2025ApJ...989..107W

Galaxies are predicted to assemble their stellar haloes through the accretion of stellar material from interactions with their cosmic environment. Observations that trace stellar halo buildup probe the processes that drive galaxy size and stellar mass growth. We investigate stellar halo assembly over 0.2 ≤ z ≤ 1.1 in a mass-complete (M_⋆ ≥ 10^9.5M_⊙) sample of 242,456 star-forming galaxies (SFGs) and 88,421 quiescent galaxies (QGs) from the CLAUDS and HSC-SSP surveys. We extract galaxy rest-frame g-band surface brightness (μ_g) profiles to study faint, extended emission in galaxy outskirts. We examine trends in galaxy assembly by analyzing the median μ_g profiles in different SFG and QG M_⋆ ranges with decreasing redshift and connecting evolution in galaxy μ_g profiles with the underlying stellar mass growth in galaxies. Since z = 1.1, the majority of evolution in the median μ_g profiles of galaxies (∼64% in SFGs and ∼71% in QGs) occurs throughout their stellar halo regions (2-10 R_e). More-massive galaxies assemble stellar halo material more rapidly at 0.2 ≤ z ≤ 1.1. Over this period, QGs grow a larger fraction of their stellar haloes than SFGs at fixed M_⋆ (factor of ∼1.2). Although star formation can account for the stellar halo growth observed in low-mass SFGs (10^9.5M_⊙ ≤ M_⋆ < 10^10.5M_⊙), high-mass SFGs (M_⋆ ≥ 10^10.5M_⊙), and both low- and high-mass QGs require an additional assembly mechanism. Our results suggest accretion via minor mergers drives additional stellar halo growth in these galaxies. The contribution from accretion is larger in more-massive galaxies (over M_⋆ ≥ 10^9.5M_⊙), and QGs exhibit larger fractional increases to their ex situ fractions over 0.2 ≤ z ≤ 1.1 than SFGs at fixed M_⋆.