The Fornax Deep Survey with VST. VIII. Connecting the accretion history with the cluster density

Context. This work is based on deep multi-band (g, r, i) data from the Fornax Deep Survey with the VLT Survey Telescope (VST). We analyse the surface brightness profiles of the 19 bright early-type galaxies (ETGs; m_B ≤ 15 mag) inside the virial radius of the Fornax cluster (R_vir ∼ 0.7 Mpc), in the mass range 8 × 10⁸ ≤ M_* ≤ 1.2 × 10¹¹ M_⊙.
Aims: The main aim of this work is to identify signatures of accretion onto galaxies by studying the presence of outer stellar haloes and to understand their nature and occurrence. Our analysis also provides a new and accurate estimate of the intra-cluster light inside the virial radius of Fornax.
Methods: We performed multi-component fits to the azimuthally averaged surface brightness profiles available for all sample galaxies. This allows us to quantify the relative weight of all components in the galaxy structure that contribute to the total light. In addition, we derived the average g - i colours in each component identified by the fit, as well as the azimuthally averaged g - i colour profiles, to correlate them with the stellar mass of each galaxy and the location inside the cluster.
Results: We find that in the most massive (10¹⁰ ≤ M ≤ 10¹¹ M_⊙) and reddest ETGs the fraction of light in, probably accreted, haloes (50%-90%) is much larger than in the other galaxies. All of these are located in the high-density region of the cluster (≤0.4R_vir ∼ 0.3 Mpc), belonging to the north-south clump (NS clump). Less massive galaxies (10⁹ ≤ M ≤ 10¹⁰ M_⊙) have an accreted mass fraction that is lower than 30%, have bluer colours, and reside in the low-density regions of the cluster. The colour profiles of the ETGs with the largest accreted mass fraction tend to flatten in the outskirts of the galaxy, that is beyond the transition radius from the central in situ to the ex situ accreted component. Inside the virial radius of the cluster (∼0.7 Mpc), the total luminosity of the intra-cluster light, compared with the total luminosity of all cluster members (bright galaxies and dwarfs), is about 34%.
Conclusions: Inside the Fornax cluster there is a clear correlation between the amount of accreted material in the stellar haloes of galaxies and the density of the environment in which those galaxies reside. By comparing this quantity with theoretical predictions and previous observational estimates, there is a clear indication that the driving factor for the accretion process is the total stellar mass of the galaxy, which agrees with the hierarchical accretion scenario. Massive galaxies in the NS clump, with the largest accreted mass fractions, went through pre-processing in a group environment before this group merged with the main cluster early on. At the present epoch of the Fornax assembly history, these galaxies are the major contribution to the stellar density in the core of the cluster.