Intracluster Light Research Articles

Identifying the discs, bulges, and intra-halo light of simulated galaxies through structural decomposition

ABSTRACT We perform a structural decomposition of galaxies identified in three cosmological hydrodynamical simulations by applying Gaussian mixture models (GMMs) to the kinematics of their stellar particles. We study the resulting disc, bulge, and intra-halo light (IHL) components of galaxies whose host dark matter haloes have virial masses in the range M200 = 1011–$10^{15}\, {\rm M_\odot }$. Our decomposition technique isolates galactic discs whose mass fractions, fdisc, correlate strongly with common alternative morphology indicators; for example, fdisc is approximately equal to κco, the fraction of stellar kinetic energy in corotation. The primary aim of our study, however, is to characterize the IHL of galaxies in a consistent manner and over a broad mass range, and to analyse its properties from the scale of galactic stellar haloes up to the intra-cluster light. Our results imply that the IHL fraction, fIHL, has appreciable scatter and is strongly correlated with galaxy morphology: at fixed stellar mass, the IHL of disc galaxies is typically older and less massive than that of spheroids. Above $M_{200}\approx 10^{13}\, {\rm M_\odot }$, we find, on average, fIHL ≈ 0.37, albeit with considerable scatter. The transition radius beyond which the IHL dominates the stellar mass of a galaxy is roughly $30\, {\rm kpc}$ for disc galaxies, but depends strongly on halo mass for spheroids. However, we find that no alternative IHL definitions – whether based on the ex situ stellar mass, or the stellar mass outside a spherical aperture – reproduce our dynamically defined IHL masses.

Exploring the correlation between dark matter, intracluster light, and globular cluster distribution in SMACS0723

We present a free-form model of SMACS0723, the first cluster observed with JWST. This model does not make any strong assumptions on the distribution of mass (mostly made up of dark matter) in the cluster and we use it to study the possible correlation between dark matter with the intracluster light and distribution of globular clusters (GCs). To explore the uncertainty in mass modeling, we derived three lens models based on spectroscopically confirmed systems and new candidate systems with redshifts predicted by the lens model derived from the spectroscopic systems. We find that beyond the radius of influence for the brightest cluster galaxy (BCG), the total mass does not trace the intracluster light (ICL), implying the need for a dark component (dark matter). Two loop-like structures observed in the intracluster light do not have any obvious correspondence with the total mass (of mostly dark matter) distribution. The radial profiles of the ICL and the distribution of GCs are similar to each other, but they are steeper than the profile of the lens model. More specifically, we find that the total mass is shallower by 1 dex in log scale than both ICL and GC profiles. This is in excellent agreement with current N-body simulations of cold dark matter.

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.

The intrinsic alignment of red galaxies in DES Y1 redMaPPer galaxy clusters

ABSTRACT Clusters of galaxies trace the most non-linear peaks in the cosmic density field. The weak gravitational lensing of background galaxies by clusters can allow us to infer their masses. However, galaxies associated with the local environment of the cluster can also be intrinsically aligned due to the local tidal gradient, contaminating any cosmology derived from the lensing signal. We measure this intrinsic alignment in Dark Energy Survey (DES) Year 1 redMaPPer clusters. We find evidence of a non-zero mean radial alignment of galaxies within clusters between redshifts 0.1–0.7. We find a significant systematic in the measured ellipticities of cluster satellite galaxies that we attribute to the central galaxy flux and other intracluster light. We attempt to correct this signal, and fit a simple model for intrinsic alignment amplitude (AIA) to the measurement, finding AIA = 0.15 ± 0.04, when excluding data near the edge of the cluster. We find a significantly stronger alignment of the central galaxy with the cluster dark matter halo at low redshift and with higher richness and central galaxy absolute magnitude (proxies for cluster mass). This is an important demonstration of the ability of large photometric data sets like DES to provide direct constraints on the intrinsic alignment of galaxies within clusters. These measurements can inform improvements to small-scale modelling and simulation of the intrinsic alignment of galaxies to help improve the separation of the intrinsic alignment signal in weak lensing studies.

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.

Intracluster light in the core of z ∼ 2 galaxy proto-clusters

ABSTRACT 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.

