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Campus Currents May 2007: Volume 4, Number 2

Are Black Holes Galactic Killers?

Supermassive black holes millions to billions of times the mass of our Sun lie at the heart of most, maybe all, large galaxies.

Some of these power brilliantly luminous, rapidly growing objects called active galactic nuclei that gather and condense enormous quantities of dust, gas and stars. Therefore many astron­omers thought active galactic nuclei might be helping to end star formation, though the evidence was always circumstantial.

That idea has now been overturned by a new survey that found active galactic nuclei in all kinds and sizes of galaxies, including young, star-making factories.

Not even light escapes the gravitational field of black holes like those at the centers of active galactic nuclei, so they can't be observed directly. But as material swirls inward, before it's sucked into the void, it releases intense radiation across the electromagnetic spectrum, including visible light. Of these, X-rays are often the brightest as they penetrate the dust and gas that sometimes obscures other wavelengths.

James Aird, a postdoctoral fellow at UC San Diego's Center for Astrophysics and Space Sciences, searched the sky for X-rays from active galactic nuclei using two orbiting telescopes and compared those signals to a separate large-scale survey of about 100,000 galaxies.

That survey, led by Alison Coil, assistant professor of physics, and colleagues at other institutions, measured the distance of each galaxy and the color of their light. As star-making ceases, and stars burn through their fuel, the color of their light shifts toward red.

Aird found hundreds of X-ray signals emanating from galaxies of every kind: massive and smaller, old red and younger blue ones. They're everywhere, which means that active galactic nuclei have been exonerated as suspects in the quenching of star formation.

—Susan Brown