Intracluster Light Fraction Research Articles

Characterising the intra-cluster light in The Three Hundred simulations

We characterise the intra-cluster light (ICL) in ensembles of full-physics cluster simulations from THE THREE HUNDRED project, a suite of 324 hydrodynamical resimulations of cluster-sized halos. We identify the ICL as those stellar particles bound to the potential of the cluster itself, but not to any of its substructures, and separate the brightest cluster galaxy (BCG) by means of a fixed 50 kpc aperture. We find the total BCG+ICL mass to be in agreement with state-of-the-art observations of galaxy clusters. The ICL mass fraction of our clusters is between 30 and 50% of the total stellar mass within R500, while the BCG represents around 10%. We further find no trend of the ICL fraction with cluster halo mass, at least not in the range [0.2, 3] 1015 h−1 M⊙ considered here. For the dynamical state, characterised both by theoretical estimators and by the recent merging history of the cluster, there is a clear correlation, such that more relaxed clusters and those that have undergone fewer recent mergers have a higher ICL fraction. Finally, we investigate the possibility of using the ICL to explore the dark matter (DM) component of galaxy clusters. We compute the volumetric density profile for the DM and ICL components and show that, up to R500, the ratio between the two can be described by a power law. Working with the velocity dispersion profiles instead, we show that the ratio can be fit by a straight line. Providing the parameters of these fits, we show how the ICL can be used to infer DM properties.

Evidence for a Redshifted Excess in the Intracluster Light Fractions of Merging Clusters at z ∼ 0.8

The intracluster light (ICL) fraction is a well-known indicator of the dynamical activity in intermediate-redshift clusters. Merging clusters in the redshift interval 0.18 < z < 0.56 have a distinctive peak in the ICL fractions measured between ∼3800 and 4800 Å . In this work, we analyze two higher-redshift, clearly merging clusters, ACT-CLJ0102-49151 and CL J0152.7-1357, at z > 0.8, using the Hubble Space Telescope optical and infrared images obtained by the RELICS survey. We report the presence of a similar peak in the ICL fractions, although wider and redshifted to the wavelength interval ∼5200–7300 Å. The fact that this excess in the ICL fractions is found at longer wavelengths can be explained by an assorted mixture of stellar populations in the ICL, direct inheritance of an ICL that was mainly formed by major galaxy mergers with the BCG at z > 1 and experienced an instantaneous burst due to the merging event. The ubiquity of the ICL fraction merging signature across cosmic time enhances the ICL as a highly reliable and powerful probe to determine the dynamical stage of galaxy clusters, which is crucial for cluster-based cosmological inferences that require relaxation of the sample.

The Connection between the Intracluster Light and its Host Halo: Formation Time and Contribution from Different Channels

We extend the analysis presented by Contini et al. to higher redshifts, up to z = 2, by focusing on the relation between the intracluster light (ICL) fraction and the halo mass, its dependence on redshift, the role played by the halo concentration, and the formation time, in a large sample of simulated galaxy groups/clusters with . Moreover, a key focus is to isolate the relative contributions provided by the main channels for the ICL formation to the total amount. The ICL fraction at higher redshift is weakly dependent on halo mass and comparable with that at the present time, in agreement with recent observations. Stellar stripping, mergers, and preprocessing are the major channels responsible for the ICL formation, with stellar stripping accounting for ∼90% of the total ICL, regardless of halo mass and redshift. Preprocessing is an important process for clusters to accrete already formed ICL. The diffuse component forms very early, at z ∼ 0.6, and its formation depends on both concentration and formation time of the halo, with more concentrated haloes that formed early assembling their ICL earlier than those that formed later. The efficiency of this process is independent of halo mass but increases with decreasing redshift, which implies that stellar stripping becomes more important with time as the concentration increases. This highlights the link between the ICL and the dynamical state of a halo: groups/clusters that have a higher fraction of diffuse light are more concentrated, relaxed, and in an advanced stage of growth.

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

We investigate the role of halo concentration in the formation of intracluster light (ICL) in galaxy groups and clusters, as predicted by a state-of-the-art semianalytic 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 . 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 halos 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.

