University of Arizona astronomers have taken the sharpest, most detailed infrared pictures ever of the nearby Andromeda galaxy. They've discovered that our neighbor spiral galaxy is a surprisingly disturbed and violent place.
Their new views from NASA's Spitzer Space Telescope and corroborating theoretical models suggest that Andromeda was completely punched through by a satellite dwarf galaxy a few million years ago and is reeling from the impact.
Astronomers have known for some time that our own Milky Way is stealing gas from two nearby small galaxies. It appears that we live in a rough neighborhood.
Andromeda, the spiral galaxy nearest our own, had been thought to be a more tranquil place. At 2 million light years away, Andromeda, or the M31 galaxy, is the farthest object in the universe visible to the naked eye and perhaps the most-studied target in the night sky.
But astronomers have been struggling to make sense of Andromeda's structure since legendary astronomer Edwin Hubble studied it more than 75 years ago. One problem is that Andromeda is highly inclined relative to our own Milky Way, so that observers see Andromeda more edge-on than face-on. It's been like trying to read a newspaper held edgewise -- you might decipher the headlines, but the story details escape you.
Spitzer's Multiband Imaging Photometer (MIPS) is very sensitive to dust between stars, which are particles the size of soot and tiny sand grains at temperatures that average minus hundreds of degrees Fahrenheit. UA Steward Observatory astronomers and their colleagues took MIPS images that traced very cold dust from the outermost edges of the Andromeda galaxy, to warmer dust at the more central star-forming region, to the warmest dust within the galaxy's nucleus. The dust clearly delineates the large structures, such as spiral arms.
"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," said Steward Observatory's Karl D. Gordon.
The new false-color Andromeda image is available at http://www.spitzer.caltech.edu/spitzer/ and http://dirty.as.arizona.edu/~kgordon/m31_press/m31_press.html
"We've been able to trace the two spiral arms all the way into the nucleus, where the spiral structures appear to start at two points near the center of the galaxy. That's a classic signature of spiral galaxies, but it's not been seen before in M31. And we've traced the spiral arms -- actually they're arm segments -- out beyond the star-forming ring."
Astronomers had seen Andromeda's star-forming ring before, but argued whether it was merely part of the spiral arm structure. The MIPS team used their high-resolution instrument at three different infrared wavelengths and discovered that the star-forming ring is a nearly circular, separate entity. The ring is offset from the center of the galaxy and split in one place, where they think that dwarf galaxy M32 blasted through on a high-speed journey through Andromeda's disk.
"What we're really seeing looks like a hole with a rim, like the first ripple flowing out when you toss a stone in a pond," said UA Professor George Rieke, head of Spitzer's MIPS team. "We think that the very compact dwarf galaxy M32 we now see hovering over the disk has just smashed its way through, disturbing the motions of the interstellar clouds and causing them to bump into each other, forming new stars and heating up the gas and dust."
Jeremy Bailin of Swinburne University, Australia, used computer calculations to test the idea that M32 and another satellite galaxy, NGC205, could interact with Andromeda to produce structures like those seen in the MIPS images. Bailin, who earned his doctorate in astronomy from UA last year, ran the most sophisticated simulations ever made for Andromeda. His results support the idea that M32 blasted entirely through the galaxy disk at high speed, and that the great collision would have shifted the star-forming ring off center, just as the MIPS observers see.
Gordon has posted a movie that shows the simulation of Andromeda and its interacting dwarf galaxy on his Web page. Click on http://dirty.as.arizona.edu/~kgordon/m31_press/m31_m32_movie_1024x768.mpeg M32, the dwarf galaxy, is shown as the blue ball. The red balls represent the gas disk, while the old stellar component is in white. The large white balls represent star formation events, leaving behind young stars (small white balls). The end of the movie shows the face-on projection of the resulting gas surface density at the present day, with M32 projected onto the middle of a hole in the ISM.
Astronomers until now have regarded beautiful Andromeda as a perfect example of galactic tranquility, a quiet, calm place where not much happens, Rieke said. "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."
"Andromeda is a far more exciting, dynamic place than we ever thought," Gordon said.
Gordon, Bailin, Rieke and others have submitted a paper describing this work, "Spitzer/MIPS Infrared Imaging of M31: Further Evidence for Interaction Induced Disturbed Spiral Structure," to the Astrophysical Journal Letters.