Accreted mass fraction vs. total stellar mass for ETGs. Black symbols correspond to other BCGs from the literature (Seigar et al. 2007; Bender et al. 2015; Iodice et al. 2016, 2020b; Spavone et al. 2017, 2018, 2020; Cattapan et al. 2019), and red circles show galaxies in this work. The red region encloses the predictions of cosmological galaxy formation simulations by Cooper et al. (2013, 2015), which are indicated as gray dots. The continuous and dashed blue regions indicate the accreted mass fraction measured within 30 kpc and outside 100 kpc, respectively, in the Illustris simulations by Pillepich et al. 2018 (see their Fig. 12).