Spectral Uniformity of Little Red Dots: A Natural Outcome of Coevolving Seed Black Holes and Nascent Starbursts

2025/09/23 by Inayoshi, Kohei, Murase, Kohta, Kashiyama, Kazumi · 7 citations
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences

paper · doi:10.48550/arxiv.2509.19422

Abstract

The birth of seeds of massive black holes (BHs) and nascent galaxies at cosmic dawn takes place in dense gaseous environments, which play a crucial role in shaping their coevolution and radiation spectra. We investigate gas accretion during the assembly of massive halos with M_{\rm h}\gtrsim 10^10-11~M_\odot at redshifts z≃ 4-10, driving both rapid BH feeding and concurrent nuclear starbursts. As the BH grows to ∼ 10^6-7~M_\odot via super-Eddington accretion, the accretion power inflates a dense envelope whose effective temperature approaches the Hayashi limit at T_{\rm eff}≃ 5000~\rm K, producing red optical emission, while a coeval young stellar population of ∼ 10⁷~M_\odot provides blue UV emission. This early coevolving system naturally reproduces the characteristic spectral features of the so-called little red dots (LRDs), a population of broad-line active galactic nuclei (AGNs), including the V-shaped UV-to-optical spectra and weakness of X-ray, infrared, and radio emission. Massive stars in the nuclear starburst soon explode as supernovae, injecting energy and momentum that expel gas from the nucleus, quench gas supply to the BH envelope, and ultimately drive a transition into normal AGN phases. For individual LRDs, the optical-to-UV luminosity ratio remains nearly constant at L_{\rm opt}/L_{\rm UV}≃ 2-10 from the onset of accretion bursts for ≃ 15~\rm Myr, one-third of the Salpeter time, until quenching by stellar feedback. While this ratio is sustained for the LRD population at z≃ 4-8, it declines toward lower redshifts as BHs can no longer maintain red envelopes, thereby losing the LRD characteristics.

Citations

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Cited by

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• (Re)solving the complex multi-scale morphology and V-shaped SED of a newly discovered strongly-lensed Little Red Dot in Abell 383
• Discovery of two little red dots transitioning into quasars
• Too Quiet for Comfort: Local Little Red Dots Lack Variability over Decades
• From nuclear star clusters to Little Red Dots: black hole growth, mergers, and tidal disruptions
• Black Hole Feedback, Galaxy Quenching and Outflows at Cosmic Dawn: Analysis of the SEEDZ Simulations
• The SEEDZ Simulations: Methodology and First Results on Massive Black Hole Seeding and Early Galaxy Growth

Related

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