arXiv subjects

S. K. Yi

Publications and source records attributed to S. K. Yi.

At least 19 recordsLinked to original sources

arXiv2025

The ALPINE-CRISTAL-JWST survey: spatially resolved star formation relations at $z\sim5$

Star formation governs galaxy evolution, shaping stellar mass assembly and gas consumption across cosmic time. The Kennicutt-Schmidt (KS) relation, linking star formation rate (SFR) and gas surface densities, is fundamental to understand star formation regulation, yet remains poorly constrained at $z > 2$ due to observational limitations and uncertainties in locally calibrated gas tracers. The [CII] $158 {\rm μm}$ line has recently emerged as a key probe of the cold ISM and star formation in the early Universe. We investigate whether the resolved [CII]-SFR and KS relations established at low redshift remain valid at $4 < z < 6$ by analysing 13 main-sequence galaxies from the ALPINE and CRISTAL surveys, using multi-wavelength data (HST, JWST, ALMA) at $\sim2$ kpc resolution. We perform pixel-by-pixel spectral energy distribution (SED) modelling with CIGALE on resolution-homogenised images. We develop a statistical framework to fit the [CII]-SFR relation that accounts for pixel covariance and compare our results to classical fitting methods. We test two [CII]-to-gas conversion prescriptions to assess their impact on inferred gas surface densities and depletion times. We find a resolved [CII]-SFR relation with a slope of $0.87 \pm 0.15$ and intrinsic scatter of $0.19 \pm 0.03$ dex, which is shallower and tighter than previous studies at $z\sim5$. The resolved KS relation is highly sensitive to the [CII]-to-gas conversion factor: using a fixed global $α_{\rm [CII]}$ yields depletion times of $0.5$-$1$ Gyr, while a surface brightness-dependent $W_{\rm [CII]}$, places some galaxies with high gas density in the starburst regime ($<0.1$ Gyr). Future inputs from both simulations and observations are required to better understand how the [CII]-to-gas conversion factor depends on local ISM properties. We need to break this fundamental limit to properly study the KS relation at $z\gtrsim4$.

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arXiv2025

Cosmic reflections I: the structural diversity of simulated and observed low-mass galaxy analogues

Dwarf galaxies serve as powerful laboratories for investigating the underlying physics of galaxy evolution including the impact of baryonic feedback processes and environmental influences. We compare the visual and structural properties of dwarf galaxies in ultra-deep HSC-SSP imaging of the COSMOS field with those measured from realistic HSC-like synthetic observations of dwarfs generated by the Illustris TNG50 and NewHorizon simulations. Using Sérsic profile fitting and non-parametric morphological metrics (Gini, $M_{20}$, asymmetry, and concentration), we evaluate the diversity of structural properties in observed and simulated galaxies. Our analysis shows that NewHorizon and TNG50 galaxies lie at opposite extremes of observed structural trends: NewHorizon produces diffuse, extended galaxies with shallow Sérsic indices, while TNG50 yields compact, concentrated systems with steep indices. Both simulations reproduce observed structural trends more closely at higher stellar masses ($M_{\star}\sim10^{9.5} {\rm M_{\odot}}$) but fail to capture the full diversity of COSMOS dwarfs at lower masses. Non-parametric metrics further show that NewHorizon galaxies exhibit more uneven, clumpy light distributions while TNG50 galaxies have smoother but excessively concentrated profiles. These structural differences reflect underlying differences in their physical prescriptions and are likely driven by differing approaches to ISM physics, supernova feedback and star formation in addition to differences in numerical resolution. Our findings highlight the unique power of low-mass galaxies to constrain differences in simulation physics, especially star formation and feedback. Upcoming surveys from facilities like the Vera C. Rubin Observatory and Euclid will enable more rigorous comparisons with simulations, offering deeper insights into the physical processes shaping galaxy evolution.

