Reduced Incidence of Little Red Dots at z < 3 from Number Density and Halo Mass Evolution

Abstract

An intriguing puzzle in extragalactic astronomy is the scarcity of Little Red Dots (LRDs) at $z<3$, compared to their higher abundance at earlier cosmic epochs. To investigate this, we measure the overdensity for 98 specpically confirmed LRDs at $3<z<7$, and find that these LRDs predominantly reside in under-dense regions at $z>4$ but shift to more typical galaxy environments at $z\sim 3.5$. Concurrently, cross-correlation analyses show that their dark matter halo masses grow rapidly, from $\lesssim 10^{10.1}{M}_{\odot }$ at $z\sim 7.5$ to $\sim 10^{11.3}{M}_{\odot }$ at $z\sim 3.5$, approaching the halo masses of normal galaxies at lower redshifts. Applying an empirical stellar-to-halo mass scaling relation, we find that LRDs still host over-massive black holes relative to their stellar masses at $z>4$, yet converge toward the local BH-stellar mass relation at lower redshifts. The coherent evolution of LRDs' large-scale environments and halo masses toward those of normal galaxies provides a plausible explanation for their declining abundance at $z<3$, even though the underlying small-scale physical mechanisms remain elusive.

20 pages, 8 Figures, in principle accepted by Nature Astronomy