NASA - National Aeronautics and Space Administration

Playing Hard to Get
In a case of elusive quarry, scientists using NASA's Chandra X-ray Observatory have found for the first time a theorized dim halo of hot gas around a spiral-shaped galaxy. This new discovery helps to justify a highly regarded, yet unproven model for spiral galaxy formation.

Image to right: A blue halo 60,000 light years across surrounds galaxy NGC 5746. Credit: NASA/CXC/U. Copenhagen/K.Pedersen et al

"The halos exist, but are so faint that an extremely sensitive telescope such as Chandra is needed to detect them," said Kristian Pedersen of the University of Copenhagen in Denmark, and lead scientist for the discovery. "Our observations solve the mystery of the missing hot halos around spiral galaxies."

The galaxy is called NGC 5746 and is the same type as our own Milky Way. The spiraling mass of stars and stellar objects sits roughly 100 million light years from Earth. According to the model, galaxies like NGC 5746 are thought to emerge from clouds of intergalactic gas that cool and collapse to form giant, spinning pinwheels of stars and debris. If the model is true, then the gas left over from the formation of the galaxies will gradually fall inwards, creating a halo of glowing gas. Despite scientists' confidence in the concept and its associated predictions, the hallmark halo has never been seen until now.

"We targeted NGC 5746 because we thought its distance and orientation would give us the best chance to detect a hot halo," said Jesper Rasmussen of the University of Birmingham in the United Kingdom and a co-researcher in the study. "What we found is in good agreement with computer simulations."

Turmoil Beneath
Like a cosmic riptide, supermassive black holes are creating a violent undertow in seemingly calm elliptical galaxies. This finding comes from astronomers reviewing data gathered by Chandra on 56 of the oblong galaxies.

Image to right: An X-ray image of NGC 5044 (left) shows turbulence not visible in optical images (right). Credit: X-ray: NASA/CXC/U. Ohio/T. Statler & S. Diehl; Optical: DSS

"Most elliptical galaxies have traditionally been considered to be quiet places, like placid lakes," said astronomer Thomas Statler from the Ohio University in Athens, Ohio. "Our results show these galaxies are a lot stormier than we thought."

When viewed through optical telescopes in space or on mountaintops, the elliptical galaxies appear docile. It's not until the galaxies are examined in X-ray light by Chandra that they take on their more unruly character. "This is another example of how valuable it is to observe the universe at different wavelengths besides just the traditional optical wavelengths," said scientist Wilt Sanders of NASA's Chandra program.

Statler and research partner Steven Diehl, also of the Ohio University, believe massive clouds of hot gas within the galaxies are being stirred up by intermittent explosions. "Something is definitely making a mess there, and pumping energy equivalent to a supernova every century into the gas," said Diehl. The source of the turbulent blasts? Statler and Diehl point to supermassive black holes at the centers of the galaxies. Black holes regularly devour nearby objects, and the two scientists believe the violent ingestion of local gases is causing the galactic rough waters.

Charlie Plain
NASA's Chandra X-ray Observatory and John F. Kennedy Space Center