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

2026/06/01 by Chenxuan Zhang, Huanian Zhang, Qingwen Wu +2
#astro-ph.GA

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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 ∼ 3.5. Concurrently, cross-correlation analyses show that their dark matter halo masses grow rapidly, from \lesssim 10^10.1 M_\odot at z ∼ 7.5 to ∼ 10^11.3 M_\odot at z ∼ 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.

Citations

• The Discovery of Little Red Dots in the Local Universe: Signatures of Cool Gas Envelopes
• The JWST UNCOVER Treasury survey: Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization
• Overview of the DESI Legacy Imaging Surveys

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• Investigating the Growth of Little Red Dot Descendants at z&lt;4 with the JWST