• La Barbera et al. (2012): https://academic.oup.com/mnras/article/426/3/2300/988992?login=false
  • This paper studied the optical-optical and optical-NIR color out to 8R_e for a sample of 673 massive early-type galaxies (ETGs) with M*>3×10^10 soloar mass, selected from SDSS-based SPIDER survey. Radial profiles of colors, as well as stellar age and metallicity derived from stellar population synthesis modelling are obtained. Generally, the outer regions are older and more metal-poor than the cores of the galaxies, and the trend is strongest for the most massive galaxies with M*>10^11 solar mass. Furthermore, the age gradient is more significant in ETGs residing in higher density environment. These results are consistent with the picture that the envelope of massive galaxies is made up of accreted small satellites (i.e. minor mergers) whose stars were born during the first stages of galaxy formation.
• Wang et al. (2019): https://ui.adsabs.harvard.edu/abs/2019MNRAS.487.1580W/abstract
  • This paper studies the stellar halo of isolated central galaxies by stacking the HSC images. The sample covers a wide range of stellar mass, 9.2<lg(M*)<11.4, and out to radii of 120kpc. A strong self-similarity of the stellar halo profiles is found, albeit the large galaxy-to-galaxy scatter. Color profiles show a minimum at some radius, beyond which the color of the outer halo turns red again. This effect is most pronounced for the most massive galaxies in their sample.
• Huang et al, (2018): https://ui.adsabs.harvard.edu/abs/2018MNRAS.475.3348H/abstract
  • The HSC images are used to probe the stellar halo of ~7000 massive galaxies at z~0.4.

• Zhang et al. (2024): https://ui.adsabs.harvard.edu/abs/2024MNRAS.531..510Z/abstract
  • This paper applied a stacking method to over 4000 galaxy clusters identified from the DES by the redMaPPer cluster finder to study the surface brightness of central galaxies and the intra-cluster light (ICL). The colour of the cluster central galaxy and ICL displays a radial gradient that becomes bluer at a larger radius, which is consistent with a stellar stripping and disruption origin of intra-cluster light as suggested by simulation studies.

• Comma, Jiménez-Teja et al. (2018): https://www.aanda.org/articles/aa/full_html/2019/02/aa33547-18/aa33547-18.html
  • The ICL is distinguished from the light of the galaxies in Comma and the fraction of ICL is obtained. The high ICL fractions and the excess in the bluer filters are indicative of a merging state. The presence of younger stars or stars with lower metallicity in the ICL suggests that the main mechanism of ICL formation for the Coma cluster is the stripping of the stars in the outskirts of infalling galaxies and possibly the disruption of dwarf galaxies during past or ongoing mergers.
• Perseus, based on Early Data Release of Euclid, https://ui.adsabs.harvard.edu/abs/2024arXiv240513503K/abstract
  • The radial surface brightness profile of the BCG+ICL is best described by a double Sérsic model, with 68±4% of the H_E-band light contained in the extended, outer component. The transition between these components occurs at ≈50 kpc, beyond which the isophotes become increasingly elliptical and off-centered.The BCG+ICL colour becomes increasingly blue with radius, consistent with the stellar populations in the ICL having subsolar metallicities [Fe/H] ∼ −0.6. The colour of the ICL, and the specific frequency and luminosity function of the ICGCs suggest that the ICL+ICGCs were tidally stripped from the outskirts of massive satellites with masses of a few ×10^10 M⊙, with an increasing contribution from dwarf galaxies at large radii.

• Horizon-AGN simulation: https://arxiv.org/abs/2409.10607
  • Satellite stripping, mergers, and pre-processing are all found to make significant contributions to the ICL, but any contribution from in-situ star-formation directly into the ICL appears negligible.
  • ~90% of the stacked ICL that is not pre-processed should come from galaxies infalling with stellar masses above 10^9 M_⊙, with roughly half coming from infalling galaxies with stellar masses within half a dex of 10^11 M_⊙. The fact that the ICL appears largely sourced from such massive objects suggests that the ICL assembly of any individual cluster may be principally stochastic.

Globular clusters (GCs) are no longer as simple as previously thought. A GC may consist of multiple stellar populations, suggesting that the formation of GCs is not limited to early times. GCs are not only found within galaxies, but also located in between galaxies in galaxy groups/clusters. The total number and mass of GCs in a galaxy cluster is found to be tightly correlated with the total dark matter mass of host halos, suggesting a tight relationship between GCs and dark halo assembly history. Thanks to JWST, we are starting to see GCs at redshift up to z~10, thus able to witness the formation and evolution of GCs across the cosmic time. However, theoretically many questions regarding GCs still remain unanswered, but surely will be answered in the next decade or so.

Reference:

• Unveiling the Cosmic Gems Arc at z~10.2 with JWST https://ui.adsabs.harvard.edu/abs/2024arXiv240410770B/abstract (This paper presents JWST imaging of the Cosmic Gems Arc at a photometric redshift of z=10.2, lensed by a galaxy cluster.)
• The Firefly Sparkle at z=8.304 with JWST https://ui.adsabs.harvard.edu/abs/2024arXiv240208696M/abstract (JWST observed a strongly lensed at a spectroscopic redshift z=8.304 exhibiting a network of massive star clusters, the Firefly Sparkle.)
• The Sparkler: Evolved globular clusters at z=1.378 with JWST https://ui.adsabs.harvard.edu/abs/2022ApJ...937L..35M/abstract (Globular clusters formed at z~7-11 are seen by JWST in a z=1.378 galaxy, strongly lensed by a z=0.39 galaxy cluster.)
• The intracluster globular clusters (ICGCs) of the Perseus Cluster with Euclid https://ui.adsabs.harvard.edu/abs/2024arXiv240513503K/abstract (Early data release of Euclid detected both intracluster light (ICL) and ICGCs in the central 500kpc of Perseus cluster.)
• Globular cluster counts around 700 nearby galaxies https://ui.adsabs.harvard.edu/abs/2024arXiv240807124L/abstract (DESI Legacy Imaging Survey is used to make a large-scale, homogeneous estimate of globular cluster abundance around 707 galaxies at distances <30Mpc. The linear relation between GC counts and dark matter halo mass, as well as radial profiles of GC counts are obtained.)
• A two-phase model of galaxy formation: the formation of globular clusters https://ui.adsabs.harvard.edu/abs/2024arXiv240518735C/abstract (The paper presents a model of globular cluster formation within the cosmological hierarchy of structure formation. Predictions are made for GCs for both the local Universe and redshift up to z~10.)
• A recent review on star clusters by Krumholtz, McKee & Bland-Hawthorn (2019, ARA&A) https://ui.adsabs.harvard.edu/abs/2019ARA%26A..57..227K/abstract