A detection of the environmental dependence of the sizes and stellar haloes of massive central galaxies

We use ~100 deg² of deep (> 28.5 mag arcsec^-2 in i band), high-quality (median 0.6 arcsec seeing) imaging data from the Hyper Suprime-Cam (HSC) survey to reveal the halo mass dependence of the surface mass density profiles and outer stellar envelopes of massive galaxies. The i-band images from the HSC survey reach ~4 mag deeper than Sloan Digital Sky Survey and enable us to directly trace stellar mass distributions to 100 kpc without requiring stacking. We conclusively show that, at fixed stellar mass, the stellar profiles of massive galaxies depend on the masses of their dark matter haloes. On average, massive central galaxies with log₁₀(M*, _{100 kpc}/M_⊙) > 11.6 in more massive haloes at 0.3 < z < 0.5 have shallower inner stellar mass density profiles (within ~10-20 kpc) and more prominent outer envelopes. These differences translate into a halo mass dependence of the mass-size relation. Central galaxies in haloes with log10(M_200b/M_⊙) > 14.0 are ~20 per cent larger in R50 at fixed M*, _{100 kpc}. Such dependence is also reflected in the relationship between the stellarmass within 10 and 100 kpc. Comparing to the mass-size relation, the M*, _{100 kpc}-M*, 10 kpc relation avoids the ambiguity in the definition of size, and can be straightforwardly compared with simulations. Our results demonstrate that, with deep images from HSC, we can quantify the connection between halo mass and the outer stellar halo, which may provide new constraints on the formation and assembly of massive central galaxies.