Galaxy Clusters in the IRAC Dark Field. I. Growth of the Red Sequence

Using three newly identified galaxy clusters at z ∼ 1 (photometric redshift) we measure the evolution of the galaxies within clusters from high redshift to the present day by studying the growth of the red cluster sequence. The clusters are located in the Spitzer Infrared Array Camera (IRAC) Dark Field, an extremely deep mid-infrared survey near the north ecliptic pole with photometry in 18 total bands from X-ray through far-IR. Two of the candidate clusters are additionally detected as extended emission in matching Chandra data in the survey area, allowing us to measure their masses to be M₅₀₀ = (6.2 ± 1.0) × 10¹³ and (3.6 ± 1.1) × 10¹³ M_☉. For all three clusters we create a composite color-magnitude diagram in rest-frame B − K using our deep HST and Spitzer imaging. By comparing the fraction of low-luminosity member galaxies on the composite red sequence with the corresponding population in local clusters at z = 0.1 taken from COSMOS, we examine the effect of a galaxy's mass on its evolution. We find a deficit of faint galaxies on the red sequence in our z ∼ 1 clusters, which implies that more massive galaxies have evolved in clusters faster than less massive galaxies, and that the less massive galaxies are still forming stars in clusters such that they have not yet settled onto the red sequence.