Bold take: galaxies don’t form stars forever; they quietly switch off, and understanding how that happens is key to unlocking how galaxies live and die. But here’s where the discussion gets intriguing and a bit controversial: the exact pathways and timescales of quenching are diverse, sometimes conflicting across different galaxies, environments, and internal dynamics. This piece reimagines how astronomers study the end of star formation by zooming in on small regions within galaxies to catch the earliest signs of quenching before the whole system goes quiet.
Title: Exploring the Early Clues of Quenching Through Spatially Resolved Star Formation Histories in UVCANDELS, with a Look at Nine Golden Galaxies
Authors and affiliations: A team led by Charlotte Olsen and colleagues, including experts from the New York City College of Technology and multiple institutions, who have submitted their work to The Astrophysical Journal. The study builds on data from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) with ultraviolet coverage provided by UVCANDELS.
How and why galaxies stop forming stars
Galaxies are laboratories for a wide range of astrophysical questions. They form, grow, and eventually quiet down, transitioning from actively star-forming to quiescent states. In the nearby universe, many galaxies appear “red and dead” because their stars are older and emit redder light. Quenching—the process by which star formation ceases—drives this transition and is a central topic in galaxy evolution. However, disentangling quenching is challenging since star formation interacts with many factors, such as a galaxy’s environment and the activity of its central supermassive black hole.
Figure 1 (conceptual): Images of nine galaxies chosen for their clear detections and variety of orientations and shapes. Each galaxy is divided into many small regions where star formation is measured. The left panel shows an example region’s star formation, while the right panel illustrates how the team spatially resolved the galaxies for detailed analysis. The figure draws from the current work’s Figures 1 and 5.
Why look at small scales? Star formation happens on much smaller scales and over shorter timescales than a galaxy’s overall lifetime. By tracking star formation in these smaller patches over time, astronomers can determine whether quenching begins in the core and propagates outward (inside-out) or starts from the outskirts due to environmental effects (outside-in). The authors utilize high-resolution ultraviolet data from UVCANDELS, paired with CANDELS, to examine nine carefully selected galaxies—the so-called golden sample—and reconstruct region-by-region star formation histories to reveal the earliest cues of quenching.
Characterizing the star formation histories
To map a galaxy’s star formation over cosmic time, the team models the integrated light emitted by a galaxy, which encodes multiple episodes of star formation, using spectral energy distribution (SED) fitting. By comparing observed light across many wavelengths to theoretical models, SED fitting estimates stellar ages and the timing of star formation events. This approach allows the study of how the star formation rate relates to stellar mass, known as the SFR–Mstellar relation. In this work, the analysis focuses on spatially resolved regions, so the researchers interpret SFR and stellar mass as surface densities, denoted ΣSFR and ΣMstellar, rather than global totals.
Using SED fitting, the authors infer the expected star formation rate and stellar mass for each region over the past 250 Myr to 1 Gyr before observation. Figure 2 presents the best-fit SFR–Mstellar relations for each galaxy’s regions, both 1 Gyr prior and at the observation epoch. Around 1 Gyr before the observations, these regions share a similar relation, but by the observation time, the overall star formation rates have declined modestly, signaling the onset of quenching. The increasing scatter across galaxies suggests that each system is following a different quenching path.
Figure 2 (conceptual): The best-fit SFR–Mstellar relations for each galaxy’s regions, with different colors representing distinct galaxies. The left panel shows the state 1 Gyr before, and the right panel shows the present state. The early similarity gives way to divergent quenching histories as time progresses.
To further probe where quenching begins, the authors examine the specific star formation rate (sSFR) of each region as a function of distance from the galaxy center. The sSFR—defined as SFR divided by the region’s stellar mass—captures how actively new stars are forming relative to the existing stellar population. Figure 3 highlights three example galaxies from the sample. Across all regions, SFR declines over the 1 Gyr interval, but the locations of the steepest declines vary—from centers to mid-disks to outskirts—indicating that different galaxies are quenching through different mechanisms.
Figure 3 (conceptual): The sSFR of regions versus their radius from the galaxy center for three galaxies. The red curve shows 1 Gyr before, the blue curve shows the present, and highlighted regions mark where the drop in star formation is most pronounced, illustrating diverse quenching patterns. This is aligned with Figure 11 in the paper.
Detecting the earliest signs of quenching
All galaxies in the study remain star-forming, yet they are on the cusp of suppression, offering a valuable glimpse into how quenching begins and why it can take multiple forms. The study also points to the future potential of new observatories. The James Webb Space Telescope provides unparalleled resolution to study distant galaxies, while the Rubin Observatory and the Nancy Grace Roman Space Telescope will generate enormous datasets containing billions of galaxies. Together, these facilities promise to reveal more about the small-scale processes that shut down star formation and shape galaxy evolution.
Astrobite edited by Catherine Slaughter.
Image note: The featured image caption references a Hubble image illustrating a star-forming galaxy transitioning toward quiescence.
Author bio: After studying astrophysics and literature at Caltech, the author pursued a Fulbright in Heidelberg. Interests include using simulations to explore supermassive black holes and galaxy evolution, with a passion for poetry and travel.
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