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arXiv2025

Dissecting the formation of gas-versus-star counter-rotating galaxies from the NewHorizon simulation

(Reduced)Using the NewHorizon simulation, we have studied ten gas-versus-star counter-rotating galaxies in field environments with a stellar mass of M*~[1-5]x10^10 Msun. For all of them, the retrograde accretion of gas either from gas stripping from a nearby companion or from the circumgalactic medium is the starting point of the formation process. This is followed by the co-existence of two distinct disks of gas rotating in opposite directions, the pre-existing one in the inner parts and the accreted gas in the outer parts of the galaxy. The latter progressively replaces the former leading to the final gas-star kinetic misalignment configuration. During the process, the star formation is first enhanced and then progressively decreases. We roughly estimate that a higher fraction of the pre-existing gas is converted into stars rather than being expelled. We also found that the black hole activity (BH) tends to be enhanced during the removal of the pre-existing gas. Furthermore, our analysis suggests that the formation of a counter-rotating gas component is always accompanied with the formation of counter-rotating stellar disks. These stellar disks can have diverse properties but host in general a younger and more metal rich population of stars with respect to the main disc, depending on the star formation history and BH activity. The central part of counter-rotating disks tend also to be characterized by a younger population, an enhanced star formation rate and a higher metallicity than their outer parts. The high metallicity comes the progressive metal enrichment of the accreted gas by mixing with the pre-existing gas and by supernovae activity as it sinks toward the center of the galaxy. In case of major mergers, a large amount of accreted stars from the companion would be distributed at large distances from the remnant center due to conservation of the initial orbital angular momentum.

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arXiv2024

New tools for studying planarity in galaxy satellite systems: Milky Way satellite planes are consistent with ΛCDM

We introduce a new concept -- termed "planarity" -- which aims to quantify planar structure in galaxy satellite systems without recourse to the number or thickness of planes. We use positions and velocities from the Gaia EDR3 to measure planarity in Milky Way (MW) satellites and the extent to which planes within the MW system are kinematically supported. We show that the position vectors of the MW satellites exhibit strong planarity but the velocity vectors do not, and that kinematic coherence cannot, therefore, be confirmed from current observational data. We then apply our methodology to NewHorizon, a high-resolution cosmological simulation, to compare satellite planarity in MW-like galaxies in a ΛCDM-based model to that in the MW satellite data. We demonstrate that kinematically supported planes are common in the simulation and that the observed planarity of MW satellites is not in tension with the standard ΛCDM paradigm.

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arXiv2024

Diffuse Light in Milky-Way like Haloes

We investigate the diffuse light (DL) content of dark matter haloes in the mass range $11.5\leq \log M_{halo}\leq13$, a range that includes also the dark matter halo of the Milky-Way, taking advantage of a state-of-the-art semi-analytic model run on the merger trees extracted from a set of high-resolution cosmological simulations. The fraction of DL in such relatively small haloes is found to progressively decrease from the high to the low mass end, in good agreement with analytic (\citealt{purcell2007}) and numerical results from simulations (\citealt{proctor2023,ahvazi2023}), in good agreement also with the fraction of the DL observed in the Milky-Way (\citealt{deason2019}) and M31 (\citealt{harmsen2017}). Haloes with different masses have a different efficiency in producing DL: $\log M_{halo} \simeq 13$ is found to be the characteristic halo mass where the production of DL is the most efficient, while the overall efficiency decreases at both larger (\citealt{contini2024}) and smaller scales (this work). The DL content in this range of halo mass is the result of stellar stripping due to tidal interaction between satellites and its host (95\%) and mergers between satellites and the central galaxy (5\%), with pre-processed material, sub-channel of mergers and stripping and so already included in the 100\%, that contributes no more than 8\% on average. The halo concentration is the main driver of the DL formation: more concentrated haloes have higher DL fractions that come from stripping of more massive satellites in the high halo mass end, while dwarfs contribute mostly in the low halo mass end.

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arXiv2024

The SAMI Galaxy Survey: galaxy spin is more strongly correlated with stellar population age than mass or environment

We use the SAMI Galaxy Survey to examine the drivers of galaxy spin, $λ_{R_e}$, in a multi-dimensional parameter space including stellar mass, stellar population age (or specific star formation rate) and various environmental metrics (local density, halo mass, satellite vs. central). Using a partial correlation analysis we consistently find that age or specific star formation rate is the primary parameter correlating with spin. Light-weighted age and specific star formation rate are more strongly correlated with spin than mass-weighted age. In fact, across our sample, once the relation between light-weighted age and spin is accounted for, there is no significant residual correlation between spin and mass, or spin and environment. This result is strongly suggestive that present-day environment only indirectly influences spin, via the removal of gas and star formation quenching. That is, environment affects age, then age affects spin. Older galaxies then have lower spin, either due to stars being born dynamically hotter at high redshift, or due to secular heating. Our results appear to rule out environmentally dependent dynamical heating (e.g. galaxy-galaxy interactions) being important, at least within $1R_e$ where our kinematic measurements are made. The picture is more complex when we only consider high-mass galaxies ($M_*\gtrsim 10^{11}$M$_{\odot}$). While the age-spin relation is still strong for these high-mass galaxies, there is a residual environmental trend with central galaxies preferentially having lower spin, compared to satellites of the same age and mass. We argue that this trend is likely due to central galaxies being a preferred location for mergers.

