Not all stars are created equal. The heaviest main-sequence stars, with masses more than 10 times that of the Sun, are bright, hot, and blue, and they burn themselves out in as little as 3 million years. At the other end of the continuum, stars less than a tenth of a solar mass are cooler and redder, with expected lifetimes of hundreds of billions of years.

In galaxies other than the Milky Way, individual stars are too far away to resolve, but their overall spectra are a record of the kinds of stars their parent galaxies contain. And therein lies a mystery- Of the most massive nearby galaxies, almost none are emitting any blue light. Not only have those galaxies’ hot, fast-burning stars all extinguished themselves, but no new ones have formed to take their place. In fact, the galaxies are forming almost no new stars at all.

How did those galaxies become so quiescent—or, more colloquially, ‘red and dead’? To find out, researchers are working to trace the history of star-formation shutdowns over the 14 billion years since the Big Bang.

Distant galaxies, whose light has taken billions of years to reach Earth, provide a window into what was happening in the universe long ago. The challenge, however, is that the farther away a galaxy is, the smaller it appears on the sky, and the less of its light can be collected by telescopes. One solution is to build up a useful signal by stacking the spectra from many similar-looking galaxies. That approach, however, risks obscuring any intergalactic diversity.

The REQUIEM collaboration (short for ‘Resolving Quiescent Magnified Galaxies’) takes a different tack. Co-led by Katherine Whitaker of the University of Massachusetts Amherst and Sune Toft of the Niels Bohr Institute in Copenhagen, the team seeks to exploit the signal-boosting effect of gravitational lensing- If a distant galaxy lies directly behind a massive foreground object, its light might be bent in such a way that more of it reaches Earth. That configuration, unsurprisingly, is rare. But from a decade-long search, the REQUIEM researchers identified 10 distant, quiescent galaxies magnified by gravitational lensing by up to a factor of 30.

Now Whitaker and colleagues have obtained complementary data on six of those galaxies from the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile. The ALMA signal, at a wavelength of 1.3 mm, comes not from the galaxies’ stars but from their interstellar dust. Dust serves as a proxy for their content of molecular hydrogen gas, the necessary ingredient for star formation.

The ALMA data showed no detectable dust in four of the six galaxies and very little in the other two. The figure shows a composite image from ALMA and the Hubble Space Telescope of one of the REQUIEM targets. The lensed galaxy is the curved object in the center of the frame. The ALMA signal, shown in purple, emanates only from other objects in the field of view.

The REQUIEM galaxies date from between 9.5 billion and 11.5 billion years ago, an epoch when there should have been plenty of H₂ gas around for galaxies to accrete and make into stars. Indeed, nonquiescent galaxies from the same period often contain more than 50% H₂ by mass.

The result presents both a tidy answer and a messy question. Early quiescent galaxies—or at least the six that REQUIEM studied—stopped forming stars because they lacked the H₂ to do so. But where did the gas go? Was it simply used up and not replenished? Was it somehow ejected from the galaxy? Or was it heated up by some energy source, such as a supermassive black hole, into an atomic or ionized form that was no longer capable of forming stars? Could the mechanism be different for different galaxies?

The REQUIEM researchers are continuing to gather data on their target galaxies at other wavelengths and using other telescopes—one of the targets is scheduled to be imaged by the soon-to-be-launched James Webb Space Telescope—in the hope of better understanding how galaxies ran out of gas. (K. E. Whitaker et al., Nature 597, 485, 2021.)