, also known as Messier 31 (M31), sits nearly 2.5 million light-years away, and is actually a fairly easy target to hunt down in a dark sky, as well as from suburban backyards when using binoculars. Fall is the best time of the year to try your hand at finding Andromeda (in the Northern Hemisphere), but it is a relatively east target in winter too, as it sets in the early evening in the northwest.

Under moderately-dark suburban skies, M31, shining at 3.4 magnitude, can be seen with the naked eye. It appears as a faint, fuzzy patch in the sky, off to the side of the constellation Andromeda. Pointing the way to the galaxy is the sloppy W-shaped constellation Cassiopeia.

Andromeda galaxy or M31, is easy to find in the northwest sky early evening in late February thanks to nearby bright constellation Cassiopeia pointing to the way. Credit: Starry Night Software / A.Fazekas
Andromeda Galaxy, or M31, is easy to find in the northwest sky in early evenings in late February, thanks to the nearby bright constellation Cassiopeia pointing the way. Credit: Starry Night Software/A. Fazekas

The galaxy begins to truly unveil itself when viewed through binoculars, or even a modest telescope. At higher magnification, Andromeda will reveal an oval, spindle shape  and fill most of the field of view. Resembling a Frisbee disk viewed nearly edge-on, we see this 150,000-light-year-wide spiral galaxy at a sharp angle. The entire body of the galaxy actually extends the width of many full moons across the sky.

While two of Andromeda’s largest companion galaxies are easy to spot even with the smallest scope, don’t expect to find Andromeda II—it shines at a feeble 14th magnitude, making it a challenge even for large instruments. Even then it will remain a faint, grayish blob for novice observers.

The excitement lies in understanding the true nature of its ghostly glow, which is provided by the survivors of an ancient starry collision.

Follow Andrew Fazekas, the Night Sky Guy, on Twitter,  Facebook, and his website.

Changing Planet

Meet the Author
Andrew Fazekas, aka The Night Sky Guy, is a science writer, broadcaster, and lecturer who loves to share his passion for the wonders of the universe through all media. He is a regular contributor to National Geographic News and is the national cosmic correspondent for Canada’s Weather Network TV channel, space columnist for CBC Radio network, and a consultant for the Canadian Space Agency. As a member of the Royal Astronomical Society of Canada, Andrew has been observing the heavens from Montreal for over a quarter century and has never met a clear night sky he didn’t like.