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arXiv2024

The formation of cores in galaxies across cosmic time -- the existence of cores is not in tension with the LCDM paradigm

The `core-cusp' problem is considered a key challenge to the LCDM paradigm. Halos in dark matter only simulations exhibit `cuspy' profiles, where density continuously increases towards the centre. However, the dark matter profiles of many observed galaxies (particularly in the dwarf regime) deviate strongly from this prediction, with much flatter central regions (`cores'). We use NewHorizon (NH), a hydrodynamical cosmological simulation, to investigate core formation, using a statistically significant number of galaxies in a cosmological volume. Halos containing galaxies in the upper (M* > 10^10.2 MSun) and lower (M* < 10^8 MSun) ends of the stellar mass distribution contain cusps. However, halos containing galaxies with intermediate (10^8 MSun < M* < 10^10.2 MSun) stellar masses are generally cored, with typical halo masses between 10^10.2 MSun and 10^11.5 MSun. Cores form through supernova-driven gas removal from halo centres, which alters the central gravitational potential, inducing dark matter to migrate to larger radii. While all massive (M* > 10^9.5 MSun) galaxies undergo a cored-phase, in some cases cores can be removed and cusps reformed. This happens if a galaxy undergoes sustained star formation at high redshift, which results in stars (which, unlike the gas, cannot be removed by baryonic feedback) dominating the central gravitational potential. After cosmic star formation peaks, the number of cores, and the mass of the halos they are formed in, remain constant, indicating that cores are being routinely formed over cosmic time after a threshold halo mass is reached. The existence of cores is, therefore, not in tension with the standard paradigm.

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arXiv2023

The Intracluster Light and its Link with the Dynamical State of the Host Group/Cluster: the Role of the Halo Concentration

We investigate on the role of the halo concentration in the formation of the intra-cluster light (ICL) in galaxy groups and clusters, as predicted by a state-of-art semi-analytic model of galaxy formation, coupled with a set of high-resolution dark matter only simulations. The analysis focuses on how the fraction of ICL correlates with halo mass, concentration and fraction of early-type galaxies (ETGs) in a large sample of groups and clusters with $13.0\leq \log M_{halo} \leq 15.0$. The fraction of ICL follows a normal distribution, a consequence of the stochastic nature of the physical processes responsible for the formation of the diffuse light. The fractional budget of ICL depends on both halo mass (very weakly) until group scales, and concentration (remarkably). More interestingly, the ICL fraction is higher in more concentrated objects, a result of the stronger tidal forces acting in the innermost regions of the haloes where the concentration is the quantity playing the most relevant role. Our model predictions do not show any dependence between the ICL and ETGs fractions and so, we instead suggest the concentration rather than the mass, as recently claimed, to be the main driver of the ICL formation. The diffuse light starts to form in groups via stellar stripping and mergers and later assembled in more massive objects. However, the formation and assembly keep going on group/cluster scales at lower redshift through the same processes, mainly via stellar stripping in the vicinity of the central regions where tidal forces are stronger.

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arXiv2022

The NewHorizon Simulation -- To Bar Or Not To Bar

We use the NewHorizon simulation to study the redshift evolution of bar properties and fractions within galaxies in the stellar masses range $M_{\star} = 10^{7.25} - 10^{11.4} \ \rm{M}_{\odot}$ over the redshift range $z = 0.25 - 1.3$. We select disc galaxies using stellar kinematics as a proxy for galaxy morphology. We employ two different automated bar detection methods, coupled with visual inspection, resulting in observable bar fractions of $f_{\rm bar} = 0.070_{-0.012}^{+0.018}$ at $z\sim$ 1.3, decreasing to $f_{\rm bar} = 0.011_{-0.003}^{+0.014}$ at $z\sim$ 0.25. Only one galaxy is visually confirmed as strongly barred in our sample. This bar is hosted by the most massive disk and only survives from $z=1.3$ down to $z=0.7$. Such a low bar fraction, in particular amongst Milky Way-like progenitors, highlights a missing bars problem, shared by literally all cosmological simulations with spatial resolution $<$100 pc to date. The analysis of linear growth rates, rotation curves and derived summary statistics of the stellar, gas and dark matter components suggest that galaxies with stellar masses below $10^{9.5}-10^{10} \ \rm{M}_{\odot}$ in NewHorizon appear to be too dominated by dark matter to form a bar, while more massive galaxies typically have formed large bulges that prevent bar persistence at low redshift. This investigation confirms that the evolution of the bar fraction puts stringent constraints on the assembly history of baryons and dark matter onto galaxies.

