Exploring Collisionless Expansion as a Mechanism for the Evolution of Little Red Dots

Abstract A growing number of compact red sources at z ≳ 6, identified with the James Webb Space Telescope and collectively known as “little red dots” (LRDs), has raised new questions about the assembly of massive systems in the early Universe. While their extreme central densities and apparently overmassive supermassive black holes (SMBHs) are now well established, the mechanisms driving their subsequent structural evolution remain unclear. Observations indicate a significant decrease in their number densities by z ∼ 4, suggesting rapid transformation toward systems resembling present-day galaxies. We perform direct N -body simulations to investigate the dynamical response of LRDs to rapid gas removal, modeled as active galactic nuclei (AGN)-driven feedback, and quantify the resulting structural changes. In the case of instantaneous and complete gas expulsion, the stellar half-mass radius of our model LRDs increases by a factor of ∼2–3. However, the stellar density within the SMBH sphere of influence remains largely unchanged, as the central potential continues to be dominated by the black hole. Partial gas removal does not have a significant impact on the LRD structure. We additionally model the combined effect of gas removal and core scouring by SMBH binaries to assess the impact on LRDs when both processes work in tandem. While gas expulsion alone drives global expansion, only the inclusion of core scouring leads to substantial reductions in stellar density both in the innermost regions and at larger radii.