Intracluster Light (ICL)

• Intracluster Light (ICL) is the diffuse stellar component stripped from galaxies, serving as a record of cluster formation and evolution.
• Advanced observational methodologies, such as 2D model subtraction and wavelet multiscale techniques, are used to detect ICL despite its low surface brightness.
• ICL properties—including fraction, color and metallicity gradients, and kinematics—offer practical insights into dark matter distribution and cluster mass estimation.

Intracluster light (ICL) is the diffuse stellar component that permeates galaxy clusters and groups, consisting of stars that are gravitationally bound to the cluster potential but not to individual galaxies. ICL spans large spatial scales—extending to hundreds of kiloparsecs beyond the luminous boundaries of the brightest cluster galaxies (BCGs)—and is a significant repository of stellar mass in clusters. Originating primarily from the stripping and disruption of galaxies during hierarchical structure formation, the ICL offers a fossil record of the assembly and dynamical evolution of massive halos, encodes metallicity and color gradients that map the history of satellite accretion and pre-processing, and increasingly serves as a luminous tracer of the cluster’s underlying dark matter distribution.

1. Observational Methodologies and Detection Challenges

The faint surface brightness of ICL makes its detection and quantitative analysis particularly challenging. Direct imaging in optical and near-infrared bands must reach depths of $\mu \gtrsim 28$–$30\,\mathrm{mag\,arcsec}^{-2}$, with careful subtraction of bright galaxy halos and diffuse backgrounds. Techniques include:

• 2D Model Subtraction and Basis Decomposition: Algorithms such as CICLE use Chebyshev–Fourier function bases to model and subtract the light of cluster galaxies (including BCGs), leaving the residual stellar component as candidate ICL (Jiménez-Teja et al., 2018, Oliveira et al., 2022, Ellien et al., 10 Mar 2025).
• Wavelet Multiscale Approaches: Software like DAWIS decomposes images into spatial frequency modes, enabling the separation of large-scale ICL from galaxy-scale features and Galactic cirrus (Ellien et al., 10 Mar 2025).
• Profile Fitting: Both 1D (isophotal) and 2D (Sérsic or de Vaucouleurs) surface brightness profile fits identify additional, more extended components (distinguished by lower Sérsic indices and larger effective radii) as ICL (Ellien et al., 10 Mar 2025, Montes et al., 2017).
• Decision Thresholds: Surface brightness cuts (e.g., $\mu_V > 26.5\,\mathrm{mag/arcsec}^{2}$) or geometric annuli (e.g. 50–200\,kpc from BCG) select ICL-dominated regions (Montes, 2022, Collaboration et al., 21 Mar 2025).
• Kinematic Decomposition: In select systems, velocity distributions of tracers (planetary nebulae, globular clusters, integrated light) disentangle BCG-bound from ICL-dominated stars by their differing dispersions and velocities (Mihos, 2015).

A significant challenge resides in the overlap between BCG halos and the ICL, the degeneracy of decomposition models (especially when overlapping Sérsic or exponential components are fitted), and the sensitivity of ICL fraction measurements to observational technique and photometric band selection (Montes, 2022).

2. Formation Mechanisms and Assembly History

ICL builds up principally via:

• Tidal Stripping of Satellites: The dominant process is the removal of stars—primarily from the outer regions of intermediate- or low-mass galaxies—that enter the deep central gravitational potential of the cluster (Brown et al., 16 Sep 2024, Contreras-Santos et al., 22 Sep 2025, Mayes et al., 19 Jun 2025). The liberated stars, owing to less binding energy, may be more metal-poor and younger.
• Major and Minor Mergers: Violent relaxation during major (mass ratio $\approx$ 1:1–1:3) and minor mergers redistributes stars, with a significant fraction added to the ICL via energetic unbinding processes (Brown et al., 16 Sep 2024). Simulations with merger trees quantify this, showing that major mergers typically contribute a median of $\sim35\%$ to the present-day ICL mass.
• Pre-processing in Groups: ICL can originate from stars that were liberated from galaxies in smaller groups before those groups accrete onto the main cluster (“intragroup light” that becomes part of the ICL on cluster infall) (Mihos, 2015, Contini, 2021).
• In-situ Star Formation: While some simulations predict a minor “in-situ” contribution directly from the diffuse intracluster medium (up to $\sim 10\%$ in some codes), the dominant channels remain stripping and merging (Contreras-Santos et al., 22 Sep 2025, Brown et al., 16 Sep 2024).

