The origin of the intra-cluster light in The Three Hundred simulations

Abstract

We investigate the origin and formation mechanisms of the intra-cluster light (ICL) in The Three Hundred simulations, a set of 324 hydrodynamically resimulated massive galaxy clusters. The ICL, a diffuse component of stars not bound to individual galaxies, serves as a key tracer of cluster formation and evolution. Using two hydrodynamics implementations, Gadget-X and Gizmo-Simba, we identify the stellar particles comprising the ICL at z=0 and trace them back in time to the moments when they were formed and accreted into the ICL. Across our 324 clusters, half of the present-day ICL mass is typically in place between $z \sim $ 0.2 and 0.5. The main ICL formation channel is the stripping of stars from subhalos after their infall into the host cluster. Within this channel, 65-80 per cent of the ICL comes from objects with stellar (infall) masses above $10^{11}$ M$_\odot$, i.e., massive galaxies, groups and clusters. Considering the ratio of the infalling halo to the total cluster mass, a median of 35 per cent of the mass is brought in major merger events, though this percentage varies significantly across clusters (15-55 per cent). Additional contributions come from minor mergers (25-35 per cent) and smooth accretion (20-50 per cent). The primary contributors typically infall at $z \leq 1$, with smaller fractions arriving at redshifts between 1 and 2. Regarding other formation channels, we find minor contributions from stars formed in subhalos after their infall and stars stripped while their contributing halo remains outside the host cluster (and can eventually fall inside or stay outside). Finally, for our two sets of simulations, we find medians of 12 (Gadget-X) and 2 (Gizmo-Simba) per cent of the ICL mass formed in-situ, that is, directly as part of the diffuse component. However, this component can be attributed to stripping of gas in high-velocity infalling satellite galaxies.

Figures (9)

• Figure 1: Top, ICL formation (solid lines) and assembly (dash-dotted lines) histories of the 324 clusters for the two different simulation runs, Gadget-X (green) and Gizmo-Simba (magenta). The lines show the median values, with the masses normalized to the total ICL mass at $z=0$, while the shaded regions correspond to the 16th to 84th percentile ranges. Bottom, ICL assembly history separating the clusters into four bins according to their formation time. Bins are selected to have approximately the same number of clusters in each bin.
• Figure 2: Distribution of the fraction of in-situ ICL mass for the 324 clusters in the Gadget-X (solid green line) and Gizmo-Simba (dash-dotted magenta line) runs.
• Figure 3: Left, distribution of distance from the in-situ ICL particles to the main branch progenitor centre at the formation time of the particle, in units of the main branch halo radius $R_{200}$. Green is for Gadget-X, magenta for Gizmo-Simba. Right, distance from the in-situ particles to the centre of the closest halo (or subhalo), in units of the radius of that halo, $R_\mathrm{subhalo}$. To ease comparison, we still show the distance to the main branch with the corresponding solid lines. Lines show the median computed stacking the 324 clusters, shaded regions are the 16th-84th percentiles.
• Figure 4: Classification of the ICL star particles according to the way they were assembled into the ICL. Stellar particles are first classified into in-situ and ex-situ following Fig. \ref{['fig:insitu-fraction']}, and then ex-situ particles are in turn classified into three different categories. On the left, schematic representation of the different channels, with the main branch depicted in red and a secondary branch in blue. On the right, mean value of the contribution from each channel for the 324 clusters in The Three Hundred sample. Top panel is for Gadget-X, bottom panel for Gizmo-Simba.
• Figure 5: Cumulative distribution of the mass contribution to the post-stripped ICL from halos of different infall masses. Left column is for stellar mass at infall, right column for the total mass ratio to the host at infall time. Lines depict the distributions obtained as the median of the individual distributions for each cluster, while the shaded regions indicate the 16th-84th percentiles. Green solid lines are for Gadget-X and magenta dash-dotted lines for Gizmo-Simba. In the right column, yellow and brown shaded regions indicate the minor (1:10 to 1:3) and major merger regimes (1:3 to 1:1). Horizontal dotted lines indicate fractions of 0.25, 0.5 and 0.75 of the post-stripped ICL.