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 SPACE.com's weekly Mystery Monday
series.
advertisement