Time-resolved simulation work demonstrates that ICL assembly accelerates at $z < 1$, with half the present-day mass typically in place between $z \sim 0.2$–$0.5$ (Contreras-Santos et al., 22 Sep 2025, Contini et al., 2013). The principal progenitors are infalling galaxies with stellar masses $\gtrsim 10^{11} M_\odot$; although low-mass satellites lose a higher fraction of their stars, the absolute contribution of massive satellites dominates due to their larger stellar mass reservoirs (Brown et al., 16 Sep 2024).

3. Physical Properties: Fractions, Gradients, and Kinematics

ICL constitutes a substantial fraction of cluster stellar mass, but its precise value depends on identification method, cluster mass, dynamical state, and redshift:

Measurement Context	ICL Fraction	Redshift	Reference
Local clusters (Coma, Virgo, Perseus)	10–25%	$z \sim 0$	(Jiménez-Teja et al., 19 Dec 2024, Jiménez-Teja et al., 2018, Oliveira et al., 2022)
Simulations (The Three Hundred Project)	30–50% (within $R_{500}$)	$z = 0$	(Contreras-Santos et al., 16 Jan 2024)
Intermediate redshift ($z\sim0.3$–0.6)	6–24%	$z\sim0.3$	(Montes et al., 2014, Ellien et al., 10 Mar 2025, Montes et al., 2017)
High redshift ($z>1$)	$\sim$17%	$1	(Joo et al., 2023)

• Color and Metallicity: The ICL is systematically bluer and more metal-poor than the BCG, with measured metallicities ranging from [Fe/H] $= -0.5$ to solar and with negative gradients (increasing blueness and decreasing metallicity with radius) indicative of accreted satellite populations (Montes et al., 2017, Mayes et al., 19 Jun 2025, Ellien et al., 10 Mar 2025). Stellar ages in the ICL are 2–6 Gyr younger than those in central BCGs (Montes et al., 2014, Montes et al., 2017).
• Spatial Structure: The ICL is spatially extended, often showing non-axisymmetric, filamentary, or clumpy morphologies; tidal features, shells, and bridges (e.g., in Coma between core and NGC 4839) are direct evidence of ongoing stripping and dynamical assembly (Jiménez-Teja et al., 19 Dec 2024).
• Kinematics: ICL velocity dispersions match or exceed those of cluster galaxies ($\sim400$–$600\,{\rm km\,s}^{-1}$), and the velocity field can be chaotic or disturbed, reflecting recent accretion, mergers, and substructure (Toledo et al., 2011, Mihos, 2015). Orbital anisotropy is more radial than for the cluster dark matter, and the specific energy distribution of ICL stars is lower than that of the DM, leading to a biased sampling of the cluster potential (Butler et al., 4 Apr 2025).

4. Role as a Tracer of Cluster Mass and Dark Matter

The ICL’s radial mass and surface brightness profiles trace the cluster’s gravitational potential and are closely linked to the distribution of dark matter:

• Density and Velocity Profiling: The ratio of ICL to dark matter densities follows a power law in radius, typically modeled as

$$\frac{\rho_{\mathrm{ICL}}(r)}{\rho_{\mathrm{DM}}(r)} = a \left(\frac{r}{R_{500}}\right)^b$$

with $b \approx -1.2$ over large scales in C-EAGLE and The Three Hundred simulations (Contreras-Santos et al., 16 Jan 2024, Asensio et al., 7 Jul 2025).

• Projected Contour Alignment: The spatial isocontours of ICL light, when compared using the Modified Hausdorff Distance, agree extremely well (within $\sim$25\,kpc) with gravitational lensing-based mass contours, demonstrating morphological alignment between ICL and DM at cluster scales (Montes et al., 2018, Contini, 2021).
• Mass Estimation: Simulation-calibrated relations between projected ICL surface density and total mass permit the recovery of cluster mass profiles directly from ICL observations; these have been validated against velocity-dispersion and lensing-derived cluster masses (Asensio et al., 7 Jul 2025). The ICL can thus serve as an independent probe of mass in deep imaging surveys, although systematic biases due to energetic differences between stellar and DM orbits must be carefully modeled (Butler et al., 4 Apr 2025).

There is, however, a recognized bias: ICL stars sample lower specific energies and more radially biased orbits than DM, leading to a steeper and more centrally concentrated ICL profile compared to the underlying DM (Butler et al., 4 Apr 2025). While the overall shape and morphology are similar, inferred DM properties from ICL must account for this sampling bias to avoid systematic errors in mass estimation.

