Why Little Red Dots Disappear at z < 3: Evolution of Number Density and Halo Mass

Abstract

A significant puzzle in extragalactic astronomy is the scarcity of Little Red Dots (LRDs) at $z < 3$, compared to their higher abundance at earlier epochs. To understand this transition, we investigate the cosmic evolution of LRD environments. We measure the overdensity for LRDs and the general galaxy population at $3<z<7$, and find that at $z > 4$, LRDs predominantly reside in under-dense regions relative to the general galaxy population. By $z \sim 3.5$, however, this environmental contrast roughly diminishes, and LRDs are found in regions of comparable density to typical galaxies. Simultaneously, the dark matter halo masses of LRDs, inferred from large-scale clustering, grow rapidly from $\lesssim 10^{10.1} \, M_{\odot}$ at $z \sim 7.5$ to $\sim 10^{11.3} \, M_{\odot}$ at $z \sim 3.5$, where the halo mass becomes close to that of normal galaxies at lower redshift. Applying an empirical stellar-to-halo mass scaling relation, we derive stellar masses for LRDs; these show that black hole masses remain over-massive relative to stellar mass at $z > 4$, but converge toward the local $M_* - M_{\rm BH}$ scaling relation by $z \sim 3.5$. The coherent evolution of LRDs&#39; large-scale environments $-$ as expressed by their overdensity and halo mass $-$ points to a distinct evolutionary pathway from that of normal galaxies. The significantly increased halo masses of LRDs lead to larger galaxy sizes, driven primarily by the potential enhancement of halo spins. Consequently, these sources are no longer as compact as typical high-redshift LRDs. Meanwhile, the depletion of dense gas and/or elevated star formation in their host galaxies would also significantly alter the spectral energy distribution of LRDs.