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Reading the Clues inGalaxy Clusters

By John Rennie

Four hundred and eighty million light years from Earth sits a great cluster of galaxies that astronomers call Abell 2052. At its heart is a giant elliptical galaxy centered on a black hole as wide as our entire solar system. At its edge is a spiraling streamer of hot gas 10 times as long as the Milky Way, born of a collision long ago with a smaller cluster. Even through the most sensitive astronomical instruments, that curving filament—like a variety of bubbles, ripples, shock waves, and other structures in the tenuous medium within the cluster—is invisible to the human eye.

Thus, one of the best places in the universe to see it is in the modest fifth-floor office of astrophysicist and Assistant Professor of Astronomy Elizabeth Blanton. Last year, she and her colleagues announced their discovery of that vast swoop of gas, which had been buried within a week’s worth of data collected by NASA’s Chandra X-ray Observatory.

Although she is always measured and precise when she talks about her work, her enjoyment shines through her descriptions of caroming clusters of galaxies. “It gives you that much, much larger perspective,” she says, gesturing excitedly at a few pixels on-screen. “And to think that our solar system is just like some little dot inside that little dot. It is mind-boggling how much is out there.”

From her laptop, she summons vividly colored images and simulations that make the dynamics and aftermath of the cluster collision apparent. During the off-center collision between the two galaxy clusters, Blanton explains, the hot, ionized gas in the core of the main cluster was tugged by the shifting gravitational influences. In effect, “it sloshed like wine in a glass,” she says, and its new angular momentum sent it into a new, wider orbit outside the central core of galaxies.

Ultimately, that sloshing has a profound effect on the evolution of the galaxies within Abell 2052: it redistributes the atoms of the elements that were forged inside exploding stars, such as iron and oxygen, which will eventually be gathered into new generations of stars and planets.

Remarkable as the wave of gas around Abell 2052 is, much of Blanton’s research focuses on a different phenomenon called AGN (Active Galactic Nucleus) feedback. Everyday physics suggests that in a galaxy cluster, gravity should naturally tend to make the ionized gas denser and hotter closer to the center. But in so-called cool core clusters like Abell 2052, that is only partly true. Instead, as the ionized gas becomes denser, it emits more X-rays, loses energy, and cools faster than the less dense gas farther out.

That unstable arrangement should cause large quantities of gas to stream inward to restore stability, and that cool, dense gas should be condensing into new stars. Yet fewer stars seem to have formed than simple models predicted.

Something must be heating the gas, and Blanton and her colleagues think they know what it is. The answer may lie with those AGNs—the supermassive black holes at the centers of elliptical galaxies. The disks of high-pressure gas swirling around those black holes give rise to jets of fast-moving particles that can be observed at radio frequencies. Blanton’s work suggests those jets may warm the cooling, radiating gas and stop much of it from falling below a certain average temperature.

That finding is important, she explains, because existing models for galaxy formation that do not include the ingredient of AGN feedback tend to predict that the universe should hold more massive galaxies than astronomers actually see. But this heating mechanism would limit star formation and keep galaxies smaller.

Blanton is excited by the opportunity afforded to her by BU’s recent investment in the 4.3-meter Discovery Channel Telescope in Arizona. Images at optical wavelengths from this telescope, along with those in infrared wavelengths from the Spitzer Space Telescope, will be vital in Blanton’s new survey of distant clusters of galaxies.

Anyone looking for clues to what helped shape Blanton into an accomplished astrophysicist might be tempted to start with her early childhood home in Altadena, California, near the California Institute of Technology and the Mt. Wilson Observatory. Yet Blanton maintained a balance of interests in the humanities and sciences well into college. At Vassar she majored in both astronomy and French, and spent a semester in Paris studying art, literature, and architecture. Her passion for astronomy was stronger, however. Graduate work at Columbia University led to postdocs at Carnegie Mellon University and the University of Virginia. She joined the faculty of BU in 2004.

What drew her to the sciences, she says, were the philosophical questions they could illuminate: “What’s the meaning of everything, and where are we going, and where did we come from?” With her recent discoveries about what shapes the evolution of galaxies and stars, she has found some very relevant answers.

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