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

Kohei Inayoshi, Kohta Murase, Kazumi Kashiyama

First listed 2025-09-23 | Last updated 2026-04-28

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 $z\simeq 4-10$, driving both rapid BH feeding and concurrent nuclear starbursts. As the BH grows to $\sim 10^{6-7}~M_\odot$ via super-Eddington accretion, the accretion power inflates a dense envelope whose effective temperature approaches the Hayashi limit at $T_{\rm eff}\simeq 5000~{\rm K}$, producing red optical emission, while a coeval young stellar population of $\sim 10^7~M_\odot$ 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 $L_{\rm opt}/L_{\rm UV}\simeq 2-10$ from the onset of accretion bursts for $\simeq 15~{\rm Myr}$, one-third of the Salpeter time, until quenching by stellar feedback. While this ratio is sustained for the LRD population at $z\simeq 4-8$, it declines toward lower redshifts as BHs can no longer maintain red envelopes, thereby losing the LRD characteristics.

Short digest

Proposes a two-component origin for LRD SEDs during assembly of Mh ≳ 1e10–11 Msun halos at z ≃ 4–10: a super-Eddington–fed seed BH (growing to ~1e6–1e7 Msun) inflates a dense envelope at Teff ≃ 5000 K that makes the red optical, while a coeval ~1e7 Msun nuclear starburst supplies the blue UV. This naturally yields the V-shaped UV–optical continuum and the weakness of X-ray/IR/radio signatures, with broad lines arising despite envelope scattering. A key prediction is a nearly constant Lopt/LUV ≃ 2–10 for ≃15 Myr (~one-third Salpeter) until SNe expel nuclear gas and quench the envelope. Population-wide, the ratio stays uniform at z ≃ 4–8 but declines toward lower z as red envelopes fail, linking LRD phases to later normal AGN states.

Key figures to inspect

• Figure 1 — Schematic: Read off how the BH envelope (Teff ≃ 5000 K) and compact starburst split the optical/UV, and how electron scattering broadens Balmer lines to observed FWHM given BLR cloud velocities.
• Figure 2 — Halo assembly tracks: Identify the Mh–z regimes where starbursts are UV-bright enough for LRDs and where BH momentum/radiative feedback limits are crossed; shows which halos can sustain the red envelope and for how long.
• Figure 3 — SED synthesis vs data: See how the envelope+10 Myr starburst reproduces the stacked LRD photometry and NIRSpec/PRISM spectra (A2744-QSO1, MoM-BH*-1, RUBIES-UDS-154183, UNCOVER-45924, CAPERS-LRD-z9), and how varying Mstar/MBH shifts Lopt/LUV and the Balmer-turnover.
• Figure 4 — Duty/termination window: Gas-retention time vs Tvir peaks at ≃15 Myr in massive halos, matching the predicted plateau in Lopt/LUV; compare solid vs dotted curves to gauge the effect of SN delay on envelope quenching.