Intracluster light as a dark matter tracer: how their spatial and kinematic relationship is shaped by satellite demographics

We investigate how the orbital evolution and mass distribution of infalling satellite galaxies shape the phase-space and radial distributions of intracluster light (ICL) relative to the underlying cluster dark matter (DM) halo. Using controlled, self-consistent N-body simulations, we follow the tidal stripping and orbital evolution of satellite galaxies as they are accreted into a live cluster halo, systematically varying satellite–to–host mass ratio and orbital circularity. From these experiments, we measure the specific orbital energy and angular momentum of stripped stellar and DM material, finding that stripped stars consistently occupy lower-energy and lower-angular momentum regions of phase-space than stripped DM. The magnitude of this difference increases strongly towards more equal satellite–to–host mass ratios, while dependence on circularity is weak. We construct a predictive model for the phase-space properties of stripped stars and DM from a given infalling satellite population and find that phase-space differences are driven primarily by the characteristic mass of the satellite stellar mass function. The ICL is always more centrally concentrated than the DM, with offset magnitude increasing towards higher characteristic masses. Comparisons with four cosmological hydrodynamical simulations show that, once the satellite stellar mass function is matched, the model reproduces the radial stellar-to-DM density profile offsets to better than inter-simulation scatter. This demonstrates that the radial ICL-DM relationship is largely governed by satellite demographics. With adequate constraints on the infalling satellite population, ICL density profiles can therefore be used as informative tracers of the underlying radial DM distribution in clusters.

We present a detailed study of the shapes and alignments of different galaxy cluster components using hydrodynamical simulations. We compute shape parameters from the dark matter (DM) distribution, the galaxy members and the intracluster light (ICL). We assess how well the DM cluster shape can be constrained by means of the identified galaxy member positions and the ICL. Further, we address the dilution factor introduced when estimating the cluster elongation using weak-lensing stacking techniques, which arises due to the misalignment between the total surface mass distribution and the distribution of luminous tracers. The dilution is computed considering the alignment between the DM and the brightest cluster galaxy, the galaxy members and the ICL. Our study shows that distributions of galaxy members and ICL are less spherical than the DM component, although both are well aligned with the semimajor axis of the latter. We find that the distribution of galaxy members hosted in more concentrated subhaloes is more elongated than the distribution of the DM. Moreover, these galaxies are better aligned with the DM component compared to the distribution of galaxies hosted in less concentrated subhaloes. We conclude that the positions of galaxy members can be used as suitable tracers to estimate the cluster surface density orientation, even when a low number of members is considered. Our results provide useful information for interpreting the constraints on the shapes of galaxy clusters in observational studies.

We use a set of simulation-based models for the dissipationless evolution of galaxies since z = 1 to constrain the fate of accreted satellites embedded in dark matter subhalos. These models assign stellar mass to dark matter halos at z = 1 by relating the observed galaxy stellar mass function (GSMF) to the halo + subhalo mass function monotonically. The evolution of the stellar mass content is then followed using halo merger trees extracted from N-body simulations. Our models are differentiated only in the fate assigned to satellite galaxies once subhalos, within which satellites are embedded, disrupt. These models are confronted with the observed evolution in the massive end of the GSMF, the z ~ 0 brightest cluster galaxy (BCG) to cluster mass relation, and the combined BCG and intracluster light (ICL) luminosity distribution—all observables expected to evolve approximately dissipationlessly since z = 1. The combined observational constraints favor a model in which the vast majority (≳80%) of satellite stars from disrupted subhalos go into the ICL (operationally defined here as light below a surface brightness cut of μi ≈ 23 mag arcsec-2). Conversely, models that leave behind a significant population of satellite galaxies once the subhalo has disrupted are strongly disfavored, as are models that put a significant fraction of satellite stars into the BCG. Our results show that observations of the ICL provide useful and unique constraints on models of galaxy merging and the dissipationless evolution of galaxies in groups and clusters.

