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

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

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

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

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

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

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

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

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

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

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

The new images will require astronomers to change how they view Andromeda. Until now, it was regarded as a perfect example of galactic tranquility, 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 to question that view, and now we know that this galaxy has just taken a punch from its little neighbor."

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

The Spitzer images were recently submitted to the journal Astrophysical Journal Letters.

This article is part of's weekly Mystery Monday series.