On the Origin of Intracluster Light based on the High-resolution Simulation, NewCluster

TL;DR

Using the high-resolution NewCluster simulation, the paper investigates the origin and stellar-population properties of intracluster light (ICL) in a Virgo-like cluster by tracking billions of stellar particles and constructing a robust merger-tree to resolve satellite histories. It partitions the BCG+ICL into four origins—stripped from surviving satellites, stripped from disrupted satellites, in-situ, and preprocessing—and finds that satellites dominate the ICL, with preprocessing forming a notable, dark-matter–like component composed of old, metal-poor, alpha-enhanced stars. The preprocessing component, along with centrally concentrated in-situ stars and extended stripped components, yields distinct density and chemical profiles; the stripping efficiency is principally set by time in the cluster and the depth of pericenter encounters, with orbital shape playing a smaller role. While offering a powerful framework to link ICL demographics with cluster assembly, the study notes limitations from a single-cluster sample and the challenges of comparing to observations, underscoring the need for mock observations and multi-cluster analyses in future work.

Abstract

Intracluster light (ICL) is a key component of galaxy clusters, with the potential to trace their dynamical assembly histories and the underlying dark matter distribution. Despite these prospects, its faint nature makes a consensus on its origin or population properties difficult to achieve, both in observations and simulations. In the hope of finding a breakthrough, we utilize the ongoing high-resolution cluster simulation, NewCluster. By classifying billions of particles in and around the cluster with a rigorous tracking procedure, we find that the majority of the ICL originates from satellites, including surviving and disrupted galaxies. Another notable finding is that the preprocessed component follows the density profile of dark matter better than the other components and has distinctive properties: old age, low metallicity, and enhanced $α$-element abundance. We further investigate the orbital dynamics, and our results demonstrate that the stripped fraction of satellites is primarily determined by the time since infall and the pericenter distance. By linking the demographic, chemical, and orbital properties of ICL stars to their origins, this work proposes a quantitative approach for tracing the assembly history of galaxy clusters from the ICL.

Figures (12)

• Figure 1: Overview of the NewCluster simulation centered on the primary cluster halo at $z=0.79$. Panel (a) presents a dark matter density map (grayscale), with the virial radii of the two cluster halos (magenta and cyan circles). The dotted line shows the virial radius (${R_{\rm vir}}$) from AdaptaHOP, and the dashed line is the ${R_{\rm 200}}$, which are described in Section \ref{['sec:2method_2halo']}. Panel (b) shows the corresponding $r$-band surface brightness map derived from the stellar density. Note that faint but complex tidal features are resolved in the surface brightness map.
• Figure 2: Schematic diagram of the merger tree. The spiral icons indicate all leaf galaxies connected to the target galaxy (z) across different snapshots, marked with alphabetical IDs. Each line represents a connected branch based on the score (described in the text). The main branch (black) has the same IDs for FIRST and HEAD, indicating a non-fragmented branch. Likewise, the same IDs for TAIL and FINAL mean that this branch is not merged into any other branch.
• Figure 3: Orbital tracks of galaxies derived from the merger tree in the NewCluster cluster. Panel (a) shows the comoving-scale tracks in the cosmological box, while panel (b) shows the physical-scale tracks centered on the main cluster at $z=0.79$ overlaid on a stellar density map (grayscale). The black dashed circles mark the ${R_{\rm 200}}$ of the main cluster halo. The red line in panel (a) traces the BCG, and the red marker in panel (b) indicates its final position. The green lines show satellites that have completely merged into the BCG. The blue circles indicate $R_{\rm 90}$, the radius enclosing 90% of its stellar mass, and the blue lines show orbits of surviving satellites. Unlike panel (a), we only present the tracks of surviving satellites more massive than $10^9\,\rm{M_{\odot}}$ in panel (b), for clarity.
• Figure 4: Illustration of the definition of the ICL within $1.5\times{R_{\rm 200}}$. Panel (a) shows all stars bound to satellite galaxies at $z=0.79$, while panel (b) presents the density map of the final BCG+ICL component, after excluding the satellite stars shown in panel (a). As described in the main text, the tight (red) and loose (blue) member stars are identified and follow the motion of their host satellite galaxy.
• Figure 5: Top (a)-(g): example of the infall member stars carried by a target surviving satellite galaxy. Panel (a) shows the overall projected smoothed trajectory (green) of the target satellite. Each position is normalized by the ${R_{\rm 200}}$ of the main cluster progenitors. The purple line highlights the time interval ($500\,\mathrm{Myr}$ before infall) used to identify member stars. Panels (b)-(g) present zoom-in views of the target satellite at each stage, showing the tight (red) and loose (blue) member stars along with other stars (grayscale). We also indicate the direction of the BCG with arrows and markers. Bottom (h)-(n): example of infall stars associated with a target disrupted galaxy. Similar to the top panels, panel (h) shows the overall projected trajectory, and the other panels present zoom-in views. Since the target does not survive to the final snapshot, we include $t_{\rm disrupted}$ (red), the epoch when the satellite is last detected.