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arXiv2021

The MAGPI Survey -- science goals, design, observing strategy, early results and theoretical framework

We present an overview of the Middle Ages Galaxy Properties with Integral Field Spectroscopy (MAGPI) survey, a Large Program on ESO/VLT. MAGPI is designed to study the physical drivers of galaxy transformation at a lookback time of 3-4 Gyr, during which the dynamical, morphological, and chemical properties of galaxies are predicted to evolve significantly. The survey uses new medium-deep adaptive optics aided MUSE observations of fields selected from the GAMA survey, providing a wealth of publicly available ancillary multi-wavelength data. With these data, MAGPI will map the kinematic and chemical properties of stars and ionised gas for a sample of 60 massive (> 7 x 10^10 M_Sun) central galaxies at 0.25 < z <0.35 in a representative range of environments (isolated, groups and clusters). The spatial resolution delivered by MUSE with Ground Layer Adaptive Optics (GLAO, 0.6-0.8 arcsec FWHM) will facilitate a direct comparison with Integral Field Spectroscopy surveys of the nearby Universe, such as SAMI and MaNGA, and at higher redshifts using adaptive optics, e.g. SINS. In addition to the primary (central) galaxy sample, MAGPI will deliver resolved and unresolved spectra for as many as 150 satellite galaxies at 0.25 < z <0.35, as well as hundreds of emission-line sources at z < 6. This paper outlines the science goals, survey design, and observing strategy of MAGPI. We also present a first look at the MAGPI data, and the theoretical framework to which MAGPI data will be compared using the current generation of cosmological hydrodynamical simulations including EAGLE, Magneticum, HORIZON-AGN, and Illustris-TNG. Our results show that cosmological hydrodynamical simulations make discrepant predictions in the spatially resolved properties of galaxies at z ~ 0.3. MAGPI observations will place new constraints and allow for tangible improvements in galaxy formation theory.

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arXiv2021

Dark-matter-deficient dwarf galaxies form via tidal stripping of dark matter in interactions with massive companions

In the standard Lambda-CDM paradigm, dwarf galaxies are expected to be dark-matter-rich, as baryonic feedback is thought to quickly drive gas out of their shallow potential wells and quench star formation at early epochs. Recent observations of local dwarfs with extremely low dark matter content appear to contradict this picture, potentially bringing the validity of the standard model into question. We use NewHorizon, a high-resolution cosmological simulation, to demonstrate that sustained stripping of dark matter, in tidal interactions between a massive galaxy and a dwarf satellite, naturally produces dwarfs that are dark-matter-deficient, even though their initial dark matter fractions are normal. The process of dark matter stripping is responsible for the large scatter in the halo-to-stellar mass relation in the dwarf regime. The degree of stripping is driven by the closeness of the orbit of the dwarf around its massive companion and, in extreme cases, produces dwarfs with halo-to-stellar mass ratios as low as unity, consistent with the findings of recent observational studies. ~30 per cent of dwarfs show some deviation from normal dark matter fractions due to dark matter stripping, with 10 per cent showing high levels of dark matter deficiency (Mhalo/M*<10). Given their close orbits, a significant fraction of dark-matter-deficient dwarfs merge with their massive companions (e.g. ~70 per cent merge over timescales of ~3.5 Gyrs), with the dark-matter-deficient population being constantly replenished by new interactions between dwarfs and massive companions. The creation of these galaxies is, therefore, a natural by-product of galaxy evolution and their existence is not in tension with the standard paradigm.

