Composite spectrum of Little Red Dot from a standard inner disk and an unstable outer disk

Chenxuan Zhang, Qingwen Wu, Xiao Fan, Luis C. Ho, Jiancheng Wu, Huanian Zhang, Bing Lyu, Xinwu Cao, Jianmin Wang

First listed 2025-05-19 | Last updated 2026-01-14

Abstract

James Webb Space Telescope (JWST) has revealed a new class of high-redshift, very red, compact broad-line sources, termed as "little red dots" (LRDs). The physical mechanism driving these properties remains elusive. We construct spectral energy distributions (SEDs) with spectroscopic redshift for 28 LRDs and find they exhibit V-shaped SEDs with a common break frequency of $ν_{\rm b}\simeq10^{14.96\pm0.06}$ Hz. We propose that the unique SEDs can be well explained by the combination of an inner standard disk and an outer gravitationally unstable accretion disk with Toomre parameter $Q\sim1$, where the outer disk has a temperature of $\sim2000-4000 K$ and mainly radiates in near-infrared to optical wavebands. The composite spectrum from this model naturally explains the V-shaped continuum and reproduces intrinsically luminous infrared-optical emission without requiring extreme dust extinction or unusual stellar populations. Even considering possible dense gas around the disk to account for pronounced Balmer breaks in some LRDs, the intrinsic optical-UV emission is only suppressed by factors of $\lesssim2-3$, which suggests that most LRDs are sub-Eddington and intrinsically weak. These results provide new insights into early-phase black hole growth and galaxy evolution.

Short digest

Builds SEDs for 29 broad-line LRDs and finds uniformly V-shaped continua with a narrow break around ν_b ≃ 10^{14.96±0.06} Hz (λ ≈ 2600–3800 Å). Models the SEDs as a composite of an inner standard thin disk plus an outer gravitationally unstable disk (Q≈1) that self-regulates to T≈2000–4000 K, naturally producing the near-IR/optical bump without invoking extreme dust. MCMC fits anchored by MBH from broad lines reproduce individual SEDs, including UNCOVER 25119 and PRIMER 33823 (whose MIRI 4.9–27.9 μm points trace the bump). Allowing for dense gas to explain strong Balmer breaks suppresses intrinsic optical–UV by only ≲2–3×, implying most LRDs are sub-Eddington and intrinsically weak.

Key figures to inspect

• Figure 1 (broken-power-law fits for UNCOVER 25119 and PRIMER 33823): Check how the V-shaped continuum is identified and where the inflection lies; note excluded line-contaminated and low-S/N points and the MIRI outliers for PRIMER 33823.
• Figure 2 (inflection-wavelength histogram and normalized SED stack): Verify the tight clustering of λ_b ≈ 2600–3800 Å and the small scatter in the composite, which drives the common break-frequency claim.
• Figure 3 (cartoon, temperature profile, and model spectrum): See how the outer Q≈1 disk attains T≈2000–4000 K and dominates the near-IR/optical bump; note the schematic showing how crossing a dense disk wind strengthens the Balmer break.
• Figure 4 (MCMC SED fits for UNCOVER 25119 and PRIMER 33823): Inspect how the composite model matches photometry/spectroscopy, the recovery of the MIRI-traced bump, and constraints on ṁ and the critical radius given MBH from broad lines.