[Spitzer News] Spitzer Sees Shining Stellar Sphere

spitzer-news at lists.ipac.caltech.edu spitzer-news at lists.ipac.caltech.edu
Thu Apr 10 14:55:31 PDT 2008

In this issue:

1) Spitzer Sees Shining Stellar Sphere
2) Spitzer Spots Ancient Cosmic Urban Sprawl
3) Cosmic Searchlights Reveal "Lost" Galaxies



Millions of clustered stars glisten like an iridescent opal in a new  
image from NASA's Spitzer Space Telescope.

Called Omega Centauri, this sparkling orb of stars is like a  
miniature galaxy. It is the biggest and brightest of the more than  
150 similar objects, called globular clusters, that orbit around the  
outside of our Milky Way galaxy. Stargazers at southern latitudes can  
spot the stellar gem with the naked eye in the constellation Centaurus.

While the visible-light observations highlight the cluster's millions  
of jam-packed stars, Spitzer's infrared eyes reveal the dustier, more  
evolved stars tossed throughout the region.

"Now we can see which stars form dust and can begin to understand how  
the dust forms and where it goes once it is expelled from a star,"  
said Martha Boyer of the University of Minnesota, Twin Cities. Boyer  
is lead author of a paper about Omega Centauri appearing in the April  
issue of the Astronomical Journal. "Surprisingly, Spitzer revealed  
fewer of these dusty stars than expected."

Globular clusters are some of the oldest objects in our universe.  
Their stars are more than 12 billion years old, and, in most cases,  
formed all at once when the universe was just a toddler. Omega  
Centauri is unusual in that its stars are of different ages and  
possess varying levels of metals, or elements heavier than boron.  
Astronomers say this points to a different origin for Omega Centauri  
than other globular clusters: they think it might be the core of a  
dwarf galaxy that was ripped apart and absorbed by our Milky Way long  

In the new picture of Omega Centauri, the red- and yellow-colored  
dots represent the stars revealed by Spitzer. These are the more  
evolved, larger, dustier stars, called red giants. The stars colored  
blue are less evolved, like our own sun, and were captured by both  
Spitzer's infrared eyes and in visible light by the National Science  
Foundation's Blanco 4-meter telescope at Cerro Tololo Inter-American  
Observatory in Chile. Some of the red spots in the picture are  
distant galaxies beyond our own.

"As stars age and mature into red giants, they form dust grains,  
which play a vital role in the evolution of the universe and the  
formation of rocky planets," said Jacco van Loon, the study's  
principal investigator at Keele University in England. "Spitzer can  
see this dust, and it was able to resolve individual red giants even  
in the densest central parts of the cluster."

NASA's Hubble Space Telescope and Gemini Observatory on Cerro Pachon  
in Chile recently found evidence that Omega Centauri is home to a  
medium-sized black hole -- http://hubblesite.org/newscenter/archive/ 
releases/2008/14/ .

Other authors of the paper include Iain McDonald and Nye Evans of  
Keele University; Robert Gehrz and Charles Woodward of the University  
of Minnesota; and Andrea Dupree of the Harvard-Smithsonian Center for  
Astrophysics, Cambridge, Mass.

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,  
also in Pasadena. Caltech manages JPL for NASA. Spitzer's infrared  
array camera, which took the Omega Centauri picture, was built by  
NASA's Goddard Space Flight Center, Greenbelt, Md. The instrument's  
principal investigator is Giovanni Fazio of the Harvard-Smithsonian  
Center for Astrophysics.

Cerro Tololo Inter-American Observatory is part of the National  
Optical Astronomy Observatory, which is operated by the Association  
of Universities for Research in Astronomy under contract with the  
National Science Foundation.




The universe's first "galactic cities" did not sprout up randomly  
across space. On the contrary, a new statistical analysis of  
observations from NASA's Spitzer Space Telescope confirms that these  
ancient galactic metropolises may have developed much like sprawling  
cities joining together into a larger urban whole.

Across the cosmos, galaxies rarely stand alone. Instead, they are  
grouped into large, densely populated communities containing  
thousands of galactic residents, called galaxy clusters.

"Previously, we only knew of a handful of galaxy clusters that  
existed when our universe was in its first few billion years. Now,  
thanks to Spitzer's superb sensitivity, we've identified over a  
hundred," says Dr. Mark Brodwin, of the National Optical Astronomy  
Observatory, in Tucson, Ariz.