A UNIONS view of the brightest central galaxies of candidate fossil groups

Context. The formation process of fossil groups (FGs) is still under debate, and, because of their relative rarity, large samples of such objects are still missing. Aims. The aim of the present paper is to increase the sample of known FGs, to analyse the properties of their brightest group galaxies (BGGs), and to compare them with a control sample of non-FG BGGs. Methods. We extracted a sample of 87 FG and 100 non-FG candidates from a large spectroscopic catalogue of haloes and galaxies. For all the objects with data available in UNIONS (initially the Canada France Imaging Survey, CFIS) in the u and r bands, and/or in an extra r-band processed to preserve all low-surface-brightness features (rLSB), we performed a 2D photometric fit of the BGG with GALFIT with one or two Sérsic components. We also analysed how the subtraction of the intracluster light (ICL) contribution modifies the BGG properties. From the SDSS spectra available for the BGGs of 65 FGs and 82 non-FGs, we extracted the properties of their stellar populations with Firefly. To complement our study, and in order to provide a detailed illustration of the possible origin of emission lines in the FG BGGs, involving the presence or absence of an AGN, we investigated the origin of the emission lines in a nearby FG that is dominated by the NGC 4104 galaxy. Results. Morphologically, a single Sérsic profile can fit most objects in the u band, while two Sérsics are needed in the r and rLSB bands, both for FGs and non-FGs. Non-FG BGGs cover a larger range of Sérsic index n. FG BGGs follow the Kormendy relation (mean surface brightness versus effective radius) previously derived for almost 1000 brightest cluster galaxies (BCGs), while the majority of non-FGs BGGs are located below this relation, with fainter mean surface brightnesses. This suggests that FG BGGs have evolved similarly to BCGs, and non-FG BGGs have evolved differently from both FG BGGs and BCGs. All the above properties can be strongly modified by the subtraction of the ICL contribution. Based on spectral fitting, the stellar populations of FG and non-FG BGGs do not differ significantly. Conclusions. FG and non-FG BGGs differ from one another in terms of their morphological properties and Kormendy relation, suggesting they have had different formation histories. However, it is not possible to trace differences in their stellar populations or in their large-scale distributions.

A newly identified galaxy group thanks to tidal streams of intragroup light

Context. In the accretion-driven growth scenario, part of the intracluster light is formed in the group environment. Aims. We report the serendipitous discovery of a group of galaxies with signs of diffuse light in the foreground of the known galaxy cluster MACS J0329−0211 at z ∼ 0.45. Methods. Our investigation began with the detection of diffuse light streams around a pair of bright galaxies in the southeastern region of a Suprime-Cam image of the galaxy cluster MACS J0329-0211. Our analysis is based on the extended CLASH-VLT redshift catalog and on new spectroscopic data obtained ad hoc with the Italian Telescopio Nazionale Galileo. We use the density reconstruction method to analyze the redshift distribution of the galaxies in the region around the galaxy pair. We also use available photometric and X-ray data to better characterize the properties of the group. Results. Thanks to the large amount of redshift data collected in this region, we have been able to discover the existence of a group of galaxies, here called GrG J0330−0218, which is associated with the pair of galaxies. These are the two brightest group galaxies (BGG1 and BGG2). We extracted 41 group members from the redshift catalog and estimate a mean redshift z = 0.1537 and a line-of-sight velocity dispersion σv ∼ 370 km s−1. In the phase-space diagram, the distribution of the galaxies of GrG J0330-0218 follows the characteristic trumpet-shaped pattern, which is related to the escape velocity of galaxy clusters, suggesting that the group is a virialized structure. Under this assumption, the mass of the group is M200 ∼ 6 × 1013 M⊙. We also measured a mass-to-light ratio of ∼130 M⊙/L⊙ and a luminosity fraction of diffuse light of ∼20% within 0.5 R200. Conclusions. We conjecture that galaxy pairs that are surrounded by diffuse light, probably due to tidal interactions, can serve as signposts for groups.

