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