New ALMA observations reveal that diverse reservoirs of dust and molecular gas can persist long after galaxies stop forming stars, challenging theoretical expectations. 

For decades, astronomers have pictured massive quiescent galaxies as truly “red and dead”, ancient stellar sites whose lights have faded after using up or expelling almost all of their key building material for new stars. In this classic view, cold molecular gas and tiny dust grains belong in active, star-forming galaxies, not in old, quiescent ones. A new study with the Atacama Large Millimeter/submillimeter Array (ALMA) paints a much more complex picture. In a paper published in The Astrophysical Journal Letters, an international team led by Giuliano Lorenzon, PhD student at the Astrophysics Division of the National Centre for Nuclear Research (NCBJ, Poland), used ALMA to carry out one of the deepest searches so far for molecular gas and dust in quiescent galaxies beyond the local Universe. The team observed massive, “retired” galaxies in the COSMOS field, seeing them as they were about four billion years ago, long after their main star-forming phase had ended. 

Dust and gas part ways after quenching 

By measuring both the faint millimeter emission from dust grains and from carbon monoxide, a key tracer of molecular hydrogen, the researchers could directly weigh how much dust and gas these galaxies still contain roughly billion years after they stopped forming stars. 

The result is surprisingly diverse. Some galaxies are indeed poor in dust and gas compared with their stellar mass, matching the classic picture of systems that have efficiently depleted or destroyed their cold interstellar medium. Others, however, are rich in dust and/or gas despite being very old, with dust-to-gas ratios similar to the “canonical” values usually assumed for active galaxies such as the Milky Way. 

“We have known for a while that some quiescent galaxies still contain dust and molecular gas,” says Lorenzon, lead author of the study and principal investigator of the ALMA program. “What is completely new here is that we can weigh both components at the same time in multiple galaxies. We show that evolutionary clocks for dust and gas are not always tightly linked after star formation shuts down.” 

The team finds that decline of dust and gas reservoirs occur on very different timescales, either fading very quickly (within a few hundred million years) or much more slowly (over several billion years). In other words, there is no single path for galaxies to shut down and age, which challenges many current theoretical models. 

Rethinking how galaxies grow old 

This complex behaviour supports a scenario where, once star formation is quenched, dust and molecular gas can evolve largely independently. Processes such as dust grain growth and destruction, the slow inflow of fresh gas, or removal by black-hole feedback, can each leave a different imprint, even when no new stars are being formed. 

“Our results confirm that a complete lack of dust and gas is not a universal fingerprint of old, passive galaxies,” says Darko Donevski, principal research scientist at NCBJ and SISSA (Trieste, Italy), who co-designed the survey and the interpretation framework. “Some of these systems manage to keep, or even rebuild, a surprisingly rich and complex interstellar medium long after their last star-formation episode. Far from being boring relics of a glorious past, they may still be evolving in subtle but important ways.” 

The study also carries a cautionary message for observers. As dust and gas can decouple after quenching, the usual practice of using dust emission as a simple proxy for molecular gas can be misleading in old galaxies: a bright dust signal does not necessarily mean a large gas reservoir, and vice versa. 

“This means we cannot safely infer how galaxies shut down their star formation by looking only at dust or only at gas,” adds Lorenzon. “To understand how galaxies lose or replenish their fuel, we need both pieces of the puzzle.” 

Future prospects 

By tripling the number of quiescent galaxies with direct dust-to-gas ratio measurements, this study opens a much clearer window onto the late lives of massive galaxies. The authors see it as a first step toward combining ALMA’s view of cold gas and dust with the James Webb Space Telescope’s detailed infrared view of old stars and warm dust. 

Professor Allison Man, from the University of British Columbia in Vancouver, co-author of the study and expert in ALMA observations, emphasizes this synergy: “Quiescent galaxies are often treated as the end point of galaxy evolution, but ALMA and JWST are showing us that their story does not simply stop. The way they hold on to, transform, or get rid of their remaining gas and dust will tell us a lot about how galaxies age in different environments.” 

“This is really just scratching the surface,” says Donevski. “With future deep observations that bring ALMA and JWST together, we will be able to follow how galaxies age, recycle material, and quietly reshape themselves long after their brightest star-forming phases are over. This will also be an important science case for future concept missions such as PRIMA.” 

The work highlights that even galaxies long labelled as “dead” can still hide a rich, evolving inner life - and that the Universe has not yet run out of surprises, even in places we thought we understood best.