Dissecting the RELICS cluster SPT-CLJ0615–5746 through intracluster light: Confirmation of the multiple merging state of the cluster formation

The intracluster light (ICL) fraction, measured at certain specific wavelengths, has been shown to provide a good marker for determining the dynamical stage of galaxy clusters, that is, merging versus relaxed, for low to intermediate redshifts. We apply it for the first time to a high-redshift system, SPT-CLJ0615–5746 at z = 0.97, using its Reionization Lensing Cluster Survey (RELICS) observations in the optical and infrared. We find the ICL fraction signature of the merging, with values ranging from 16 to 37%. A careful reanalysis of the X-ray data available for this cluster indicates at least one current and plausibly a second merger. These two results contradict previous works based on X-ray data, which claimed a relaxed state for SPT-CLJ0615–5746, and the results confirm the evidence presented by kinematic analyses. We also found an abnormally high ICL fraction in the rest-frame near-ultraviolet wavelengths, which may be attributed to the combination of several phenomena such as an ICL injection during recent mergers of stars with average early-type spectra, the reversed star formation-density relation found at this high redshift in comparison with lower-redshift clusters, and projection effects.

Does the virial mass drive the intra-cluster light?

In this Letter, we revisit the relationship between the fraction of the intra-cluster light (ICL) as well as the virial mass and the fraction of early-type galaxies in the host halo. This exploration is based on a statistically significant and homogeneous sample of 22 groups and clusters of galaxies in the Local Universe (z ≤ 0.05), obtained with the VST Early-type GAlaxy Survey (VEGAS). Taking advantage of the long integration time and large area of the VEGAS images, we are able to map the galaxy outskirts and ICL down to μg ≥ 29–30 mag arcsec−2 and out to hundreds of kpc. With this data set, we have expanded the sample of ICL measurements, doubling the previous measures available from the literature for z ≤ 0.05. The main result of this work is the apparent lack of any significant trend between the fraction of ICL and the virial mass of the host environment, covering a wide range of virial masses (∼1012.5 ≤ Mvir ≤ 1015.5 M⊙), in full agreement with a number of theoretical studies. Since the new data points have all been derived based on the same methodology and from the same observational setup, all with comparable depths, the large observed scatter indicates an intrinsic variation in the ICL fraction. On the other hand, there is a weak relationship between the fraction of ICL and the fraction of early-type galaxies in the host halo, where a larger fraction of ICL is found in groups and clusters of galaxies dominated by earlier morphological types, indicating a connection between the ICL and the dynamical state of the host system.

Intracluster light is already abundant at redshift beyond unity.

Intracluster light (ICL) is diffuse light from stars that are gravitationally bound not to individual member galaxies, but to the halo of galaxyclusters. Leading theories1,2 predict that the ICL fraction, defined by the ratio of the ICL to the total light, rapidly decreases with increasing redshift, to the level of a few per cent at z > 1. However, observational studies have remained inconclusive about the fraction beyond redshift unity because, to date, only two clusters in this redshift regime have been investigated. One shows a much lower fraction than the mean value at low redshift3, whereas the other possesses a fraction similar to the low-redshift value4. Here we report an ICL study of ten galaxy clusters at 1 ≲ z ≲ 2 based on deep infrared imaging data. Contrary to the leading theories, our study finds that ICL is already abundant at z ≳ 1, with a mean ICL fraction of approximately 17%. Moreover, no significant correlation between cluster mass and ICL fraction or between ICL colour and cluster-centric radius is observed. Our findings suggest that gradual stripping can no longer be the dominant mechanism of ICL formation. Instead, our study supports the scenario wherein the dominant ICL production occurs in tandem with the formation and growth of the brightest cluster galaxies and/or through the accretion of preprocessed stray stars.

Independent Evidence for Earlier Formation Epochs of Fossil Groups of Galaxies through the Intracluster Light: The Case for RX J100742.53+380046.6

Fossil groups (FG) of galaxies still present a puzzle to theories of structure formation. Despite the low number of bright galaxies, they have relatively high velocity dispersions and ICM temperatures often corresponding to cluster-like potential wells. Their measured concentrations are typically high, indicating early formation epochs as expected from the originally proposed scenario for their origin as being older undisturbed systems. This is, however, in contradiction with the typical lack of expected well developed cool cores. Here, we apply a cluster dynamical indicator recently discovered in the intracluster light fraction (ICLf) to a classic FG, RX J1000742.53+380046.6, to assess its dynamical state. We also refine that indicator to use as an independent age estimator. We find negative radial temperature and metal abundance gradients, the abundance achieving supersolar values at the hot core. The X-ray flux concentration is consistent with that of cool core systems. The ICLf analysis provides an independent probe of the system’s dynamical state and shows that the system is very relaxed, more than all clusters, where the same analysis has been performed. The specific ICLf is about 6 times higher, than any of the clusters previously analyzed, which is consistent with an older noninteractive galaxy system that had its last merging event within the last ∼5 Gyr. The specific ICLf is predicted to be an important new tool to identify fossil systems and to constrain the relative age of clusters.