5. Environmental Dependence, Dynamical Evolution, and Progenitor Population

Environmental and dynamical state modulate the ICL properties:

• Relaxed vs. Unrelaxed Systems: Relaxed, early-forming clusters tend to exhibit higher ICL fractions due to cumulative stripping over long timescales; recently merged or dynamically active clusters show lower fractions but enhanced clumpy or blue ICL features (Contreras-Santos et al., 16 Jan 2024, Jiménez-Teja et al., 2018, Jiménez-Teja et al., 19 Dec 2024).
• Substructure Contribution: Subgroups (e.g., in Abell 2390, the northwest and southeast subclusters) contribute measurably to the ICL, with identifiable filamentary connections supporting ongoing accretion and pre-processing (Ellien et al., 10 Mar 2025).
• Progenitor Mass Function: Although lower-mass galaxies lose a larger fraction of their stars to the ICL, the overall mass budget is dominated by the more massive progenitors (typically those with $M_*\gtrsim10^{11} M_\odot$) (Brown et al., 16 Sep 2024). This leads to a stochastic element in ICL assembly: rare, massive infall events can dominate the ICL composition in individual clusters.
• Co-evolution with BCGs: The ICL and BCGs have intertwined, but not identical, progenitor pools—both draw from accreted galaxies, but the ICL is enriched by stripping from intermediate-mass satellites, while the BCG mainly grows via major mergers and in-situ star formation, resulting in bluer and more metal-poor ICL compared to the BCG (Mayes et al., 19 Jun 2025).

Negative metallicity and color gradients across BCG+ICL systems reflect sequential assembly: BCGs are preferentially enriched at the center, while increasing blueness outward records the increasing contribution from stripped, lower-metallicity satellites. No significant difference in stellar age is found between ICL and BCG populations in some simulations, underscoring the complexity and scatter in assembly histories (Mayes et al., 19 Jun 2025).

6. Cosmological Evolution and Scaling with Redshift

Contrary to earlier theoretical expectations, the ICL fraction is already substantial ($\sim$17%) at $z \gtrsim 1$, indicating that much of the ICL forms concurrently with the assembly of BCGs and large-scale accretion rather than by late-time, gradual stripping alone (Joo et al., 2023). No significant correlation is observed between cluster mass and ICL fraction, nor between ICL color and cluster-centric radius at high redshifts. These findings necessitate revision to hierarchical stripping-centric models, emphasizing coeval production of ICL and BCG stellar components during major build-up phases and the significance of star-rich pre-processed group infall.

Simulation and observational studies agree that more than half of the ICL is typically in place by $z\sim0.5$–$0.2$, with some cluster-to-cluster variation depending on accretion history and major merger events (Contreras-Santos et al., 22 Sep 2025, Contreras-Santos et al., 16 Jan 2024).

7. Future Prospects and Open Challenges

The ICL is poised to become an indispensable probe of cluster evolution, halo assembly, and the mapping of dark matter with upcoming deep wide-field surveys (Euclid, LSST, Roman):

• Survey Forecasts: Euclid is projected to detect ICL at $S/N>3$ in up to $\sim80,000$ clusters between $z=0.3$ and $1.5$, enabling statistically significant studies of ICL evolution, assembly bias, and substructure (Collaboration et al., 21 Mar 2025).
• Mass Estimation and Cosmology: Mass profiles of clusters determined from ICL density maps offer an independent cross-check of lensing and X-ray methods; the precision and systematics of these approaches will improve with larger survey footprints and refined simulation calibrations (Asensio et al., 7 Jul 2025).
• Improved Definitions: Continued convergence on robust, reproducible ICL definitions—blending photometric, kinematic, and phase-space criteria—remains essential for meaningful comparison across datasets and theoretical models (Montes, 2022, Contini, 2021).
• Tracing the Assembly History: Analysis of color/metallicity gradients, the role of in-situ star formation, and the quantification of pre-processing in infalling groups demand multiwavelength imaging and integral-field spectroscopy at faint surface brightness levels (Ellien et al., 10 Mar 2025).

Open questions remain regarding the degree to which ICL traces the dark matter halo outskirts (including the “splashback” radius), the precise correlation with cluster dynamical state, and the stochastic nature of ICL assembly. A consistent observational census of ICL across redshift and mass, with careful multi-method calibration to simulations, is required to fully exploit the diagnostic power of diffuse stellar light for cluster and cosmological studies.