[Spitzer News] Even Thin Galaxies Can Grow Fat Black Holes

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Thu Jan 10 12:41:48 PST 2008

In this issue:

1) Even Thin Galaxies Can Grow Fat Black Holes
2) One Million Downloads for the Hidden Universe



NASA's Spitzer Space Telescope has detected plump black holes where  
least expected -- skinny galaxies.

Like people, galaxies come in different shapes and sizes. There are  
thin spirals both with and without central bulges of stars, and more  
rotund ellipticals that are themselves like giant bulges. Scientists  
have long held that all galaxies except the slender, bulgeless  
spirals harbor supermassive black holes at their cores. Furthermore,  
bulges were thought to be required for black holes to grow.

The new Spitzer observations throw this theory into question. The  
infrared telescope surveyed 32 flat and bulgeless galaxies and  
detected monstrous black holes lurking in the bellies of seven of  
them. The results imply that galaxy bulges are not necessary for  
black hole growth; instead, a mysterious invisible substance in  
galaxies called dark matter could play a role.

"This finding challenges the current paradigm. The fact that galaxies  
without bulges have black holes means that the bulges cannot be the  
determining factor," said Shobita Satyapal of the George Mason  
University, Fairfax, Va. "It's possible that the dark matter that  
fills the halos around galaxies plays an important role in the early  
development of supermassive black holes."

Satyapal presented the findings today at the 211th meeting of the  
American Astronomical Society in Austin, Texas. A study from Satyapal  
and her team will be published in the April 10 issue of the  
Astrophysical Journal.

Our own Milky Way is an example of a spiral galaxy with a bulge; from  
the side, it would look like a plane seen head-on, with its wings out  
to the side. Its black hole, though dormant and not actively  
"feeding," is several million times the mass of our sun.

Previous observations had suggested that bulges and black holes  
flourished together like symbiotic species. For instance,  
supermassive black holes are almost always about 0.2 percent the mass  
of their galaxies' bulges. In other words, the more massive the  
bulge, the more massive the black hole. Said Satyapal, "Scientists  
reasoned that somehow the formation and growth of galaxy bulges and  
their central black holes are intimately connected."

But a wrinkle appeared in this theory in 2003, when astronomers at  
the University of California, Berkeley, and Observatories of the  
Carnegie Institution of Washington, Pasadena, Calif., discovered a  
relatively "lightweight" supermassive black hole in a galaxy lacking  
a bulge. Then, earlier this year, Satyapal and her team uncovered a  
second supermassive black hole in a similarly svelte galaxy.

In the latest study, Satyapal and her colleagues report the discovery  
of six more hefty black holes in thin galaxies with minimal bulges,  
further weakening the "bulge-black hole" theory. Why hadn't anybody  
seen these black holes before? According to the scientists, bulgeless  
galaxies tend to be very dusty, letting little visible light escape.  
But infrared light can penetrate dust, so the team was able to use  
Spitzer's infrared spectrograph to reveal the "fingerprints" of  
active black holes lurking in galaxies millions of light years away.

"A feeding black hole spits out high-energy light that ionizes much  
of the gas in the core of the galaxy," said Satyapal. "In this case,  
Spitzer identified the unique fingerprint of highly ionized neon --  
only a feeding black hole has the energy needed to excite neon to  
this state." The precise masses of the newfound black holes are unknown.

If bulges aren't necessary ingredients for baking up supermassive  
black holes, then perhaps dark matter is. Dark matter is the  
enigmatic substance that permeates galaxies and their surrounding  
halos, accounting for up to 90 percent of a galaxy's mass. So-called  
normal matter makes up stars, planets, living creatures and  
everything we see around us, whereas dark matter can't be seen. Only  
its gravitational effects can be felt. According to Satyapal, dark  
matter might somehow determine the mass of a black hole early on in  
the development of a galaxy.

"Maybe the bulge was just serving as a proxy for the dark matter mass  
-- the real determining factor behind the existence and mass of a  
black hole in a galaxy's center," said Satyapal.

Other authors of this study include: D. Vega of the George Mason  
University; R.P. Dudik of the George Mason University and NASA  
Goddard Space Flight Center, Greenbelt, Md.; N.P. Abel of the  
University of Cincinnati, Ohio; and Tim Heckman of the Johns Hopkins  
University, Baltimore, Md.

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  
spectrograph was built by Cornell University, Ithaca, N.Y. Its  
development was led by Jim Houck of Cornell.




It's official -- "The Hidden Universe" is no longer a secret. The  
video podcast series from NASA's Spitzer Space Telescope Science  
Center has reached one million downloads!

This series takes viewers on a visually stunning voyage into the  
infrared universe lurking beyond the realm of human vision.

When the series originally launched in May 2006, it was the first  
video podcast for a NASA mission. In March 2007, the series was the  
first astronomy podcast offered in high-definition. Over the next few  
months, ratings for the high-definition feed surged -- and in  
September 2007, The Hidden Universe HD was the number one podcast in  
US iTunes music store, since then it has lingered among the top 25 of  
all podcasts.

The Hidden Universe shows come in two formats: "Gallery Explorer" and  
"Hidden Universe: Showcase." Gallery Explorer offers a slideshow of  
striking infrared images with commentary, while Showcase gives an in- 
depth documentary-style look at science in the infrared universe.

In addition to The Hidden Universe, the Spitzer Science Center also  
produces a general astronomy video podcast for elementary school  
audiences called "Ask an Astronomer." This particular series has now  
reached over three million downloads.

Spitzer also has an audio podcast, which features conversations with  
leading astronomers on the hottest topics, as well as news and  
information about NASA's Spitzer Space Telescope.



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