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The Dusty Atmospheres of Spiral Galaxies

High-altitude clouds on Earth affect our climate through their interaction with incoming sunlight. Their properties can also reflect unusual activity that occurred earlier on the ground, such as the eruption of a major volcano.

NGC4013, a large, nearby, edge-on spiral galaxy. Click for a 598x400 pixel image.

NGC4013, a large, nearby, edge-on spiral galaxy. Taken at the WIYN Telescope

Looking far beyond Earth, astronomers are gathering increasing evidence that dusty clouds high in the "atmospheres" of spiral galaxies - shaped like our own Milky Way - hold important clues about how these galaxies evolve over time. These dust clouds may even confuse the scientific interpretation of exotic objects located behind them in the distant reaches of the universe.

Dr. Chris Howk of The Johns Hopkins University in Baltimore and Dr. Blair Savage of the University of Wisconsin-Madison have used the Wisconsin-Indiana-Yale-NOAO (WIYN) 3.5-meter telescope on Kitt Peak to take images of these clouds around spiral galaxies in the local universe, a region within about 100 million light-years of the Milky Way.

In galactic terms, these clouds are considered "high altitude" since they are found very far above the main disk of the host spiral galaxy. The existence of these dusty clouds has been noted for decades, but most of them have not been studied in any great detail before now.

"The galaxies have to be 'big-on-the-sky' and have their central band set edge-on as we view it from Earth for us to study them properly," Howk explains.

About 70 percent of the edge-on spiral galaxies in the local universe show evidence for dusty high-altitude clouds far from the thin galactic disks. Some of the clouds are quite dense. The total mass of material in one of them can reach more than 100,000 times the mass of the Sun.

One way for this dust to emerge from the main disk of the host galaxy is through an interstellar "chimney" effect, where hot, high-pressure gas from multiple exploding stars, or supernovae, provides enough kinetic energy to lift material out. However, the recent observations with WIYN show a more clumpy, random and diverse distribution of cloud shapes than such a model produces.

NGC891, another large, nearby, edge-on spiral galaxy. Click for a 400x400 pixel image.

NGC891, another large, nearby, edge-on spiral galaxy Taken at the WIYN Telescope

This suggests more complex processes are likely at play, perhaps involving the relatively cool intersections of several of these hot chimneys, or interlocking shock fronts created as the expelled dusty material falls back toward the galaxy.

Indeed, as much as 10 percent of the dust of a galaxy may be ejected, carrying fundamental material like iron outward, and perhaps creating star-forming regions thousands of light years distant from the main spiral disk.

"The balance of how much of this dust rains back down onto the disk of a galaxy and how much is expelled altogether has major implications for how the galaxy will evolve over time," Howk explains.

Howk and Savage study the gaseous interstellar medium in the extended atmospheres of galaxies, and these dust clouds may have major implications for their efforts to better understand the flow of gas away from spiral galaxy disks. The actual expulsion of dust from a galaxy can even result in the 'pollution' of the intergalactic environment of the host galaxy.

"The high density of some of these clouds is quite surprising," Howk says. "If the dust is far enough away from the galaxy, it can even extinguish some of the light from objects like quasars and supernovae in the background, potentially changing the way astronomers interpret their properties. For example, the dust can redden and dim the light of the distant object, an effect similar to our view of the Sun at sunset."

Howk and Savage plan to use a spectrographic instrument on the National Science Foundation's 4-meter Mayall telescope at Kitt Peak National Observatory to further investigate the properties of these strange high-altitude clouds, and the stars that may be forming within them.

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