Researchers appear to have an explanation for a longstanding question in astrophysics: why did the pace of star formation in the Universe slow down 11 billion years ago?

Galaxies reached their busiest star-making pace about 11 billion years ago, then slowed down.

Astrophysicists have puzzled for years over the question of what happened.

“The answer was energy feedback from quasars within the galaxies where stars are born,” explain the researchers who report their findings in the Monthly Notices of the Royal Astronomical Society.

“That is, intense radiation and galaxy-scale winds emitted by the quasars heats up clouds of dust and gas. The heat prevents that material from cooling and forming more dense clouds, and eventually stars.”

The scientists looked at information on 17,468 galaxies and found a tracer of energy known as the Sunyaev-Zel’dovich effect (SZ effect).

The phenomenon, named for two physicists who predicted it nearly 50 years ago, appears when high-energy electrons disturb the Cosmic Microwave Background (CMB), a remnant from the superheated birth of the Universe some 13.7 billion years ago.

“The thermal energy levels were analyzed to see if they rise above predictions for what it would take to stop star formation,” said lead author Devin Crichton, from Johns Hopkins University. “A large number of galaxies were studied to give the study statistical heft.”

“For feedback to turn off star formation, it must be occurring broadly,” he added.

To take the faint temperature measurements that would show the SZ effect, the team used information gathered by two ground-based telescopes – an optical telescope at the Apache Point Observatory in New Mexico and the Atacama Cosmology Telescope in northern Chile – and the Spectral and Photometric Imaging Receiver (SPIRE) onboard ESA’s Herschel Space Observatory.

“Using several instruments with different strengths in search of the SZ effect is relatively new. It’s a pretty wild sort of thermometer,” said co-author Dr. Tobias Marriage, also from Johns Hopkins University.

“I would argue that this is the first convincing observational evidence of the presence of quasar feedback when the Universe was only a quarter of its present age, when the cosmic star formation was most vigorous,” he said.

“While the findings are not conclusive, the evidence is very compelling and has scientists excited.”

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Devin Crichton et al. 2016. Evidence for the thermal Sunyaev-Zel’dovich effect associated with quasar feedback. MNRAS 458 (2): 1478-1492; doi: 10.1093/mnras/stw344