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

TL;DR

This work addresses how intracluster light (ICL) traces the cluster dark matter (DM) halo by examining differences in phase-space between stripped stars and DM from infalling satellites. Using controlled N-body simulations with varying satellite mass ratios and orbital circularities, the authors develop a coregionalized Gaussian-process model to predict the orbital energy and angular momentum of stripped material for individual satellites and then convolve these results over plausible infalling satellite populations described by Schechter mass functions and beta-distributed circularities. They find that stripped stars consistently reside at lower energies and angular momenta than stripped DM, with the magnitude of the offset governed primarily by the characteristic satellite mass scale and the timing of stripping; metallicity and radial profiles of the ICL steepen with increasing satellite mass. When the model is conditioned on satellite populations from four cosmological simulations, it reproduces the observed stellar–DM radial density offsets within inter-simulation scatter, indicating that satellite demographics largely drive the ICL–DM relationship. The study implies that robust inference of cluster DM properties from ICL requires constraints on the infalling satellite population, but that ICL remains a powerful tracer for the radial DM distribution when such demographics are accounted for.

Abstract

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 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. We measure the specific orbital energy and angular momentum of stripped stellar and DM material, finding that the stripped stars consistently occupy lower-energy and lower-angular momentum regions of phase-space than the stripped DM. The magnitude of this difference increases strongly towards more equal satellite--to--host mass ratios, while the dependence on orbital circularity is weak. We construct a predictive model for the phase-space properties of stripped stars and DM from a whole infalling satellite population and find that the resulting phase-space difference between the components are driven primarily by the characteristic mass of the infalling satellite stellar mass function. We find that the ICL is always more centrally concentrated than the DM. The magnitude of this offset depends on the characteristic mass and increases towards higher characteristic masses. Comparisons with four independent cosmological hydrodynamical simulations show that, once the infalling satellite stellar mass function is matched, the model reproduces the radial stellar-to-DM density profile offsets to better than the inter-simulation scatter. This demonstrates that the radial relationship between the ICL and the DM distribution 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.

Figures (17)

• Figure 1: Scaling relations used to assign structural properties to the cluster and satellite models. Panels show: (a) the infall orbital circularity distribution ($\eta$) adopted from Wetzel2011, (b) the cumulative contribution of satellites to the ICL as a function of satellite stellar mass from Brown2024, (c) the stellar mass--size relation from Mowla2019, (d) the stellar--to--halo mass relation from Moster2013, and (e) the halo concentration--mass relation from Prada2012. Relations are shown at both $z\sim1$ and $z\sim0$ to illustrate redshift evolution. Satellite properties are assigned using the $z=1$ relations, while the cluster is modelled at its formation redshift $z=0.6$, leading to a small offset relative to the plotted scaling relations.
• Figure 2: Final projected distributions of stripped stars (green) and DM (purple) for a grid of satellite mass ratios $M_{\rm host} / M_{\rm host}$ and orbital circularities $\eta$. Each panel spans $2\times2$ Mpc. Columns vary orbital circularity; rows vary mass ratio. An interactive version of these plots showing the distribution for all combinations of mass ratio and orbital circularity can be found here: https://garrethmartin.github.io/interactive-profiles-ICL/index.html#energy-am.
• Figure 3: The cluster-centric specific orbital energy evolution of different mass ratio satellites. Thin lines show individual orbits, terminated by a cross where the satellite is completely destroyed, and thick lines show the weighted average orbital energy over all orbital configurations. For the purpose of averaging the profiles, all satellites are included, and for those that are disrupted, the last available orbital energy is assumed to persist. The time when each satellite reaches its first pericentric passage is marked by a filled circle.
• Figure 4: Contour plots showing the final distributions of specific orbital energy ($\varepsilon$) and angular momentum ($h$) for stars (green) and DM (purple) stripped from the satellite by the end of the simulation. Marginal distributions in $\varepsilon$ and $h$ are shown along the horizontal and vertical axes, respectively. Mean values and $1\sigma$ dispersions are indicated for stars (green) and DM (purple) error bars. The coloured track traces the evolution of the mean $(\varepsilon,h)$ of stripped particles, from earlier (blue) to later (red) stripping times. An interactive version of these plots showing the distribution for all combinations of mass ratio and orbital circularity can be found here: https://garrethmartin.github.io/interactive-profiles-ICL/index.html#energy-am.
• Figure 5: Top: ratio of the mean specific orbital energy of stripped stars to that of stripped DM as a function of satellite--to--host mass ratio. Bottom: equivalent ratio for specific orbital angular momentum. Error bars show the $1\sigma$ dispersion across values of orbital circularity, weighted by the circularity probability distribution. Fainter square markers and error bars indicate the result when particles within 100 kpc of the cluster central galaxy are excluded. Solid and dashed black line indicate best-fit to $y(x)=A+B\exp(-kx)$ for all stripped particles and stripped particles at $r>100$ kpc respectively with shaded grey regions showing the 16--84th percentile envelope of the posterior. Reported parameter values are the maximum a posteriori estimates, with uncertainties equal to half the 16--84th percentile credible interval.