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A new study reveals how often some of the biggest black holes are active.

These results come from a massive survey of galaxies using data from the Chandra X-ray Observatory and the Sloan Digital Sky Survey.

The study has important implications for how a galaxy's environment affects the growth of the black hole at its center.

This two-panel graphic contains two composite images of galaxies used in a recent study of supermassive black holes. In each of the galaxies, data from NASA's Chandra X-ray Observatory are blue, and optical data from the Sloan Digital Sky survey are colored red, green and blue. The galaxy on the left, Abell 644, is in the center of a galaxy cluster that lies about 1.1 billion light years from Earth. On the right is an isolated, or "field," galaxy named XS04107B6_001, which is located about 900 million light years away. At the center of both of these galaxies is a growing supermassive black hole, called an active galactic nucleus (AGN) by astronomers, which is pulling in large quantities of gas.

A newly published study from Chandra tells scientists how often the biggest black holes in field galaxies like XS04107B6_001 have been active over the last few billion years. This has important implications for how environment affects black hole growth. The scientists found that only about one percent of field galaxies with masses similar to the Milky Way contain supermassive black holes in their most active phase. They also found that the most massive galaxies are the most likely to host these AGN, and that there is a gradual decline in the AGN fraction with cosmic time. Finally, the AGN fraction for field galaxies was found to be indistinguishable from that for galaxies in dense clusters, like Abell 644.

This study involves a survey called the Chandra Multiwavelength Project, or ChaMP, which covers 30 square degrees on the sky, the largest area covered of any Chandra survey to date. Combining Chandra's X-ray images with optical images from the Sloan Digital Sky Survey, about 100,000 galaxies were analyzed. Out of those, about 1,600 were bright in X-ray light, signaling possible AGN activity.

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