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Intracluster light is thought to originate from stars that were ripped away from their parent galaxies by gravitational tides and galaxy interactions during the build up of the cluster. The stars from such interactions will accumulate over time, so semi-analytic models suggest that the abundance of intracluster stars is negligible in young proto-clusters at z ∼ 2 and grows to around a quarter of the stellar mass in the oldest, most mature clusters. In contrast to these theoretical expectations, we report on the detection of intracluster light within two proto-clusters at z = 2 using deep HST images. We use the colour of the intracluster light to estimate its mass-to-light ratio in annuli around the brightest cluster galaxies (BCG), up to a radius of 100 kpc. We find that 54 ± 5 per cent and 71 ± 3 per cent of the stellar mass in these regions is located more than 10 kpc away from the BCGs in the two proto-clusters. This low concentration is similar to BCGs in lower redshift clusters, and distinct from other massive proto-cluster galaxies. This suggests that intracluster stars are already present within the core 100 kpc of proto-clusters. We compare these observations to the Hydrangea hydrodynamical galaxy cluster simulations and find that intracluster stars are predicted to be a generic feature of group-sized haloes at z = 2. These intracluster stars will gradually move further away from the BCG as the proto-cluster assembles into a cluster. [ABSTRACT FROM AUTHOR]

Minor mergers are thought to be responsible for the size growth of quiescent field galaxies with decreasing redshift. We test this hypothesis using the cluster environment as a laboratory. Satellite galaxies in clusters move at high velocities, making mergers between them rare. The stellar mass–size relation in 10 clusters and in the field is measured and compared at |$z$| ∼ 1. Our cluster sample contains 344 spectroscopically confirmed cluster members with Gemini/Gemini Multi-Object Spectrographs and 182 confirmed with Hubble Space Telescope /Wide Field Camera 3 G141 grism spectroscopy. On average, quiescent and star-forming cluster galaxies are smaller than their field counterparts by (0.08 ± 0.04) and (0.07 ± 0.01) dex, respectively. These size offsets are consistent with the average sizes of quiescent and star-forming field galaxies between 1.2 ≤ |$z$| ≤ 1.5, implying the cluster environment has inhibited size growth between this period and |$z$| ∼ 1. The negligible differences measured between the |$z$| ∼ 0 field and cluster quiescent mass–size relations in other works imply that the average size of quiescent cluster galaxies must rise with decreasing redshift. Using a toy model, we show that the disappearance of the compact cluster galaxies might be explained if, on average, |${\sim }40{{\ \rm per\ cent}}$| of them merge with their brightest cluster galaxies (BCGs) and |${\sim }60{{\ \rm per\ cent}}$| are tidally destroyed into the intracluster light (ICL) between 0 ≤ |$z$| ≤ 1. This is in agreement with the observed stellar mass growth of BCGs between 0 ≤ |$z$| ≤ 1 and the observed ICL stellar mass fraction at |$z$| ∼ 0. Our results support minor mergers as the cause for the size growth in quiescent field galaxies, with cluster-specific processes responsible for the similarity between the field and cluster quiescent mass–size relations at low redshift. [ABSTRACT FROM AUTHOR]