'Ghost of Mirach' Materializes in Space Telescope Image

Feature October 31, 2008


'Ghost of Mirach' Materializes in Space Telescope Image

NASA's Galaxy Evolution Explorer has lifted the veil off a ghost known to haunt the local
universe, providing new insight into the formation and evolution of galaxies.

The eerie creature, called NGC 404, is a type of galaxy known as "lenticular." Lenticular
galaxies are disk-shaped, with little ongoing star formation and no spiral arms. NGC 404 is the
nearest example of a lenticular galaxy, and therefore of great interest. But it lies hidden in the
glare from a red giant star called Mirach. For this reason, NGC 404 became known to
astronomers as the "Ghost of Mirach."

When the Galaxy Evolution Explorer spied the galaxy in ultraviolet light, a spooky ring
materialized.

"We thought this celestial ghost was essentially dead, but we've been able to show that it has an
extended ring of new stars. The galaxy has a hybrid character in which the well-known, very old
stellar population tells only part of the story," said David Thilker of Johns Hopkins University in
Baltimore. "It's like the living dead."

Thilker and members of the Galaxy Evolution Explorer team spotted the Ghost of Mirach in
images taken during the space telescope's all-sky survey. The Galaxy Evolution Explorer is a
relatively low-cost NASA mission, launched in 2003, with an ambitious charge to survey the
entire visible sky in ultraviolet light, a job never before accomplished. Because Earth's
atmosphere absorbs ultraviolet photons -- a good thing for us living creatures who are susceptible
to the damaging light -- ultraviolet telescopes must operate from space.

The first images of the Ghost of Mirach taken by the Galaxy Evolution Explorer hinted at a
surrounding ultraviolet-bright extended structure. Subsequent, longer exposure observations
indeed show that the lenticular galaxy is surrounded by a clumpy, never-before-seen ring of
stars.

What is this mysterious ultraviolet ring doing around an otherwise nondescript lenticular galaxy?
As it turns out, previous imaging with the National Science Foundation's Very Large Array radio
telescope in New Mexico had discovered a gaseous ring of hydrogen that matches the ultraviolet
ring observed by the Galaxy Evolution Explorer. The authors of this Very Large Array study
attributed the gas ring to a violent collision between NGC 404 and a small neighboring galaxy
900 million years ago.

The ultraviolet observations demonstrate that, when the hydrogen from the collision settled into
the plane of the lenticular galaxy, stars began to form in a ghostly ring. Young, relatively hot
stars forming in stellar clusters sprinkled throughout NGC 404's ring give off the ultraviolet light
that the Galaxy Evolution Explorer was able to see.

"Before the Galaxy Evolution Explorer image, NGC 404 was thought to contain only very old
and evolved red stars distributed in a smooth elliptical shape, suggesting a galaxy well into its
old age and no longer evolving significantly," said Mark Seibert of the Observatories of the
Carnegie Institution of Washington in Pasadena, Calif. "Now we see it has come back to life, to
grow once again."

"The Ghost of Mirach has been lucky enough to get a new lease on life through the rejuvenating,
chance merger with its dwarf companion," added Thilker.

The findings indicate that the evolution of lenticular galaxies might not yet be complete. They
may, in fact, continue to form stars in a slow, piecemeal fashion as they suck the raw, gaseous
material for stars from small, neighboring galaxies. It seems the Ghost of Mirach might act more
like a vampire than a ghost.

Caltech leads the Galaxy Evolution Explorer mission and is responsible for science operations
and data analysis. NASA's Jet Propulsion Laboratory in Pasadena, Calif., manages the mission
and built the science instrument. The mission was developed under NASA's Explorers Program
managed by the Goddard Space Flight Center, Greenbelt, Md. Researchers sponsored by Yonsei
University in South Korea and the Centre National d'Etudes Spatiales (CNES) in France
collaborated on this mission.

Graphics and additional information about the Galaxy Evolution Explorer is online at
http://www.nasa.gov/galex/ and http://www.galex.caltech.edu .

Media contact: Whitney Clavin/JPL
818-354-4673

-end-

To remove yourself from this mailing, please go to http://www.kintera.org/TR.asp?a=glLYLhMXLhKYLrI&s=hrJWLYNBKdKIIXNHIrF&m=feKIIQNxEiKVF

To remove yourself from all mailings from NASA Jet Propulsion Laboratory, please go to http://www.kintera.org/TR.asp?a=chKQJ5OHIeLII6I&s=hrJWLYNBKdKIIXNHIrF&m=feKIIQNxEiKVF