NASA's Spitzer Finds Galaxy With Split Personality

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News release: 2012-115 April 24, 2012

NASA's Spitzer Finds Galaxy With Split Personality

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2012-115&cid=release_2012-115

PASADENA, Calif. -- While some galaxies are rotund and others are slender disks like our
spiral Milky Way, new observations from NASA's Spitzer Space Telescope show that the
Sombrero galaxy is both. The galaxy, which is a round elliptical galaxy with a thin disk
embedded inside, is one of the first known to exhibit characteristics of the two different types.
The findings will lead to a better understanding of galaxy evolution, a topic still poorly
understood.

"The Sombrero is more complex than previously thought," said Dimitri Gadotti of the European
Southern Observatory in Chile and lead author of a new paper on the findings appearing in the
Monthly Notices of the Royal Astronomical Society. "The only way to understand all we know
about this galaxy is to think of it as two galaxies, one inside the other."

The Sombrero galaxy, also known as NGC 4594, is located 28 million light-years away in the
constellation Virgo. From our viewpoint on Earth, we can see the thin edge of its flat disk and a
central bulge of stars, making it resemble a wide-brimmed hat. Astronomers do not know
whether the Sombrero's disk is shaped like a ring or a spiral, but agree it belongs to the disk
class.

"Spitzer is helping to unravel secrets behind an object that has been imaged thousands of times,"
said Sean Carey of NASA's Spitzer Science Center at the California Institute of Technology in
Pasadena. "It is intriguing Spitzer can read the fossil record of events that occurred billions of
years ago within this beautiful and archetypal galaxy."

Spitzer captures a different view of the galaxy than visible-light telescopes. In visible views, the
galaxy appears to be immersed in a glowing halo, which scientists had thought was relatively
light and small. With Spitzer's infrared vision, a different view emerges. Spitzer sees old stars
through the dust and reveals the halo has the right size and mass to be a giant elliptical galaxy.

While it is tempting to think the giant elliptical swallowed a spiral disk, astronomers say this is
highly unlikely because that process would have destroyed the disk structure. Instead, one
scenario they propose is that a giant elliptical galaxy was inundated with gas more than nine
billion years ago. Early in the history of our universe, networks of gas clouds were common, and
they sometimes fed growing galaxies, causing them to bulk up. The gas would have been pulled
into the galaxy by gravity, falling into orbit around the center and spinning out into a flat disk.
Stars would have formed from the gas in the disk.

"This poses all sorts of questions," said Rubén Sánchez-Janssen from the European Southern
Observatory, co-author of the study. "How did such a large disk take shape and survive inside
such a massive elliptical? How unusual is such a formation process?"

Researchers say the answers could help them piece together how other galaxies evolve. Another
galaxy, called Centaurus A, appears also to be an elliptical galaxy with a disk inside it. But its
disk does not contain many stars. Astronomers speculate that Centaurus A could be at an earlier
stage of evolution than the Sombrero and might eventually look similar.

The findings also answer a mystery about the number of globular clusters in the Sombrero
galaxy. Globular clusters are spherical nuggets of old stars. Ellipticals typically have a few
thousand, while spirals contain a few hundred. The Sombrero has almost 2,000, a number that
makes sense now but had puzzled astronomers when they thought it was only a disk galaxy.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope
mission for NASA's Science Mission Directorate, Washington. Science operations are conducted
at the Spitzer Science Center at the California Institute of Technology in Pasadena. Data
are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis
Center at Caltech. Caltech manages JPL for NASA. For more information about Spitzer, visit
http://spitzer.caltech.edu and http://www.nasa.gov/spitzer .

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