The intracluster light on Frontier Fields clusters Abell 370 and Abell S1063

ABSTRACT We analysed the contribution of the intracluster light (ICL) to the total luminosity of two massive galaxy clusters observed by the Hubble Space Telescope within the Frontier Fields program, Abell 370 (z ∼ 0.375) and Abell S1063 (z ∼ 0.348), in order to correlate it with the dynamical stage of these systems. We applied an algorithm based on the Chebyshev–Fourier functions called CICLE, specially developed to disentangle the ICL from the light of galaxies and measure the ICL fraction. We measured the ICL fraction in three broad-band optical filters, F435W, F606W, and F814W, without assuming any prior hypothesis about the ICL physical properties or morphology. The results obtained from the ICL fraction vary between $\sim 7\!-\!25{{\ \rm per\ cent}}$, and $\sim 3\!-\!22{{\ \rm per\ cent}}$ for both A370 and AS1063, respectively, which are consistent with theoretical predictions for the total amount of ICL obtained by ICL formation and evolution simulations. We found enhanced ICL fractions in the intermediate filter F606W for both clusters and we suggest that this is due to the presence of an excess of younger/lower metallicity stars in the ICL compared to the cluster galaxies. We conclude that both Abell 370 and Abell S1063 are merging systems since they exhibit a similar feature as merging CLASH and Frontier Fields clusters sub-sample previously analysed. We compare these results to the dynamical indicators obtained through different methods and we reinforce the use of ICL as a new and independent method to determine the dynamical state of clusters of galaxies.

RELICS: ICL Analysis of the z = 0.566 Merging Cluster WHL J013719.8–08284

Abstract We present a pilot study of the intracluster light (ICL) in massive clusters using imaging of the z = 0.566 cluster of galaxies WHL J013719.8–08284 observed by the RELICS project with the Hubble Space Telescope. We measure the ICL fraction in four optical ACS/WFC filters (F435W, F475W, F606W, and F814W) and five infrared WFC3/IR bands (F105W, F110W, F125W, F140W, and F160W). The ICL maps are calculated using the free-of-a-priori-assumptions algorithm CICLE, and the cluster membership is estimated from photometric properties. We find optical ICL fractions that range between ∼6% and 19%, in nice agreement with the values found in previous works for merging clusters. We also observe an ICL fraction excess between 3800 Å and 4800 Å, previously identified as a signature of merging clusters at 0.18 &lt; z &lt; 0.55. This excess suggests the presence of an enhanced population of young/low-metallicity stars in the ICL. All indicators thus point to WHL J013719.8–08284 as a disturbed cluster with a significant amount of recently injected stars, bluer than the average stars hosted by the cluster members and likely stripped out from infalling galaxies during the current merging event. Infrared ICL fractions are ∼50% higher than optical ones, which could be signatures of an older and/or higher-metallicity ICL population that can be associated with the buildup of the brightest cluster galaxy, passive evolution of previously injected young stars, or preprocessing in infalling groups. Finally, investigating the photometry of the cluster members, we tentatively conclude that WHL J013719.8–08284 fulfills the expected conditions for a fossil system progenitor.

Intracluster light properties in a fossil cluster at z = 0.47

ABSTRACT Galaxy clusters contain a diffuse stellar component outside the cluster’s galaxies, which is observed as faint intracluster light (ICL). Using Gemini/GMOS-N deep imaging and multiobject spectroscopy of a massive fossil cluster at a redshift of z = 0.47, RX J105453.3+552102 (J1054), we improve the observational constraints on the formation mechanism of the ICL. We extract the ICL surface brightness and colour profiles out to 155 kpc from the brightest cluster galaxy (BCG) with a detection limit of 28.7 mag arcsec−2 (1σ, 4.8 × 4.8arcsec2; i band). The colour of the diffuse light is similar to that of the BCG and central bright galaxies out to ∼ 70 kpc, becoming slightly bluer toward the outside. We find that the ICL distribution shows better agreement with the spatial distribution of member galaxies than with the BCG-dominated cluster luminosity distribution. We report the ICL fraction of J1054 as $15.07 \pm 4.57 {{\ \rm per\ cent}}$ in the range of 60 ∼ 155 kpc from the BCG, which appears to be higher than the ICL fraction-redshift trend in previous studies. Our findings suggest that intracluster stars seems not to be explained by one dominant production mechanism. However, a significant fraction of the ICL of J1054 may have been generated from the outskirts of infalling/satellite galaxies more recently rather than by the BCG at the early stage of the cluster.