The halo occupation distribution (HOD) describes the bias between galaxies and dark matter by specifying (a) the probability P(N|M) that a halo of virial mass M contains N galaxies of a particular class and (b) the relative distributions of galaxies and dark matter within halos. We calculate predicted HODs for a Lambda-CDM cosmological model using an SPH hydrodynamic simulation and a semi-analytic (SA) galaxy formation model. Although the two methods predict different galaxy mass functions, their HOD predictions agree remarkably well. For mass-selected samples, the mean occupation <N(M)> exhibits a sharp cutoff at low halo masses, a slowly rising plateau for <N>~1-2, and a more steeply rising high occupancy regime. At low <N>, the mean pair and triple counts are well below Poisson expectations, with important consequences for small scale behavior of 2- and 3-point correlation functions. The HOD depends strongly on galaxy age, with high mass halos populated mainly by old galaxies and low mass halos by young galaxies. The SPH simulation supports several simplifying assumptions about HOD bias: the most massive galaxy in a halo usually lies close to the center and moves near the halo's mean velocity; satellite galaxies have the same radial profile and velocity dispersion as the dark matter; and the mean occupation at fixed halo mass is independent of the halo's larger scale environment. By applying the SPH and SA HODs to a large volume N-body simulation, we show that both methods predict slight, observable departures from a power-law galaxy correlation function. The predicted HODs are closely tied to the underlying galaxy formation physics, they offer useful guidance to theoretical models of galaxy clustering, and they will be tested empirically by ongoing analyses of galaxy redshift surveys. (Shortened)

Using a cosmological $N$-body simulation, we investigate the origin and distribution of stars in the intracluster light (ICL) of a Fornax-like cluster. In a dark matter only simulation we identify a halo which, at $z=0$, has $M_200 \simeq 4.1 \times 10^{13}M_{sun}$ and $r_{200} = 700kpc$, and replace infalling subhalos with models that include spheroid and disc components. As they fall into the cluster, the stars in some of these galaxies are stripped from their hosts, and form the ICL. We consider the separate contributions to the ICL from stars which originate in the haloes and the discs of the galaxies. We find that disc ICL stars are more centrally concentrated than halo ICL stars. The majority of the disc ICL stars are associated with one initially disc-dominated galaxy that falls to the centre of the cluster and is heavily disrupted, producing part of the cD galaxy. At radial distances greater than 200kpc, well beyond the stellar envelope of the cD galaxy, stars formerly from the stellar haloes of galaxies dominate the ICL. Therefore at large distances, the ICL population is dominated by older stars.

Cosmological N-body simulations represent an excellent tool to study the formation and evolution of dark matter (DM) haloes and the mechanisms that have originated the universal profile at the largest mass scales in the Universe. In particular, the combination of the velocity dispersion σv with the density ρ can be used to define the pseudo-entropy $S(r)=\sigma _\mathrm{v}^2/\rho ^{\, 2/3}$, whose profile is well described by a simple power law $S\propto \, r^{\, \alpha }$. We analyse a set of cosmological hydrodynamical re-simulations of massive galaxy clusters and study the pseudo-entropy profiles as traced by different collisionless components in simulated galaxy clusters: DM, stars, and substructures. We analyse four sets of simulations, exploring different resolution and physics (N-body and full hydrodynamical simulations) to investigate convergence and the impact of baryons. We find that baryons significantly affect the inner region of pseudo-entropy profiles as traced by substructures, while DM particles profiles are characterized by an almost universal behaviour, thus suggesting that the level of pseudo-entropy could represent a potential low-scatter mass-proxy. We compare observed and simulated pseudo-entropy profiles and find good agreement in both normalization and slope. We demonstrate, however, that the method used to derive observed pseudo-entropy profiles could introduce biases and underestimate the impact of mergers. Finally, we investigate the pseudo-entropy traced by the stars focusing our interest in the dynamical distinction between intracluster light and the stars bound to the brightest cluster galaxy: the combination of these two pseudo-entropy profiles is well described by a single power law out to almost the entire cluster virial radius.

Strong lensing studies can provide detailed mass maps of the inner regions even in dynamically active galaxy clusters. Proper modelling of the intracluster medium can play an important role. We discuss a parametric method which exploits X-ray and optical observations. The mass model accounts separately for cluster-sized dark matter halos, galaxies and the intracluster medium. This gives an unbiased look at each matter component. The method has been applied to AC 114, an irregular X-ray cluster. The dark matter distribution is shifted and rotated with respect to the gas but follows the galaxy density, which hints at its collisionless nature. Despite the dynamical activity, the matter distribution seems to be in agreement with predictions from N-body simulations. A universal cusped profile provides a good description of either the overall or the dark matter distribution and the cluster is not affected by the lensing over-concentration bias, as further stressed by an analysis of the triaxial structure.

