Why galaxies stop forming stars

Chuck Bednar for redOrbit.com – @BednarChuck

Supernovae may play a role in bringing an end to a galaxy’s star-forming days, working with supermassive black holes and sweeping out gas to prevent the process from restarting, recent research from Michigan State University has revealed.

The study, published in a recent edition of The Astrophysical Journal Letters, explains that the black holes found at the center of a galaxy launch streams of charged particles, stirring up gas and interrupting star formation. Typically, that gas would cool down, and star formation would resume in that galaxy – meaning that something else has to be involved.

“Star formation is conspicuously absent from massive elliptical galaxies,” Mark Voit, a physics and astronomy professor in the MSU College of Natural Science and lead investigator on the study, told redOrbit via email. “Astronomers have known for a while that supernova explosions alone are not powerful enough to shut off star formation in very massive galaxies. The odds-on favorite these days for shutting off star formation is outbursts from a central black hole.”

“However, black-hole outbursts can’t be the whole story, because aging stars continue to shed gas and dust into interstellar space long after a black-hole outburst shuts off the star formation,” he added. “Somehow the galaxy needs to continually sweep out that gas. Otherwise, it will accumulate and start forming stars again. That’s where supernova sweeping comes in.”

Solving the mystery of why galaxies stop forming stars

Voit explained that he and his colleagues found that there are two different types of massive elliptical galaxies: One that has cooler clouds intermixed with hot gas that fills the galaxy, and a second that has only hot gas. The former is known as a multiphase medium, and the latter is called a single-phase medium.

“We found that supernova sweeping can keep cold gas from accumulating in the single-phase galaxies by continually expelling the gas that comes off of dying stars. Once the black hole shuts off star formation, supernova sweeping can keep it turned off,” he told redOrbit.

In multiphase elliptical, however, cooler gas clouds can accumulate because the supernova sweeping process is less effective in that type of galaxy, Voit continued. He and his co-authors were able to discover that multiphase elliptical galaxies were designed in such a way to trigger a black-hole outburst whenever too much cold gas begins to accumulate.

“This helps to solve a long-standing puzzle in astronomy:  Why did star formation stop a long time ago in most of the universe’s largest galaxies?”

“Our answer comes in two parts: (1) black-hole outbursts halt star formation by heating and ejecting cool gas from massive galaxies, and (2) supernova sweeping prevents star formation from starting again by preventing more cold gas from accumulating.”

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