Density-protected states in active matter under virtual confinement

We investigate photo-responsive structure formation in a minimal model of dry active nematics. Combining microscopic simulations with the analysis of the corresponding hydrodynamic theory, we show that the system generically self-assembles into a dense, nematically ordered ring at the boundary of compact illumination patterns. Remarkably, this boundary structure gives rise to a disordered core whose density is self-selected and independent of the global particle density. Our analysis reveals that these protected states emerge from a generic interplay between local nematic alignment and curvature-driven active currents. These results identify a robust route to boundary-induced structure formation in active matter and provide experimentally testable predictions.