A new study offers the first direct evidence that supermassive black holes grow along with the galaxies they inhabit. Astronomers had suspected as much, but because most galaxies today are billions of years old, the notion was difficult to test.

In their work, Michael R. Merrifield of the University of Nottingham in England and his colleagues sought galaxies spanning a range of ages. In this way, they could determine how black holes that lurk at the hearts of these galaxies evolved over time.

To measure galactic age, the astronomers analyzed the color and intensity of starlight. Massive stars, for instance, live briefly and emit lots of blue light. If a galaxy isn't in the throes of starbirth yet is radiating strongly at the blue end of the spectrum, it's likely to be relatively young.

Merrifield's team found that Andromeda and 22 other nearby galaxies believed to house central black holes range in age from 4 billion to 12 billion years. Moreover, the researchers determined that the lightest black holes reside in the youngest galaxies, and the heaviest ones in the oldest.

Black holes apparently bulk up as they dine on stars and gas in their host galaxies. Mergers between galaxies in the young cosmos may have hastened that process by driving gas toward the core black holes, notes Abraham Loeb of Harvard University.

Merrifield, Duncan A. Forbes of the University of Birmingham in England, and Alejandro I. Terlevich of Swinburne University in Hawthorn, Australia, report their study in the April 1 Monthly Notices of the Royal Astronomical Society.

"This new work is further evidence that the growth of the hole is related to the evolution of the galaxy," says Martin J. Rees of the University of Cambridge in England.

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