# On the Origin of the Scatter in the Red Sequence: An Analysis of Four CLASH Clusters

## TL;DR

This study investigates the causes of color scatter in the red sequence of galaxy clusters, finding that stellar age variations significantly contribute to the observed scatter beyond metallicity effects.

## Contribution

It introduces a method to reduce red sequence color scatter by adjusting galaxy colors based on stellar population models, highlighting age as a key factor.

## Key findings

- Stellar age correlates strongly with red sequence offset.
- Adjusting for age reduces color scatter by nearly half.
- Color precision allows resolving stellar population age spread.

## Abstract

In clusters of galaxies, the red sequence is believed to be a consequence of a correlation between stellar mass and chemical abundances, with more massive galaxies being more metal-rich and, as a consequence, redder. However, there is a color scatter around the red sequence that holds even with precision photometry, implying that the galaxy population is more complicated than as described by a mass-metallicity relation. We use precision photometry from the Cluster Lensing and Supernova survey with Hubble (CLASH) to investigate what drives this scatter. In four CLASH clusters at $z=0.355 \pm 0.007$, we find that the optical-IR galaxy colors confirm the previously known trend of metallicity along the red sequence but also show a strong connection between stellar age and red sequence offset, with ages ranging from 3 to 8 Gyr. Starting with fixed-age color-magnitude relations motivated by the mass-metallicity correlations of CLASH cluster galaxies, and by adjusting galaxy colors through stellar population models to put them all at the age of our red sequence, we are able to reduce the, e.g., F625W $-$ F814W scatter from 0.051 mag to 0.026 mag with median photometric errors of 0.029 mag. While we will extend this analysis to the full CLASH sample, in four clusters our technique already provides a color precision in near-total-light apertures to resolve the spread in stellar population formation ages that drives the scatter in the red sequence.