Using Spitzer's substantial sample of distant galaxy clusters,  
Brodwin and his colleagues were able to conduct a statistical  
analysis showing that distant galaxy clusters tend to huddle together  
-- like cosmic urban sprawl with neighborhoods growing together into  
a larger and larger community.

"The clustering of clusters indicates that these ancient galactic  
cities are not randomly distributed across space," says Brodwin.
Astronomers have long suspected that the first galaxy clusters grew  
in very special regions of space, where pockets of hot gas began  
collecting hundreds of thousands of years after the big bang. The  
fact that ancient galaxy clusters clump together could be evidence  
that they formed in such dense regions in the early universe.

Brodwin notes that this observed clumping matches current  
astronomical theories about galaxy cluster formation. Theory  
predicted the density of these cosmic cities in the distant past, and  
the recent measurements of his team have confirmed these predictions.

"This analysis is helping us understand how the largest structures in  
the universe came to be," says Brodwin.

Brodwin's paper was published in the December 20, 2007 issue of  
Astrophysical Journal Letters. Co-authors on this paper are Leonidas  
Moustakas, Peter Eisenhardt, and Daniel Stern, of NASA's Jet  
Propulsion Laboratory in Pasadena, Calif.; Anthony Gonzalez, of the  
University of Florida, Gainsville, Fla.; Adam Stanford, of the  
University of California at Santa Cruz; and Michael Brown of Monash  
University in Clayton, Australia.




Millions of faint galaxies are hovering near the edge of our  
universe, too dim to be detected by most telescopes -- but some huge  
cosmic explosions and the supersensitive infrared eyes of NASA's  
Spitzer Space Telescope are bringing many of these muted galaxies to  

Located approximately 12.5 billion light-years away from Earth, the  
distant galaxies exist in an era when our universe was just one  
billion years old. With Spitzer's sensitive infrared eyes,  
astronomers can finally snap infrared portraits and even "weigh" many  
of these otherwise invisible galaxies.

"A few billion years after the big bang, 90 percent of the stars  
being born were occurring in these types of faint galaxies. By  
identifying this population, we hope to gain insights into the  
environments where the universe's first stars formed," says Dr. Ranga  
Ram Chary, of the Spitzer Science Center, Pasadena, Calif.

Finding Hidden Galaxies

How did astronomers find these elusive galaxies? Like a searchlight  
directing people to a high-profile event, astronomers followed an  
afterglow from huge explosions, called "gamma ray bursts" to the  
faint distant galaxies. They suspect that gamma ray bursts appear  
when a very massive star dies and becomes a black hole.

Gamma ray bursts are fleeting events -- lasting anywhere from a  
fraction of a second, to a few minutes. This is not enough time for  
astronomers to directly identify their source. However, as the gamma  
ray light fades, a lingering afterglow can be seen at other  
wavelengths of light. In fact, Chary's team used ground-based  
telescopes to follow the infrared afterglow from several of these  
events back to their dim host galaxies, months after the initial  
explosions occurred.

The afterglow occurs when energetic electrons spiral around magnetic  
fields, and release light. In its explosive death, material shooting  
out of the massive star smashes into surrounding gas. This violent  
collision heats nearby gas and energizes its electrons.

Once coordinates of the faint galaxies were determined, Chary's team  
then used Spitzer's supersensitive infrared array camera to snap a  
picture of the faint galaxy. The amount of light from the galaxies  
allowed Chary to weigh the galaxies. They found these distant  
galaxies were cosmic "lightweights", or not very massive compared to  
mature galaxies we see nearby.

"Understanding the mass and chemical makeup of the universe's first  
galaxies and then taking snapshots of galaxies at different ages,  
gives us a better idea of how gas, dust and metals-- the material  
that went into making our Sun, solar system, and Earth --has changed  
throughout the Universe's history," says Chary.

Unlike the galaxies of today, Chary says that galaxies living in the  
one billion year old universe were much more pristine -- comprised  
primarily of hydrogen and helium gas and containing less than 10% of  
the heavier elements we see in the local Universe, and even on Earth.  
The stars that formed and lived in these galaxies eventually forged  
heavier chemical elements in their cores. In death, the stars spit  
their chemical creations into space. Some of that material went into  
making another generation of stars and eventually planets in the  
galaxies while a fraction of the metals were ejected entirely out of  
the galaxy.

Chary's paper was published in the December 10, 2007 issue of the  
Astrophysical Journal. Co-authors on this paper include Dr. Edo  
Berger, of Princeton University, Princeton, NJ, and Dr. Len Cowie, of  
the University of Hawaii.



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