Using N-body simulations, we have modeled the production and evolution of substructures in the intracluster light (ICL) of a simulated galaxy cluster. We use a density-based definition of ICL, where ICL consists of luminous particles which are at low densities, to identify ICL particles and track their evolution. We have implemented a friends-of-friends-type clustering algorithm which finds groups of particles correlated in both position and velocity space to identify substructures in the ICL, hereafter referred to as ``streams''. We find that ~40% of the cluster's ICL is generated in the form of these massive (M > 7.0x10^8 Msun), dynamically cold streams. The fraction of the ICL generated in streams is greater early in the cluster's evolution, when galaxies are interacting in the group environment, than later in its evolution when the massive cluster potential has been assembled. The production of streams requires the strong tidal fields associated with close interactions between pairs of galaxies, and is usually associated with merging pairs of galaxies, or fast, close encounters with the cluster's central galaxy. Once streams are formed, they begin to decay as they are disrupted by the tidal field of the cluster. We find that streams have decay timescales which are ~1.5 times their dynamical time in the cluster.

The dark matter distribution in the very inner region of our Galaxy is still\nin debate. In the N-body simulations a cuspy dark matter halo density profile\nis favored. Several dissipative baryonic processes however are found to be able\nto significantly flatten dark matter distribution and a cored dark matter halo\ndensity profile is possible. The baryons dominate the gravitational potential\nin the inner Galaxy, hence a direct constrain on the abundance of the dark\nmatter particles is rather challenging. Recently, a few groups have identified\na tentative 130 GeV line signal in the Galactic center, which could be\ninterpreted as the signal of the dark matter annihilation. With current 130 GeV\nline data and adopting the generalized Navarro-Frenk-White profile of the dark\nmatter halo, for local dark matter density \\rho_0=0.4 GeV cm^{-3} and r_s=20\nkpc we obtain a 95% confidence level lower (upper) limit on the inner slope of\ndark matter density distribution \\alpha = 1.06 (the cross section of dark\nmatter annihilation into gamma-rays &lt;\\sigma v &gt;_{\\chi\\chi -&gt; \\gamma\\gamma}=\n1.3\\times 10^{-27} cm^3 s^{-1}). Such a slope is consistent with the results of\nsome N-body simulations, and if the signal is due to dark matter, suggests that\nbaryonic processes may be unimportant.\n

The growth and virialization of spherical top-hat fluctuations, in coupled dark energy models, causes segregation between dark matter (DM) and baryons, as the gravitational infall into the potential well proceeds more slowly for the baryons than for DM. As a consequence, after attaining their turnaround and before full virialization, halos have outer layers rich of baryons. Accordingly, a natural ambiguity exists on the definition of the virial density contrast. In fact, when the outer baryon layers infall onto the DM-richer core, they carry with them DM materials outside the original fluctuation; hence, no time exists when all materials originally belonging to the fluctuation--and only they--have virialized. Baryon-DM segregation can have various astrophysical consequences on different length scales. The smallest halos may loose up to 50% of the original baryonic contents and become hardly visible. Subhalos in cluster-size halos may loose much baryonic materials, which could then be observed as intracluster light. Isolated halos, in general, can be expected to have a baryon component richer than the cosmological proportions, due to the cosmic enrichement of baryons lost in small halo encounters.

The integration of Machine Learning (ML) and mathematical modeling has emerged as a transformative approach in advancing dark matter research, a domain that remains one of the most profound mysteries in modern astrophysics. Traditional analytical and simulation-based methods often face limitations due to the vast complexity, high dimensionality, and scarcity of observable data associated with dark matter phenomena. In this context, ML techniques such as deep learning, neural networks, and probabilistic models provide powerful tools for pattern recognition, anomaly detection, and predictive analysis in large astrophysical datasets. Mathematical modeling, on the other hand, offers a rigorous theoretical framework to describe the behavior, distribution, and interaction of dark matter through equations derived from gravitational dynamics, particle physics, and cosmological principles. The integration of these two approaches enables more accurate simulations of cosmic structures, improved detection of weak signals in observational data, and enhanced interpretation of experimental results from telescopes and particle detectors. Furthermore, ML algorithms can optimize parameter estimation in complex mathematical models, reducing computational costs and increasing efficiency. This interdisciplinary synergy not only accelerates the discovery process but also refines theoretical predictions, bridging the gap between empirical observations and fundamental physics. Consequently, the combined application of machine learning and mathematical modeling holds significant potential to unravel the nature, properties, and distribution of dark matter, thereby contributing to a deeper understanding of the universe's composition and evolution.