DAWIS: a detection algorithm with wavelets for intracluster light studies

Context. Large numbers of deep optical images will be available in the near future, allowing statistically significant studies of low surface brightness structures such as intracluster light (ICL) in galaxy clusters. The detection of these structures requires efficient algorithms dedicated to this task, which traditional methods find difficult to solve. Aims. We present our new detection algorithm with wavelets for intracluster light studies (DAWIS), which we developed and optimized for the detection of low surface brightness sources in images, in particular (but not limited to) ICL. Methods. DAWIS follows a multiresolution vision based on wavelet representation to detect sources. It is embedded in an iterative procedure called synthesis-by-analysis approach to restore the unmasked light distribution of these sources with very good quality. The algorithm is built so that sources can be classified based on criteria depending on the analysis goal. We present the case of ICL detection and the measurement of ICL fractions. We test the efficiency of DAWIS on 270 mock images of galaxy clusters with various ICL profiles and compare its efficiency to more traditional ICL detection methods such as the surface brightness threshold method. We also run DAWIS on a real galaxy cluster image, and compare the output to results obtained with previous multiscale analysis algorithms. Results. We find in simulations that DAWIS is on average able to separate galaxy light from ICL more efficiently, and to detect a greater quantity of ICL flux because of the way sky background noise is treated. We also show that the ICL fraction, a metric used on a regular basis to characterize ICL, is subject to several measurement biases on galaxies and ICL fluxes. In the real galaxy cluster image, DAWIS detects a faint and extended source with an absolute magnitude two orders brighter than previous multiscale methods.

Photometric Dissection of Intracluster Light and Its Correlations with Host Cluster Properties

Abstract We explore several ways to dissect brightest cluster galaxies (BCGs) and their surrounding intracluster light (ICL) using a surface brightness (SB) cut, a luminosity cut, excess light above a de Vaucouleurs profile, or a double Sérsic decomposition. Assuming that all light above is attributable to the ICL, we find that an average fraction of of all diffuse light centered on the BCG belongs to the ICL. Likewise, if we assume that all light fainter than mag arcsec−2 belongs to the ICL, the average ICL fraction is . After fitting a de Vaucouleurs profile to the inner parts of the SB profile, we detect excess light at large radii, corresponding to an average ICL fraction of . Finally, by decomposing the SB profile into two Sérsic functions, we find an average ICL fraction of associated with the outer Sérsic component. Our measured ICL and BCG+ICL luminosities agree well with predictions from high-resolution simulations where the outer Sérsic component traces the unrelaxed, accreted stellar material. BCG and ICL properties defined in this way are correlated with cluster parameters to study the coevolution of BCGs, ICL, and their host clusters. We find positive correlations between BCG+ICL brightness and cluster mass, cluster velocity dispersion, cluster radius, and integrated satellite brightness, confirming that BCG/ICL growth is indeed coupled with cluster growth. On average, the ICL is better aligned than the BCG with the host cluster in terms of position angle, ellipticity, and centering. That makes it a potential dark-matter tracer.

The complex case of MACS J0717.5+3745 and its extended filament: intra-cluster light, galaxy luminosity function, and galaxy orientations

Context. The properties of galaxies are known to be affected by their environment, but although galaxies in clusters and groups have been quite thoroughly investigated, little is known about galaxies belonging to filaments of the cosmic web, or about the properties of the filaments themselves. Aims. Here we investigate the properties of the rich cluster MACS J0717.5+3745 and its extended filament by analyzing the distribution and fractions of intra-cluster light (ICL) in its core and by trying to detect intra-filament light (IFL) in the filament. We analyze the galaxy luminosity function (GLF) of the cluster core and of the filament. We also study the orientations of galaxies in the filament to better constrain the filament properties. Methods. This work is based on Hubble Space Telescope (HST) archive data, both from the Hubble Frontier Fields in the F435W, F606W, F814W, and F105W bands, and from a mosaic of images in the F606W and F814W bands. The spatial distribution of the ICL was determined with our new wavelet-based software, DAWIS. The GLFs were extracted in the F606W and F814W bands, with a statistical subtraction of the background, and fit with Schechter functions. The galaxy orientations in the filaments were estimated with SExtractor after correction for the point spread function. Results. We detect a large amount of ICL in the cluster core, but no IFL in the cosmic filament. The fraction of ICL in the core peaks in the F606W filter before decreasing with wavelength. Though relatively noisy, the GLFs in the filament are notably different from those of field galaxies, with a flatter faint end slope and an excess of bright galaxies. We do not detect a significant alignment of the galaxies in the filament region analyzed.

