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

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 haloes of arbitrary masses. We explore the IHSC mass fraction - total halo's stellar mass, f(M*,IHSC) - M-*, relation, and the physical drivers of its scatter. We find that on average, the f(M*,IHSC) increases with total stellar mass, with the scatter decreasing strongly with mass from 2 dex at M-*,M-tot similar or equal to 10(11)( )M(circle dot) to 0.3 dex at group masses. At high masses, M-*,M-tot > 10(11.5) M-circle dot, f(M*,IHSC) increases with the number of substructures, and with the mass ratio between the central galaxy and largest satellite, at fixed M-*,M-tot. From mid-size groups and systems below M-*,M-tot < 10(12) M-circle dot, we find that the central galaxy's stellar rotation-to-dispersion velocity ratio, V/sigma, displays the strongest (anti)-correlation with f(M*,IHSC )at fixed M-*,(tot) of all the galaxy and halo properties explored, transitioning from f(M*,IHSC) < 0.1 per cent for high V/sigma, to f(M*,IHSC) approximate to 5 per cent for low V/sigma galaxies. By studying the f(M*,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*,IHSC) population displays a much more active merger history. In the case of massive groups and galaxy clusters, M-*,M-tot greater than or similar to 10(12) M-circle dot, a fraction f(M*,IHSC) approximate to 10 - 20 per cent is reached at z approximate to 1 and then they evolve across lines of constant f(M*,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.