Small Galaxy Punches Hole In Andromeda

Small Galaxy Punches Hole In Andromeda
Image of Andromeda Galaxy taken in visible light. (Image credit: NASA/JPL/K. Gordon)

Sometime in the distant past, the dwarf galaxy M32 hurleditself at its much larger neighbor Andromeda, delivering an explosive uppercutpunch that left a jagged hole nearly 10,000 light-years across in Andromeda'splane of stars, one that millions of years later has yet to fully heal.

New infrared images from NASA's Spitzer Space Telescoperecently revealed the hole, which is hidden to optical telescopes behindAndromeda's veils of cosmic dust and gas.

The Spitzer images also revealed other features of Andromedathat have never been seen before, including bright, new stars and spiral arcsswirling out from the galaxy's center.

"Andromeda is a far more exciting, dynamic place than weever thought," said Karl Gordon, an astronomer at the University ofArizona's Steward Observatory and lead author of a new study detailing thefindings.

Andromeda, also known as Messier Object 31 or M31, is about 200thousand light-years across and about 1.5 times more massive than the MilkyWay. It is the largest spiral galaxy of the so-called Local Group, a collectionof about 30 galaxies that includes our own, and it has a central supermassiveblack hole that is encircled by hundreds of hot,blue stars.

At about 2.5 million light-years away, it is the farthestobject that can be observed with the naked eye on a dark night. And it is themost studied galaxy besides the Milky Way. But examining Andromeda has alwaysbeen a challenge. The galaxy is highly inclined relative to our own, soobserving it is like trying to read a newspaper held edgewise. Another problemis that large parts of Andromeda are obscured by a haze of cosmic dust and gas.

The Spitzer Space Telescope gets around the latter problem byactually using the cosmic dust to create its image. Its Multiband ImagingPhotometer (MIPS) is designed to detect the infrared radiation, or heat, givenoff by the dust particles. Cosmic dust near Andromeda's center gives off moreheat than dust at the galaxy's periphery and therefore glows brighter in theinfrared.  

By tracing the patterns made by the dust, MIPS acts like aninfra-red X-ray to reveal Andromeda's underlying structure in detail for thefirst time.

In images taken with visiblelight, Andromeda looks like a milky blur. In Spitzer's infraredimages, the haze disappears and Andromeda's skeleton becomes visible.Revealed is a framework of glowing, twisting arms and an overall structure thatlooks less like a spiral than concentric rings of fire.

"Spitzer's tracing of the dust is the most complete viewwe've seen so far of interstellar gas and dust in this galaxy, especiallywithin the nucleus, where it's been hardest to look," Gordon said.

The new images will require astronomers to change how they viewAndromeda. Until now, it was regarded as a perfect example of galactictranquility, a quiet, calm place where not much happens, said George Rieke,head of Spitzer's MIPS team. "Our pictures were good enough that we had toquestion that view, and now we know that this galaxy has just taken a punchfrom its little neighbor."

Galactic collisions like that between Andromeda and M32 areactually quite common. In fact, Andromeda will collide with our own Milky WayGalaxy in about 3 billion years. The violence of that impact will make the M32incident seem minor. Neither Andromeda or our own galaxy are expected tosurvive that collision with their spiral shapes intact. Instead, the two willmerge to form a giant elliptical galaxy.

The Spitzer images were recently submitted to the journal AstrophysicalJournal Letters.

This article is part of's weekly Mystery Mondayseries.

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Staff Writer

Ker Than is a science writer and children's book author who joined as a Staff Writer from 2005 to 2007. Ker covered astronomy and human spaceflight while at, including space shuttle launches, and has authored three science books for kids about earthquakes, stars and black holes. Ker's work has also appeared in National Geographic, Nature News, New Scientist and Sky & Telescope, among others. He earned a bachelor's degree in biology from UC Irvine and a master's degree in science journalism from New York University. Ker is currently the Director of Science Communications at Stanford University.