Dark matter plays a fundamental role in theories of formation and evolution of galaxies, so every attempt to model galaxy formation and evolution has to consider the presence of dark halos. Moreover, mergers and accretions appear to be fundamental driving mechanisms in determining the present day properties of galaxies. Our project studies the ultra‐fine distribution of dark matter in the Milky Way and the implications for the next generation of dark matter particle detectors. We have developed a model for halo mergers in a Milky Way‐like galaxy. The signals expected in the lab‐based dark matter detection experiments depend on the phase‐space distribution on submilliparsec scales: with our numerical technique it is possible to resolve structures, produced by mergers of subhalos with a larger parent halo, inaccessible to conventional N‐body simulations. When applied in a cosmological context, this method becomes a powerful instrument to reproduce and analyse the complete multiple merger history of a Milky Way‐like system. Preliminary results suggest that fine‐grained features from mergers that occurred a few Gyrs ago survive until now. These features may raise also the possibility of using them to unravel the complete merger history of the Milky Way.

Contemporarily, dark matter exerts dominant impacts on galaxy formation, which is the spine bone of the galaxy. Herein, we investigate the attraction effect of initial dark matter distribution on the normal matter (hydrogen and other gases), which will form the dark matter halos. Subsequently, an introduction to the properties of dark matter halos and some disputes about it are demonstrated. Besides, the cold dark matter (CDM) is chosen to fit the current observation and interpret the influences of dark matter on galaxy formation. Overall, this paper offers a clear procession of the impact of dark matter on galaxy formation, which will be guideline for future development of both galaxy formation theory as well as dark matter detection.

Observations of the low-mass satellites in the Local Group have shown high fractions of gas-poor, quiescent galaxies relative to isolated dwarfs, implying that the host halo environment plays an important role in the quenching of dwarf galaxies. In this work, we present measurements of the quenched fractions and quenching timescales of dwarf satellite galaxies in the DC Justice League suite of four high-resolution cosmological zoom-in simulations of Milky Way–mass halos. We show that these simulations accurately reproduce the satellite luminosity functions of observed nearby galaxies, as well as the variation in satellite quenched fractions from M * ∼ 105 M ⊙ to 1010 M ⊙. We then trace the histories of satellite galaxies back to z ∼ 15 and find that many satellites with M * ∼ 106−108 M ⊙ quench within ∼2 Gyr of infall into the host halo, while others in the same mass range remain star-forming for as long as 5 Gyr. We show that this scatter can be explained by the satellite’s gas mass and the ram pressure it feels at infall. Finally, we identify a characteristic stellar mass scale of 108 M ⊙ above which infalling satellites are largely resistant to rapid environmental quenching.

The search for a particle dark matter signal in terms of radiation produced by dark matter annihilation or decay has to cope with the extreme faintness of the predicted signal and the presence of masking astrophysical backgrounds. It has been shown that using the correlated information between the dark matter distribution in the Universe with the fluctuations of the cosmic radiation fields has the potential to allow setting apart a pure dark matter signal from astrophysical emissions, since spatial fluctuations in the radiation field due to astrophysical sources and dark matter emission have different features. The cross-correlation technique has been proposed and adopted for dark matter studies by looking at dark matter halos (overdensities). In this paper we extend the technique by focusing on the information on dark matter distribution offered by cosmic voids, and by looking specifically at the gamma-ray dark matter emission: we show that, while being underdense and therefore producing a reduced emission as compared to halos, nevertheless in voids the relative size of the cross-correlation signal due to decaying dark matter vs. astrophysical sources is significantly more favourable, producing signal-to-background ratios S/B (even significantly) larger than 1 for decay lifetimes up to 2 × 1030 s. This is at variancewith the case of halos, where S/B is typically (even much) smaller than 1. We show that forthcoming galaxy surveys such as Euclid combined with future generation gamma-ray detectors with improved specifications have the ability to provide a hint of such a signal with a predicted significance up to 5.7σ for galaxies and 1.6σ for the cosmic shear. The bound on the dark matter lifetime attainable exploiting voids is predicted to improve on current bounds in a mass range for the WIMP of 25 ÷ 900 GeV.