Living in a Galaxy Cluster Slows Down Star Formation

Living in a crowded cluster neighborhood can have a big impact on a galaxy. New research looks to galaxies beyond the local universe to test how living in a galaxy cluster affects galaxies’ stellar populations.

Cluster Influence

spiral galaxy Messier 83

Spiral galaxy Messier 83 is alight with pink and blue star forming regions. Observations suggest that cluster membership can slow a galaxy’s star formation. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA); Acknowledgement: W. Blair (STScI/Johns Hopkins University) and R. O’Connell (University of Virginia)]

Galaxy clusters contain hundreds or thousands of galaxies, and the static-seeming galaxy clusters we see in still images are actually dynamic systems, constantly welcoming new galaxies to the neighborhood. Observations of galaxy clusters in the nearby universe show that when a galaxy joins a cluster, it fundamentally changes the properties of that galaxy. Namely, cluster membership seems to slow the formation of new stars.

That’s what happens in the local universe — does the same story hold for galaxies farther away?

A Higher-Redshift Investigation

Keunho Kim (University of Cincinnati) and collaborators used observations of galaxy clusters spanning 5 billion years of the universe’s history to test this finding farther back in time. The team analyzed South Pole Telescope and Atacama Cosmology Telescope observations of 105 galaxy clusters at redshifts 0.26 < z < 1.13 (about 5.4–10.7 billion years after the Big Bang).

diagram showing the orbit of an infalling galaxy and where certain places on the orbit land the galaxy on a plot relating velocity and radius

Left: Diagram showing the orbit of a galaxy as it falls in to a cluster. Right: Corresponding locations on the position–velocity plot. Click to enlarge. [Kim et al. 2023]

Part of the challenge of studying the effects of cluster membership at such a large distance is that it’s hard to tell which galaxies belong to the cluster; galaxies billions of light-years away from the cluster can get in the way and be mistaken for cluster members. To get around this issue, Kim’s team analyzed not just the on-sky location of the apparent cluster galaxies but also their velocities relative to the center of the galaxy cluster. This analysis revealed not just whether a galaxy belonged to the cluster, but how long since it joined; the slow orbit of an infalling galaxy places galaxies in distinct locations on a position–velocity plot over time.

Slowing Down Star Formation

plot of age of stellar populations versus infall time

Demonstration of how stellar populations become older as time since infall increases. This result holds for galaxies of all luminosities studied. [Kim et al. 2023]

Combining estimates of when the galaxies joined their home clusters with spectroscopically determined ages of their stellar populations, Kim’s team found that the longer galaxies have spent in a cluster, the older their stars are. For the galaxies studied, the difference in average stellar age between those that have been cluster-bound the longest and the late arrivals is about 700 million years.

If the average age of a galaxy’s stars increases with time spent in a cluster, this means that living in a galaxy cluster suppresses star formation — just as was found for local galaxies. This result holds regardless of the luminosity of the galaxy or its redshift, giving us clues about the process responsible for stopping star formation. Given the time frames involved, Kim and collaborators suggest that the process responsible for quenching star formation proceeds slowly, such as the removal of the circumgalactic medium that would normally replenish a galaxy’s star-forming fuel.


“A Gradual Decline of Star Formation since Cluster Infall: New Kinematic Insights into Environmental Quenching at 0.3 < z < 1.1,” Keunho J. Kim et al 2023 ApJ 955 32. doi:10.3847/1538-4357/acecff