In the local universe, the galaxy population in the high-density environment comprises mainly red, passive galaxies that are no longer actively forming stars. The red galaxies in the highest-density environment, i.e., galaxy clusters, reside in a distinct region of the color–magnitude space known as the red sequence. Understanding how these red-sequence galaxies form and evolve and the physical processes involved remains one of the primary goals in extragalactic astronomy.
The extent of the evolution of the faint red-sequence population is under debate. Various studies have revealed that clusters at intermediate and high redshifts show a continual decrease in the fraction of the faint red-sequence population with redshift, which indicates a gradual buildup of the faint red-sequence population over time. Contrary to the studies mentioned above, several studies have reported that there is little or no evolution of the faint end of the red-sequence cluster luminosity function up to redshift z~1.5, which in turn suggests an early formation of the faint end.
In a study I led in 2019, I investigated the abundance of faint red-sequence galaxies in a sample of galaxy clusters taken from the Gemini Observation of Galaxies in Rich Early Environments (GOGREEN) survey at 1.0<z<1.3. By looking into the rest-frame H-band luminosity function of the clusters, we showed that there is a deficit of faint red-sequence galaxies in clusters at 1.0<z<1.3. By comparing our sample with a sample of clusters at z~0.6, we find an evolution of the faint end of the red sequence over the ∼2.6 Gyr between the two samples, with the mean faint-end red-sequence luminosity growing by more than a factor of 2. The ratio of faint to luminous red sequence galaxies (faint-to-luminous ratio) of our sample is consistent with the trend of decreasing ratio with increasing redshift proposed in previous studies. We also found that the faint-to-luminous ratios in clusters are consistent with those in the field at z~1.15 and exhibit a stronger redshift dependence. Our results support the picture that the buildup of faint red-sequence galaxies occurs gradually over time and suggest that faint cluster galaxies, similar to bright cluster galaxies, already experience the quenching effect induced by the environment at z~1.15.