The growth of brightest cluster galaxies in the TNG300 simulation: dissecting the contributions from mergers andin situstar formation

ABSTRACTWe investigate the formation of brightest cluster galaxies (BCGs) in the TNG300 cosmological simulation of the IllustrisTNG project. Our cluster sample consists of 700 haloes with $M_{\rm 200}\ge 5 \times 10^{13} \, {\rm M}_{\odot }$ at z = 0, along with their progenitors at earlier epochs. This includes 280 systems with $M_{\rm 200}\ge 10^{14} \, {\rm M}_{\odot }$ at z = 0, as well as three haloes with $M_{\rm 200}\ge 10^{15} \, {\rm M}_{\odot }$. We find that the stellar masses and star formation rates of our simulated BCGs are in good agreement with observations at z ≲ 0.4, and that they have experienced, on average, ∼2 (∼3) major mergers since z = 1 (z = 2). Separating the BCG from the intracluster light (ICL) by means of a fixed 30 kpc aperture, we find that the fraction of stellar mass contributed by ex situ (i.e. accreted) stars at z = 0 is approximately 70, 80, and 90 per cent for the BCG, BCG + ICL, and ICL, respectively. Tracking our simulated BCGs back in time using the merger trees, we find that they became dominated by ex situ stars at z ∼1–2, and that half of the stars that are part of the BCG at z = 0 formed early (z ∼ 3) in other galaxies, but ‘assembled’ onto the BCG until later times (z ≈ 0.8 for the whole sample, z ≈ 0.5 for BCGs in $M_{\rm 200}\ge 5 \times 10^{14} \, {\rm M}_{\odot }$ haloes). Finally, we show that the stellar mass profiles of BCGs are often dominated by ex situ stars at all radii, with stars from major mergers being found closer to the centre, while stars that were tidally stripped from other galaxies dominate the outer regions.

Characterizing the intracluster light over the redshift range 0.2 < z < 0.8 in the DES-ACT overlap

ABSTRACT We characterize the properties and evolution of bright central galaxies (BCGs) and the surrounding intracluster light (ICL) in galaxy clusters identified in the Dark Energy Survey and Atacama Cosmology Telescope Survey (DES-ACT) overlapping regions, covering the redshift range 0.20 < z < 0.80. Over this redshift range, we measure no change in the ICL’s stellar content (between 50 and 300 kpc) in clusters with log10(M200m,SZ/M⊙) >14.4. We also measure the stellar mass–halo mass (SMHM) relation for the BCG+ICL system and find that the slope, β, which characterizes the dependence of M200m,SZ on the BCG+ICL stellar mass, increases with radius. The outskirts are more strongly correlated with the halo than the core, which supports that the BCG+ICL system follows a two-phase growth, where recent growth (z < 2) occurs beyond the BCG’s core. Additionally, we compare our observed SMHM relation results to the IllustrisTNG300-1 cosmological hydrodynamic simulations and find moderate qualitative agreement in the amount of diffuse light. However, the SMHM relation’s slope is steeper in TNG300-1 and the intrinsic scatter is lower, likely from the absence of projection effects in TNG300-1. Additionally, we find that the ICL exhibits a colour gradient such that the outskirts are bluer than the core. Moreover, for the lower halo mass clusters (log10(M200m,SZ/M⊙) < 14.59), we detect a modest change in the colour gradient’s slope with lookback time, which combined with the absence of stellar mass growth may suggest that lower mass clusters have been involved in growth via tidal stripping more recently than their higher mass counterparts.

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.

The Formation of the Brightest Cluster Galaxy and Intracluster Light in Cosmological N-body Simulations with the Galaxy Replacement Technique

We investigate the formation channels of the intracluster light (ICL) and the brightest cluster galaxy (BCG) in clusters at z = 0. For this, we perform multi-resolution cosmological N-body simulations using the “galaxy replacement technique.” We study the formation channels of the ICL and BCG as a function of distance from the cluster center and the dynamical state of the clusters at z = 0. To do this, we trace back the stars of the ICL and BCG, and identify the stellar components in which they existed when they first fell into the clusters. We find that the progenitors of the ICL and BCG in the central region of the cluster fell earlier and with a higher total mass ratio of the progenitors to the cluster compared to the outer region. This causes a negative radial gradient in the infall time and total mass ratio of the progenitors. Although stellar mass of the progenitors does not show the same radial gradient in all clusters, massive galaxies (M gal > 1010 M ⊙ h−1) are the dominant formation channel of the ICL and BCG for all clusters, except for our most relaxed cluster. For clusters that are dynamically more unrelaxed, we find that the progenitors of the ICL and BCG fall into their clusters more recently, and with a higher mass and mass ratio. Furthermore, we find that the diffuse material of massive galaxies and group-mass halos that is formed by preprocessing contributes significantly to the ICL in the outer region of the unrelaxed clusters.

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.

