The Next Generation Fornax Survey (NGFS): Satellite Environmental Regions.

Abstract All satellite galaxies experience environmental secular evolution as they are accreted onto their host systems. Observations and simulations indicate that the closer they are to their hosts, the higher their dark matter densities are. Using the NGFS deep photometric observations of 630 satellites in the Fornax galaxy cluster we discover that their mean stellar densities also show such anti-correlation with their distances to the cluster center, correlating with the host tidal density field, up to a transition radius at ℜ★ ≈ 0.4𝑹vir (400 kpc), beyond which the stellar densities increase again. Comparisons with satellites of a Fornax-type cluster in the TNG50 simulation reveal a similar break at ℜ★ ≈ 0.5𝑹vir in deprojection, while their mean dark matter densities keep decreasing with distance to their host. Moreover, the relative fraction of gas-rich satellites and gaseous densities decrease significantly within ℜ★. We expand our analysis to satellites in different environments probing the more massive Virgo cluster, and also the less massive Milky Way and Andromeda systems, finding a similar behavior and transition radius in units of 𝑹vir for each system of satellites, with ℜ★ ∼ 0.4 − 0.6𝑹vir. This transition radius, if universal, could provide a useful observational diagnostic tool to identify the regions where the cluster environment would have processed the baryonic component of the accreted satellites more strongly, and where the more pristine population would be located. Finally, we suggest the utility of calibrating ℜ★ as a possible photometric virial mass estimator.