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arXiv2021

The origin of low-surface-brightness galaxies in the dwarf regime

Low-surface-brightness galaxies (LSBGs) -- defined as systems that are fainter than the surface-brightness limits of past wide-area surveys -- form the overwhelming majority of galaxies in the dwarf regime (M* < 10^9 MSun). Using NewHorizon, a high-resolution cosmological simulation, we study the origin of LSBGs and explain why LSBGs at similar stellar mass show the large observed spread in surface brightness. New Horizon galaxies populate a well-defined locus in the surface brightness -- stellar mass plane, with a spread of ~3 mag arcsec^-2, in agreement with deep SDSS Stripe data. Galaxies with fainter surface brightnesses today are born in regions of higher dark-matter density. This results in faster gas accretion and more intense star formation at early epochs. The stronger resultant supernova feedback flattens gas profiles at a faster rate which, in turn, creates shallower stellar profiles (i.e. more diffuse systems) more rapidly. As star formation declines towards late epochs (z<1), the larger tidal perturbations and ram pressure experienced by these systems (due to their denser local environments) accelerate the divergence in surface brightness, by increasing their effective radii and reducing star formation respectively. A small minority of dwarfs depart from the main locus towards high surface brightnesses, making them detectable in past wide surveys. These systems have anomalously high star-formation rates, triggered by recent, fly-by or merger-driven starbursts. We note that objects considered extreme/anomalous at the depth of current datasets, e.g. `ultra-diffuse galaxies', actually dominate the predicted dwarf population and will be routinely visible in future surveys like LSST.

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arXiv2020

The role of mergers and interactions in driving the evolution of dwarf galaxies over cosmic time

Dwarf galaxies (M*<10^9 Msun) are key drivers of mass assembly in high mass galaxies, but relatively little is understood about the assembly of dwarf galaxies themselves. Using the \textsc{NewHorizon} cosmological simulation (40 pc spatial resolution), we investigate how mergers and fly-bys drive the mass assembly and structural evolution of around 1000 field and group dwarfs up to z=0.5. We find that, while dwarf galaxies often exhibit disturbed morphologies (5 and 20 per cent are disturbed at z=1 and z=3 respectively), only a small proportion of the morphological disturbances seen in dwarf galaxies are driven by mergers at any redshift (for 10^9 Msun, mergers drive only 20 per cent morphological disturbances). They are instead primarily the result of interactions that do not end in a merger (e.g. fly-bys). Given the large fraction of apparently morphologically disturbed dwarf galaxies which are not, in fact, merging, this finding is particularly important to future studies identifying dwarf mergers and post-mergers morphologically at intermediate and high redshifts. Dwarfs typically undergo one major and one minor merger between z=5 and z=0.5, accounting for 10 per cent of their total stellar mass. Mergers can also drive moderate star formation enhancements at lower redshifts (3 or 4 times at z=1), but this accounts for only a few per cent of stellar mass in the dwarf regime given their infrequency. Non-merger interactions drive significantly smaller star formation enhancements (around two times), but their preponderance relative to mergers means they account for around 10 per cent of stellar mass formed in the dwarf regime.

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arXiv2020

The Roles of Mass and Environment in the Quenching of Galaxies. II

We take advantage of an analytic model of galaxy formation coupled to the merger tree of an N-body simulation to study the roles of environment and stellar mass in the quenching of galaxies. The model has been originally set in order to provide the observed evolution of the stellar mass function as well as reasonable predictions of the star formation rate-stellar mass relation, from high redshift to the present time. We analyse the stellar mass and environmental quenching efficiencies and their dependence on stellar mass, halo mass (taken as a proxy for the environment) and redshift. Our analysis shows that the two quenching efficiencies are redshift, stellar and halo mass dependent, and that the halo mass is also a good proxy for the environment. The environmental quenching increases with decreasing redshift and is inefficient below $\log M_* \sim 9.5$, reaches the maximum value at $\log M_* \sim 10.5$, and decreases again, becoming poorly efficient at very high stellar mass ($\log M_* \gtrsim 11.5$). Central and satellites galaxies are mass quenched differently: for the former, the quenching efficiency depends very weakly on redshift, but strongly on stellar mass; for the latter, it strongly depends on both stellar mass and redshift in the range $10\lesssim \log M_* \lesssim 11$. According to the most recent observational results, we find that the two quenching efficiencies are not separable: intermediate mass galaxies are environmental quenched faster, as well as intermediate/massive galaxies in more massive haloes. At stellar masses lower than $\log M_* \lesssim 9.5$ both quenching mechanisms become inefficient, independently of the redshift.

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arXiv2019

The Roles of Mass and Environment in the Quenching of Galaxies

We study the roles of stellar mass and environment in quenching the star formation activity of a large set of simulated galaxies by taking advantage of an analytic model coupled to the merger tree extracted from an N-body simulation. The analytic model has been set to match the evolution of the global stellar mass function since redshift $z\sim 2.3$ and give reasonable predictions of the star formation history of galaxies at the same time. We find that stellar mass and environment play different roles: the star formation rate/specific star formation rate-$M_*$ relations are independent of the environment (defined as the halo mass) at any redshift probed, $0 1$ as generally claimed, while the environment has a minimal role. All the physical processes linked to the environment must act on very short timescales, such that they do not influence the star formation of active galaxies, but increase the probability of a given galaxy to become quiescent.

