Dust in Quiet Galaxies May Reveal How Their Star Formation Shut Down

Dust should face a difficult future when a galaxy stops forming stars. The stellar sources that supply fresh grains fade, while hot gas and shocks can destroy what remains. Yet astronomers have found galaxies with little apparent star formation that still carry considerable dust. A new study suggests that understanding this mismatch requires looking beyond when star formation stopped to how it declined.

In a preprint posted to arXiv on November 4, Sarah Bodansky of the University of Massachusetts Amherst and colleagues examine 13 strongly gravitationally lensed galaxies selected for spectra consistent with quiet stellar populations. Their results suggest that rapid shutdowns can accompany severe dust destruction, whereas more extended declines may leave conditions in which dust survives or grows again. The authors emphasize that this interpretation is not unique.

Natural magnification reveals a dusty divide

The team combined new observations of eight galaxies with five archival systems from the COOL-LAMPS and REQUIEM surveys. The new measurements used the Atacama Large Millimeter/submillimeter Array, or ALMA, to observe emission at a wavelength of about 1.3 millimeters. At the galaxies’ distances, this probes long-wavelength radiation from cold dust.

Foreground gravitational lenses brighten and stretch the background galaxies, making otherwise faint dust easier to study. The researchers paired the ALMA measurements with optical and near-infrared observations, reconstructing the galaxies’ star formation histories and estimating their stellar masses.

The inferred dust-to-stellar-mass ratios span more than a hundredfold, including upper limits for galaxies whose dust was not detected. Among the eight newly observed targets, three show clear dust detections. Two, J1201-3952 and J2359-0208, have extended dust emission rather than an unresolved source.

The contrast is not simply between younger and older stellar populations. The team found no statistically significant association between dust fraction and stellar age. Instead, comparing dust content with the reconstructed shutdown histories provided a more informative, though model-dependent, picture.

Different shutdowns, different outcomes

The researchers compared their measurements with models of dust evolution after quenching—the suppression of star formation. Roughly half the sample is consistent with harsh conditions that destroy dust efficiently. These systems preferentially lie at higher redshift, meaning they are observed at earlier cosmic epochs, although some lower-redshift galaxies also fit that picture.

In the models, energy released by an active galactic nucleus can help keep surrounding gas hot. Collisions with particles in that gas can erode dust grains. The observations are consistent with such destruction, but do not establish that this mechanism operated in every dust-poor galaxy.

The two galaxies with extended, abundant dust are harder to explain. Their dust fractions exceed even the mildest destruction tracks considered. J1201-3952 stands out because its reconstructed history places quenching about 1.8 billion years before observation, yet it has the highest dust fraction in the sample.

If it truly stopped forming stars, simply retaining dust briefly after shutdown is an inadequate explanation. The authors instead point to grains growing through the accumulation of material in the interstellar medium, continued dust production by evolved stars, or a combination. Intermediate-age stars in the thermally pulsing asymptotic giant branch phase could continue supplying dust long after the main star-forming episode.

J2359-0208 presents a different complication. Its inferred history includes renewed star formation followed by a recent steep decline, although that reconstruction is uncertain because its spectrum is noisy. It also does not formally meet the study’s numerical quiescence threshold, underscoring that spectral selection does not guarantee a completed shutdown.

Hidden star formation remains a possibility

The central limitation is that one ALMA band cannot determine dust temperature. Allowing temperatures between 15 and 35 kelvin shifts the inferred dust fractions by about a factor of two in either direction. The same measurement cannot reliably establish the total infrared luminosity needed to assess obscured star formation.

Consequently, the two dust-rich galaxies could still be forming stars behind dust. Their extended emission argues against a compact hidden starburst dominating the signal, but smaller buried components could remain unresolved.

The sample is also small and biased by how strong lenses are discovered. Dust and stars could receive different magnifications if they occupy different regions. The suggested evolution toward gentler quenching at later epochs therefore remains tentative.

Multi-band, higher-resolution ALMA observations could constrain temperatures and separate compact from extended dust. Infrared spectroscopy could test both buried star formation and dust production by evolved stars. For now, the surviving dust offers a promising diagnostic—not a definitive verdict—on how galaxies become quiet.