Trying to understand the life of a whale by observing different whales at different ages is tricky. That is similar to what astronomers deal with when they study galaxy clusters over time.

But what if you could actually follow the same whale? Or in this case, the same cluster?

A team of astrophysicists has developed a method that does exactly that. By combining observations of galaxy clusters with computer simulations of dark matter evolution, they've created a cosmic family tree. One that lets them trace how massive red galaxies evolved as the universe aged from 7.8 billion years ago to 3.5 billion years ago.

The breakthrough lies in acknowledging uncertainty. Previous studies stumbled because they treated cluster mass estimates as fixed numbers. But these measurements scatter. A cluster catalog might list richness (the count of massive red member galaxies), yet that observable only approximates true mass. Two clusters with identical richness could differ substantially in actual mass.

The new approach embraces this ambiguity. Instead of assigning each observed cluster a single mass value, the method uses probability distributions. For any given richness, there's a range of possible masses, each with its own likelihood. The researchers then match these probability-weighted clusters to dark matter halos extracted from the Millennium Run simulation, a massive computational model tracking how cosmic structure formed and evolved.

Dark matter merger trees provide the evolutionary scaffolding. These trees chart how halos grow through mergers and accretion across billions of years. By matching observed clusters at high redshift to halos in the simulation, then tracing those halos forward to lower redshift and matching descendant halos to lower-redshift clusters, the method statistically connects progenitor and descendant populations.

The research team applied this technique to galaxy clusters detected by the CAMIRA algorithm using data from the Hyper Suprime-Cam Subaru Strategic Program. Over 780 square degrees of sky, they identified 14,224 clusters between redshift 0.3 and 1.0. They divided these into five time slices, each spanning the same comoving volume.

They also split clusters into three mass bins based on their predicted mass at redshift 0.3. Low-mass clusters: those destined to weigh between 1.58 and 2.51 trillion solar masses. Intermediate: 2.51 to 3.98 trillion. High-mass: anything above 3.98 trillion.

What did the cosmic family albums reveal?

First, stellar mass content grew steadily. In high-mass clusters, the total stellar mass locked in red member galaxies increased from 1.9 trillion solar masses at redshift 0.95 to 2.5 trillion at redshift 0.42. That's 32 percent growth over 4.3 billion years. Intermediate-mass clusters showed 32 percent growth. Low-mass clusters: 35 percent.

The consistency is striking. Regardless of final cluster mass, the fractional increase in red galaxy stellar content tracks closely with total mass growth. This suggests stellar mass content serves as a reliable mass proxy across cosmic time, maintaining a stable relationship as clusters evolve.

Brightest cluster galaxies tell a different story. These massive elliptical galaxies sit at cluster centers like anchors. BCG growth varied by cluster mass. In low-mass clusters, BCG stellar mass increased 17 percent from redshift 0.99 to 0.3. Intermediate clusters: 12 percent. High-mass clusters? Essentially no growth.

Downsizing in action. BCGs in the most massive clusters assembled their stellar mass earlier than their counterparts in smaller systems. By redshift 0.6, BCG growth had largely ceased across all mass ranges. Meanwhile, the total stellar mass content continued rising as smaller red galaxies joined the cluster population or grew through star formation.

This divergence drives another finding: BCG dominance declines over time. At redshift 0.99, BCGs in high-mass clusters contained a larger fraction of total red galaxy stellar mass compared to redshift 0.3. The ratio dropped as the cluster aged, simply because BCGs stopped growing while total stellar content kept climbing.

At any given epoch, BCGs also become less dominant as cluster mass increases. In more massive clusters, the central galaxy claims a smaller share of total stellar mass. This pattern held constant across cosmic time, consistent with earlier findings from nearby cluster surveys and extending the trend to higher redshift.

The method also enables a fresh perspective on scaling relations. By examining how observables relate to mass at different epochs, the researchers measured evolutionary trends. The stellar mass-cluster mass relation maintains a slope around 0.68 across all five redshift bins. No evolution detected. The relationship appears locked in place from redshift 1 to 0.3.

BCG stellar mass scales with cluster mass too, but with a shallower slope between 0.24 and 0.39, depending on redshift. Richness-mass scaling shows similar stability, with slopes hovering around 0.79 regardless of cosmic epoch.

Why doesn't the stellar mass-cluster mass slope evolve? That remains unsolved. The answer likely connects to intracluster light, the diffuse glow of stars stripped from galaxies and scattered throughout the cluster. As galaxies merge and interact, stars get redistributed. The interplay between these processes and the overall growth of cluster mass may conspire to preserve the scaling relation's slope even as individual components evolve.

The method's power extends beyond red galaxies and BCGs. It could track star-forming galaxy populations, trace different types of active galactic nuclei, or follow the evolution of intracluster gas using X-ray observations. The early eROSITA cluster sample offers mass-X-ray scaling relations that could reveal how baryon content evolves for clusters of different final masses.

Current analysis stops at redshift 1 due to practical constraints. Survey volume becomes limited at lower redshift. CAMIRA cluster detection loses reliability above redshift 1. But future surveys will push boundaries. When Euclid combines forces with the Legacy Survey of Space and Time, cluster evolution studies could extend to redshift 2, probing when massive clusters were still assembling their core populations.

The technique offers something previous methods couldn't: a way to fairly compare clusters across cosmic time while accounting for measurement uncertainty. No single cluster is followed individually. Instead, statistical populations are matched through merger trees, allowing ensemble evolution to emerge from the noise.

Think of it as demographic tracking rather than individual biography. We can't follow one specific cluster from redshift 1 to 0.3 the way we might track a person from age 30 to 60. But we can follow populations. We can ask: what happens, on average, to clusters of a given mass as the universe ages?

The answer reveals both continuity and change. Some relationships remain frozen. Others shift. BCGs largely finish their assembly before redshift 0.6. Total stellar mass keeps climbing. The scaling between stellar content and cluster mass holds steady, a fixed relationship carved early and preserved through billions of years of cosmic evolution.

Galaxy clusters, it turns out, remember their past more faithfully than we might have guessed.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.3847/1538-3881/adc0f8