Over the hill ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt

OUR galaxy is past middle age and could be gradually dying. A team of stellar archaeologists have found the first evidence that the Milky Way suddenly stopped giving birth to stars after it formed a thick saucer-like disc around 8 billion years ago, suggesting such “quenching” can happen even before a galaxy runs out of gas.

Galactic life cycles were thought to be driven largely by how much gas a galaxy has to build new stars with. But it was unclear whether most galaxies lose their raw material abruptly when it is ejected by supernovae or a central black hole, burn through their reserves slowly, or stop growing for some other reason.

Misha Haywood at the Paris Observatory and his colleagues decided that studying our own galaxy is the best way to address this question. They probed the chemical signatures of tens of thousands of stars – a clue to their ages – recorded by a high-resolution spectrograph at the Sloan Digital Sky Survey telescope in New Mexico. With the data, they were able to reconstruct a record of the Milky Way’s past.


They found that our galaxy’s star formation rate dropped by an order of magnitude between 10 billion and 8 billion years ago. It resumed forming stars after this sudden die-off, but at a much slower rate.

This epoch was also when our galaxy formed its bulging disc and bar-like concentration of stars at its centre. “There seems to be a connection between quenching in the Milky Way and its thick disc,” Haywood says.

That means a galaxy can stop growing even while it has reservoirs of gas, as the Milky Way does. Haywood argues that the disc and bar structures could disrupt growth by stirring up the gas, making it too hot to form new stars (arxiv.org/abs/1601.03042). Other spiral galaxies too distant to be probed by these methods could be ageing in a similar fashion.

“It was unclear whether most galaxies lose their fuel abruptly or burn through reserves slowly“

“Star formation boils down to a battle between gravity and other things, like turbulence,” says Katherine Alatalo at the Carnegie Observatories in Pasadena, California. Our galaxy’s disc and bar cause the gas to become turbulent, injecting energy that prevents it from collapsing and initiating star formation, she says. “Their results with the Milky Way give us subtle clues about what we should be looking for in other evolving spiral galaxies, too.”

This article appeared in print under the headline “Milky Way retired early from star-making”