J-PLUS: Analysis of the intracluster light in the Coma cluster

Context. The intracluster light (ICL) is a luminous component of galaxy clusters composed of stars that are gravitationally bound to the cluster potential, but do not belong to the individual galaxies. Previous studies of the ICL have shown that its formation and evolution are intimately linked to the evolutionary stage of the cluster. Thus, the analysis of the ICL in the Coma cluster will give insights into the main processes driving the dynamics in this highly complex system.Aims. Using a recently developed technique, we measure the ICL fraction in Coma at several wavelengths, using the J-PLUS unique filter system. The combination of narrow- and broadband filters provides valuable information on the dynamical state of the cluster, the ICL stellar types, and the morphology of the diffuse light.Methods. We used the Chebyshev-Fourier intracluster light estimator (CICLE) to distinguish the ICL from the light of the galaxies, and to robustly measure the ICL fraction in seven J-PLUS filters.Results. We obtain the ICL fraction distribution of the Coma cluster at different optical wavelengths, which varies from ∼7%−21%, showing the highest values in the narrowband filtersJ0395, J0410, andJ0430. This ICL fraction excess is a distinctive pattern that has recently been observed in dynamically active clusters (mergers), indicating a higher amount of bluer stars in the ICL than in cluster galaxies.Conclusions. 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.

Evidence for the Existence of Abundant Intracluster Light at z = 1.24

Abstract Intracluster stars are believed to be unbound from their progenitor galaxies and diffused throughout the galaxy cluster, creating intracluster light (ICL). However, when and how these stars form is still under debate. To directly constrain the origin, one powerful method is to study clusters at the epoch when mature galaxy clusters began to appear. We report measurements of the spatial distribution, color, and quantity of diffuse intracluster stars for a massive galaxy cluster at a redshift of 1.24. This is the most distant galaxy cluster to date for which those three properties of the ICL have been quantified simultaneously. Our detection of the significant ICL fraction in this unprecedentedly high-redshift regime strongly indicates that intracluster stars, contrary to most previous studies, might have formed during a short period and early in the history of the Virgo-like massive cluster formation and might be concurrent with the formation of the brightest cluster galaxy.

An Investigation of Intracluster Light Evolution Using Cosmological Hydrodynamical Simulations

Abstract Intracluster light (ICL) in observations is usually identified through the surface brightness limit (SBL) method. In this paper, for the first time we produce mock images of galaxy groups and clusters, using a cosmological hydrodynamical simulation to investigate the ICL fraction and focus on its dependence on observational parameters, e.g., the SBL, the effects of cosmological redshift-dimming, point-spread function (PSF), and CCD pixel size. Detailed analyses suggest that the width of the PSF has a significant effect on the measured ICL fraction, while the relatively small pixel size shows almost no influence. It is found that the measured ICL fraction depends strongly on the SBL. At a fixed SBL and redshift, the measured ICL fraction decreases with increasing halo mass, while with a much fainter SBL, it does not depend on halo mass at low redshifts. In our work, the measured ICL fraction shows a clear dependence on the cosmological redshift-dimming effect. It is found that there is more mass locked in the ICL component than light, suggesting that the use of a constant mass-to-light ratio at high surface brightness levels will lead to an underestimate of ICL mass. Furthermore, it is found that the radial profile of ICL shows a characteristic radius that is almost independent of halo mass. The current measurement of ICL from observations has a large dispersion due to different methods, and we emphasize the importance of using the same definition when observational results are compared with theoretical predictions.

