The hidden satellites of massive galaxies and quasars at high-redshift

Using cosmological, radiation-hydrodynamic simulations targeting a rare $\approx \, 2 \times 10^{12} \, \rm M_\odot$ halo at $z \, = \, 6$, we show that the number counts and internal properties of satellite galaxies within the massive halo are sensitively regulated by a combination of local stellar radiative feedback and strong tidal forces. Radiative feedback operates before the first supernova explosions erupt and results in less tightly-bound galaxies. Satellites are therefore more vulnerable to tidal stripping when they accrete onto the main progenitor and are tidally disrupted on a significantly shorter timescale. Consequently, the number of satellites with $M_{\rm \star} > 10^{7} \, \rm M_\odot$ within the parent system's virial radius drops by up to $60 \%$ with respect to an identical simulation performed without stellar radiative feedback. Radiative feedback also impacts the central galaxy, whose effective radius increases by a factor $\lesssim 3$ due to the presence of a more extended and diffuse stellar component. We suggest that the number of satellites in the vicinity of massive high-redshift galaxies is an indication of the strength of stellar radiative feedback and and can be anomalously low in the extreme cosmic environments of high-redshift quasars.