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arXiv2019

Physical properties of SDSS satellite galaxies in projected phase-space

We investigate how environment affects satellite galaxies using their location within the projected phase-space of their host haloes from the Wang et al.'s group catalogue. Using the Yonsei Zoom in Cluster Simulations, we derive zones of constant mean infall time T_inf in projected phase-space, and catalogue in which zone each observed galaxy falls. Within each zone we compute the mean observed galaxy properties including specific star formation rate, luminosity-weighted age, stellar metallicity and [alpha/Fe] abundance ratio. By comparing galaxies in different zones, we inspect how shifting the mean infall time from recent infallers (mean T_inf < 3 Gyr) to ancient infallers (mean T_inf > 5 Gyr) impacts galaxy properties at fixed stellar and halo mass. Ancient infallers are more quenched, and the impact of environmental quenching is visible down to low host masses (< group masses). Meanwhile, the quenching of recent infallers is weakly dependent on host mass, indicating they have yet to respond strongly to their current environment. [alpha/Fe] and especially metallicity are less dependent on host mass, but show a dependence on mean T_inf. We discuss these results in the context of longer exposure times for ancient infallers to environmental effects, which grow more efficient in hosts with a deeper potential well and a denser intracluster medium. We also compare our satellites with a control field sample, and find that even the most recent infallers (mean T_inf < 2 Gyr) are more quenched than field galaxies, in particular for cluster mass hosts. This supports the role of pre-processing and/or faster quenching in satellites.

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arXiv2018

The SAMI Galaxy Survey: Observing the environmental quenching of star formation in GAMA groups

We explore the radial distribution of star formation in galaxies in the SAMI Galaxy Survey as a function of their local group environment. Using a sample of galaxies in groups (with halo masses less than $ \simeq 10^{14} \, \mathrm{M_{\odot}}$) from the Galaxy And Mass Assembly Survey, we find signatures of environmental quenching in high-mass groups ($M_{G} > 10^{12.5} \, \mathrm{M_{\odot}}$). The mean integrated specific star formation rate of star-forming galaxies in high-mass groups is lower than for galaxies in low-mass groups or that are ungrouped, with $Δ\log(sSFR/\mathrm{yr^{-1}}) = 0.45 \pm 0.07$. This difference is seen at all galaxy stellar masses. In high-mass groups, star-forming galaxies more massive than $M_{*} \sim 10^{10} \, \mathrm{M_{\odot}}$ have centrally-concentrated star formation. These galaxies also lie below the star-formation main sequence, suggesting they may be undergoing outside-in quenching. Lower mass galaxies in high-mass groups do not show evidence of concentrated star formation. In groups less massive than $M_{G} = 10^{12.5} \, \mathrm{M_{\odot}}$ we do not observe these trends. In this regime we find a modest correlation between centrally-concentrated star formation and an enhancement in total star formation rate, consistent with triggered star formation in these galaxies.

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arXiv2018

Theoretical Predictions of Colors and Metallicity of the Intra-Cluster Light

We study colors and metallicities of the Brightest Cluster Galaxies (BCGs) and Intra-Cluster Light (ICL) in galaxy groups and clusters, as predicted by a semi-analytic model of galaxy formation, coupled with a set of high-resolution N-body simulations. The model assumes stellar stripping and violent relaxation processes during galaxy mergers to be the main channels for the formation of the ICL. We find that BCGs are more metal-rich and redder than the ICL, at all redshifts since the ICL starts to form ($z\sim 1$). In good agreement with several observed data, our model predicts negative radial metallicity and color gradients in the BCG+ICL system. By comparing the typical colors of the ICL with those of satellite galaxies, we find that mass and metals in the ICL come from galaxies of different mass, depending on the redshift. Stripping of low mass galaxies, $9<\log M_* <10$, is the most important contributor in the early stage of the ICL formation, but the bulk of the mass/metals contents are given by intermediate/massive galaxies, $10<\log M_* <11$, at lower redshift. Our analysis supports the idea that stellar stripping is more important than galaxy mergers in building-up the ICL, and highlights the importance of colors/metallicity measurements for understanding the formation and evolution of the ICL.

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