How Massive, Elliptical Galaxies Die from the Inside-Out

Giant galaxies may actually die from the inside out. Astronomers have shown for the first time how star formation in "dead" galaxies sputtered out billions of years ago.

Scientists have long wondered how massive, elliptical galaxies, common in the modern universe, quenched their furious rates of star formation. These huge galaxies, often called spheroids, typically pack in stars ten times as densely in the central regions as in our home galaxy, the Milky Way.

Astronomers refer to these big galaxies as red and dead, since they possess many ancient red stars, but lack young blue stars and show no evidence of new star formation. It's estimated that these host galaxies ceased to create new stars about ten billion years ago.

"Massive dead spheroids contain about half of all the stars that the universe has produced during its entire life," said Sandro Tacchella, lead author of the new study, in a news release. "We cannot claim to understand how the universe evolved and became as we see it today unless we understand how these galaxies come to be."

The researchers studied 22 galaxies, spanning a range of masses, from a time period that was about three billion years after the Big Bang. The researchers found that the most massive galaxies in the sample kept up a steady production of new stars in their peripheries. In their centers, though, star formation had already stopped.

"The newly demonstrated inside-out nature of star formation shutdown in massive galaxies should shed light on the underlying mechanisms involved, which astronomers have long debated," said Alvio Renzini, one of the researchers.

The findings reveal a bit more about galaxy evolution and show that massive, star-ridden galaxies die from the inside-out.

The findings are published in the journal Science.

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