Kinematics of the diffuse intragroup and intracluster light in groups and clusters of galaxies in the local universe within 100 Mpc distance

Nearly all intragroup (IGL) and intracluster light (ICL) comes from stars that are not bound to any single galaxy but were formed in galaxies and later unbound from them. In this review we focus on the physical properties–phase space properties, metallicity and age distribution–of the ICL and IGL components of the groups and clusters in the local universe, within 100 Mpc distance. Kinematic–information on these very low surface brightness structures mostly comes from discrete tracers such as planetary nebulae and globular clusters, showing highly unrelaxed velocity distributions. Cosmological hydrodynamical simulations provide key predictions for the dynamical state of IGL and ICL and find that most IC stars are dissolved from galaxies that subsequently merge with the central galaxy. The increase of the measured velocity dispersion with radius in the outer halos of bright galaxies is a physical feature that makes it possible to identify IGL and ICL components. In the local groups and clusters, IGL and ICL are located in the dense regions of these structures. Their light fractions relative to the total luminosity of the satellite galaxies in a given group or cluster are between a few to ten percent, significantly lower than the average values in more evolved, more distant clusters. IGL and ICL in the Leo I and M49 groups, and the Virgo cluster core around M87, has been found to arise from mostly old (≥10 Gyr) metal-poor ([Fe/H] <-1.0) stars of low-mass progenitor galaxies. New imaging facilities such as LSST, Euclid, and the “big eyes’’ on the sky–ELT and JWST with their advanced instrumentation–promise to greatly increase our knowledge of the progenitors of the IGL and ICL stars, their ages, metal content, masses and evolution, there by increasing our understanding of this enigmatic component.

A New Era of Intracluster Light Studies with JWST

Still largely unexplored, the diffuse light in clusters of galaxies traces the past and ongoing buildup of these massive structures. Here we present the first comprehensive study of the intracluster light (ICL) of the cluster SMACS J0723.3–7327 (z = 0.39) using the JWST Early Release Observations. These deep and high spatial resolution images allow the study of the ICL with a high signal-to-noise ratio up to a radial distance of ∼400 kpc, twice as far with respect to previous HST studies of intermediate-redshift clusters. This opens up the possibility of exploring the rich mixture of processes that are building the ICL. We find that the inner parts of this cluster (R < 100 kpc) are built through a major merger, while the outer parts (R > 100 kpc) are mainly produced by the tidal stripping of Milky Way–like satellites. We also find that the slope of the stellar mass density radial profile of the ICL of this cluster (α 3D = −2.47 ± 0.13) closely follows the predicted dark matter halo slope (α 3D,DM = −2.6 to −2), supporting the idea that both components have a similar shape and thus the potential of using the ICL as a tracer of the dark matter distribution in clusters of galaxies. Future JWST studies of the ICL are set to revolutionize our understanding of cluster formation and will be crucial to improve the gravitational lensing mass maps of these structures and thus accurately characterize the properties of the first galaxies.

Shapes and Centroids of 39 Strong Lensing Galaxy Clusters from the Sloan Giant Arcs Survey

AbstractStrong lensing galaxy clusters provide a powerful observational test of Cold Dark Matter (CDM) structure predictions derived from simulation. Specifically, the shape and relative alignments of the dark matter halo, stars, and hot intracluster gas tells us the extent to which theoretical structure predictions hold for clusters in various dynamical states. We measure the position angles, ellipticities, and locations/centroids of the brightest cluster galaxy (BCG), intracluster light (ICL), the hot intracluster medium (ICM), and the core lensing mass for a sample of strong lensing galaxy clusters from the SDSS Giant Arcs Survey (SGAS). We use iterative elliptical isophote fitting methods and GALFIT modeling on HST WFC3/IR imaging data to extract ICL and BCG information and use CIAO’s Sherpa modeling on Chandra ACIS-I X-ray data to make measurements of the ICM. Using this multicomponent approach, we attempt to constrain the physical state of these strong lensing clusters and evaluate the different observable components in terms of their ability to trace out the gravitational potential of the cluster.

