NASA'S SPITZER FINDS GALAXY WITH SPLIT PERSONALITY

WASHINGTON -- 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 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, Calif.. 
"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 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.