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Punched by a neighbor

The Andromeda Galaxy has a hole in its disk where a satellite galaxy blasted through.
Andromeda Galaxy hole
The Andromeda Galaxy's star-forming ring has a gaping hole (arrow) seen in this Spitzer image.
NASA/JPL-Caltech/K. Gordon (University of Arizona)
October 27, 2005
New images from NASA's Spitzer Space Telescope shed light on the Andromeda Galaxy's violent past. It appears one of Andromeda's satellite galaxies, the dwarf spheroidal M32, blasted through Andromeda's disk a few million years ago.

Astronomers have known the Milky Way Galaxy interacts with two satellites, but scientists thought Andromeda was a more tranquil environment. Because Andromeda is inclined relative to us, studying the galaxy's structure is a difficult task. A team of astronomers led by Karl D. Gordon of Steward Observatory used Spitzer's Multiband Imaging Photometer (MIPS) to take infrared photographs of Andromeda. The MIPS is sensitive to dust and provides the most detailed images ever of the dust distribution in another galaxy.

From these images, astronomers inferred that the Andromeda Galaxy has two spiral arms and a prominent star-forming ring, and that these are separate structures. The images also show the ring "splits" into two arcs in one section, creating a hole. This hole is where the team believes M32 punched through Andromeda's galactic disk. "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," says George Rieke, head of Spitzer's MIPS team.
Andromeda Galaxy multiwavelength
By combining images from three wavelengths, astronomers analyzed the dust's temperature. In this composite, blue/white shows the hottest dust (detected in 24 microns) in star-forming regions and in Andromeda's nucleus, while the red regions (using 160 microns) show cooler dust farther away from the galaxy's center.
NASA/JPL-Caltech/K. Gordon (University of Arizona)
Jeremy Bailin of Swinburne University, Australia, made detailed computer simulations of Andromeda interacting with different satellite galaxies. His findings support the theory that M32 crashed through Andromeda.

Spitzer took images in three different infrared wavelengths: 24, 70, and 160 microns. The image where Andromeda's structure is most prominent is a composite of roughly 11,000 photographs taken with the 24-micron detector. The astronomers also combined images from all three detectors to analyze the dust's temperature. The warmest dust is brightest at 24 microns, while the coolest is brightest at 160 microns. All images were taken August 25, 2004, exactly one year after Spitzer's launch.

The Andromeda Galaxy is about 2.5 million light-years away and is a favorite target among amateur astronomers.

The researchers have submitted their findings to The Astrophysical Journal.
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