Detection of Intracluster Globular Clusters in the First JWST Images of the Gravitational Lens Cluster SMACS J0723.3–7327 at z = 0.39

We present a survey of globular clusters (GCs) in the massive gravitational lens cluster SMACS J0723.3–7327 at z = 0.39 based on the early released JWST/NIRCam images. In the color–magnitude diagrams of the point sources, we clearly find a rich population of intracluster GCs that are spread over a wide area of the cluster. Their ages, considering the cluster redshift, are younger than 9.5 Gyr. The F200W (AB) magnitudes of these GCs, 26.5 mag < F200W0 < 29.5 mag, correspond to −15.2 mag < M F200W < −12.2 mag, showing that they belong to the brightest GCs (including ultracompact dwarfs). The spatial distributions of these GCs show a megaparsec-scale structure elongated along the major axis of the brightest cluster galaxy. In addition, they show a large number of substructures, some of which are consistent with the substructures seen in the map of diffuse intracluster light. The GC number density map is, in general, consistent with the dark matter mass density map based on the strong lensing analysis in the literature. The radial number density profile of the GCs in the outer region is steeper than the dark matter mass profile obtained from lensing models. These results are consistent with those for the GCs found in the deep HST images of A2744, another massive cluster at z = 0.308, and in simulated galaxy clusters. This shows that the intracluster GCs are an excellent independent tool to probe the dark matter distribution in galaxy clusters, as well as reveal the cluster assembly history in the JWST era.

Physical properties of more than one thousand brightest cluster galaxies detected in the Canada-France-Hawaii Telescope Legacy Survey

Context. Brightest cluster galaxies (BCGs) are very massive elliptical galaxies found at the centres of clusters. Their study gives clues to the formation and evolution of the clusters in which they are embedded. Aims. We analyse here in a homogeneous way the properties of a sample of more than 1000 BCGs in the redshift range 0.15 &lt; z &lt; 0.7, based on images from the Canada-France-Hawaii Telescope Legacy Survey. Methods. Based on a recent catalogue of 1371 clusters, we applied our automatic BCG detection algorithm and successfully identified 70% of the BCGs in our sample. We analysed their 2D photometric properties with GALFIT. We also compared the position angles of the BCG major axes with those of the overall cluster to which they belong. Results. We find no evolution of the BCG properties with redshift up to z = 0.7, in agreement with previous results by Chu et al. (2021, A&amp;A, 649, A42), who analysed a sample an order of magnitude smaller, but reaching a redshift z = 1.8. The Kormendy relation for BCGs is tight and consistent with that of normal elliptical galaxies and BCGs measured by other authors. The position angles of the BCGs and of the cluster to which they belong agree within 30 degrees for 55% of the objects with well-defined position angles. Conclusions. The study of this very large sample of more than 1000 BCGs shows that they were mainly formed before z = 0.7 as we find no significant growth for the luminosities and sizes of central galaxies. We discuss the importance of the intracluster light in the interpretation of these results. We highlight the role of image depth in the modelling of the luminosity profiles of BCGs, and give evidence of the presence of an inner structure which can only be resolved on deep surveys with limiting apparent magnitude at 80% completeness m80 &gt; 26 mag arcsec−2.

Unscrambling the Lensed Galaxies in JWST Images behind SMACS 0723

The first deep field images from the James Webb Space Telescope (JWST) of the galaxy cluster SMACS J0723.3-7327 reveal a wealth of new lensed images at uncharted infrared wavelengths, with unprecedented depth and resolution. Here we securely identify 14 new sets of multiply imaged galaxies totaling 42 images, adding to the five sets of bright and multiply imaged galaxies already known from Hubble Space Telescope data. We find examples of arcs crossing critical curves, allowing detailed community follow-up, such as JWST spectroscopy for precise redshift determinations, and measurements of the chemical abundances and of the detailed internal gas dynamics of very distant, young galaxies. One such arc contains a pair of compact knots that are magnified by a factor of hundreds, and features a microlensed transient. We also detect an Einstein cross candidate only visible thanks to JWST’s superb resolution. Our parametric lens model is available through the following link (https://www.dropbox.com/sh/gwup2lvks0jsqe5/AAC2RRSKce0aX-lIFCc9vhBXa?dl=0) and will be regularly updated using additional spectroscopic redshifts. The model is constrained by 16 of these sets of multiply imaged galaxies, three of which have spectroscopic redshifts, and reproduces the multiple images to better than an rms of 0.″5, allowing for accurate magnification estimates of high-redshift galaxies. The intracluster light extends beyond the cluster members, exhibiting large-scale features that suggest a significant past dynamical disturbance. This work represents a first taste of the enhanced power JWST will have for lensing-related science.

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.