Unveiling the Dynamical State of Massive Clusters through the ICL Fraction

Abstract We have selected a sample of 11 massive clusters of galaxies observed by the Hubble Space Telescope in order to study the impact of the dynamical state on the intracluster light (ICL) fraction, the ratio of total integrated ICL to the total galaxy member light. With the exception of the Bullet cluster, the sample is drawn from the Cluster Lensing and Supernova Survey and the Frontier Fields program, containing five relaxed and six merging clusters. The ICL fraction is calculated in three optical filters using the CHEFs ICL estimator, a robust and accurate algorithm free of a priori assumptions. We find that the ICL fraction in the three bands is, on average, higher for the merging clusters, ranging between ∼7% and 23%, compared with the ∼2%–11% found for the relaxed systems. We observe a nearly constant value (within the error bars) in the ICL fraction of the regular clusters at the three wavelengths considered, which would indicate that the colors of the ICL and the cluster galaxies are, on average, coincident and, thus, so are their stellar populations. However, we find a higher ICL fraction in the F606W filter for the merging clusters, consistent with an excess of lower-metallicity/younger stars in the ICL, which could have migrated violently from the outskirts of the infalling galaxies during the merger event.

THE BURRELL SCHMIDT DEEP VIRGO SURVEY: TIDAL DEBRIS, GALAXY HALOS, AND DIFFUSE INTRACLUSTER LIGHT IN THE VIRGO CLUSTER

ABSTRACT We present the results of a deep imaging survey of the Virgo cluster of galaxies, concentrated around the cores of Virgo subclusters A and B. The goal of this survey was to detect and study very low surface brightness features present in Virgo, including discrete tidal features, the faint halos of luminous galaxies, and the diffuse intracluster light (ICL). Our observations span roughly 16 degrees2 in two filters, reaching a 3σ limiting depth of = 29.5 and = 28.5 mag arcsec−2. At these depths, our limiting systematic uncertainties are astrophysical: variations in faint background sources as well as scattered light from galactic dust. We show that this dust-scattered light is well traced by deep far-infrared imaging, making it possible to separate it from true diffuse light in Virgo. We use our imaging to trace and measure the color of the diffuse tidal streams and ICL in the Virgo core near M87, in fields adjacent to the core including the M86/M84 region, and to the south of the core around M49 and subcluster B, along with the more distant W cloud around NGC 4365. Overall, the bulk of the projected ICL is found in the Virgo core and within the W cloud; we find little evidence for an extensive ICL component in the field around M49. The bulk of the ICL we detect is fairly red in color (B − V = 0.7–0.9), indicative of old, evolved stellar populations. Based on the luminosity of the observed ICL features in the cluster, we estimate a total Virgo ICL fraction of 7%–15%. This value is somewhat smaller than that expected for massive, evolved clusters, suggesting that Virgo is still in the process of growing its extended ICL component. We also trace the shape of M87's extremely boxy outer halo out to ∼150 kpc, and show that the current tidal stripping rate from low luminosity galaxies is insufficient to have built M87's outer halo over a Hubble time. We identify a number of previously unknown low surface brightness structures around galaxies projected close to M86 and M84. The extensive diffuse light seen in the infalling W cloud around NGC 4365 is likely to be subsumed in the general Virgo ICL component once the group enters the cluster, illustrating the importance of group infall in generating ICL. Finally, we also identify another large and extremely low surface brightness ultradiffuse galaxy, likely in the process of being shredded by the cluster tidal field. With the survey complete, the full imaging data set is now available for public release.

DISENTANGLING THE ICL WITH THE CHEFs: ABELL 2744 AS A CASE STUDY

ABSTRACT Measurements of the intracluster light (ICL) are still prone to methodological ambiguities, and there are multiple techniques in the literature to address them, mostly based on the binding energy, the local density distribution, or the surface brightness. A common issue with these methods is the a priori assumption of a number of hypotheses on either the ICL morphology, its surface brightness level, or some properties of the brightest cluster galaxy (BCG). The discrepancy in the results is high, and numerical simulations just place a boundary on the ICL fraction in present-day galaxy clusters in the range 10%–50%. We developed a new algorithm based on the Chebyshev–Fourier functions to estimate the ICL fraction without relying on any a priori assumption about the physical or geometrical characteristics of the ICL. We are able to not only disentangle the ICL from the galactic luminosity but mark out the limits of the BCG from the ICL in a natural way. We test our technique with the recently released data of the cluster Abell 2744, observed by the Frontier Fields program. The complexity of this multiple merging cluster system and the formidable depth of these images make it a challenging test case to prove the efficiency of our algorithm. We found a final ICL fraction of 19.17 ± 2.87%, which is very consistent with numerical simulations.