Kavli Affiliate: Luis Ho
| Summary:
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.
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