From Stellar Halos to Intracluster Light: the physics of the Intra-Halo Stellar Component in cosmological hydrodynamical simulations

Abstract

We study the Intra-Halo Stellar Component (IHSC) of Milky Way-mass systems up to galaxy clusters in the Horizon-AGN cosmological hydrodynamical simulation. We identify the IHSC using an improved phase-space galaxy finder algorithm which provides an adaptive, physically motivated and shape-independent definition of this stellar component, that can be applied to halos of arbitrary masses. We explore the IHSC mass fraction-total halo's stellar mass, $f_{M\ast ,IHSC}-M\ast$, relation and the physical drivers of its scatter. We find that on average the $f_{M\ast ,IHSC}$ increases with $M_{\ast ,tot}$, with the scatter decreasing strongly with mass from 2 dex at $M_{\ast ,tot}\sim 10^{11}{M}_{\odot }$ to 0.3 dex at group masses. At high masses, $M_{\ast ,tot}>10^{11.5}{M}_{\odot }$, $f_{M\ast ,IHSC}$ increases with the number of substructures, and with the mass ratio between the central galaxy and largest satellite, at fixed $M_{\ast ,tot}$. From mid-size groups and systems below $M_{\ast ,tot}<10^{12}{M}_{\odot }$, we find that the central galaxy's stellar rotation-to-dispersion velocity ratio, V/σ, displays the strongest (anti)-correlation with $f_{M\ast ,IHSC}$ at fixed $M_{\ast ,tot}$ of all the galaxy and halo properties explored, transitioning from $f_{M\ast ,IHSC}$<0.1% for high V/σ, to $f_{M\ast ,IHSC}\sim 5$% for low V/σ galaxies. By studying the $f_{M\ast ,IHSC}$ temporal evolution, we find that, in the former, mergers not always take place, but if they did, they happened early (z>1), while the high $f_{M\ast ,IHSC}$ population displays a much more active merger history. In the case of massive groups and galaxy clusters, $M_{\ast ,tot}>10^{12}{M}_{\odot }$, a fraction $f_{M\ast ,IHSC}\sim$10-20% is reached at $z\sim 1$ and then they evolve across lines of constant $f_{M\ast ,IHSC}$ modulo some small perturbations. Because of the limited simulation's volume, the latter is only tentative and requires a larger sample of simulated galaxy clusters to confirm.

21 pages, 17 figures. Submitted to MNRAS. Comments are welcomed