Little red dots as embryos of active galactic nuclei

TL;DR

This work proposes that JWST-detected little red dots (LRDs) are embryonic active galactic nuclei where central massive black holes ($M_{ullet}\lesssim 10^{6}M_{\odot}$) are accompanied by a large population of stellar-mass black holes embedded in a cMBH-disk (the spt@ cMBH-disk). Accretion onto the sMBHs powers the rest-frame optical continuum, while UV emission arises from the slim disk around the cMBH and associated nuclear starbursts; radiation-pressure-driven outflows form clumped envelopes that produce Balmer absorption features, giving the observed Balmer-line profiles. The model yields V-shaped SEDs and broad H$\beta$ lines whose widths reflect the collective gravity of the sMBH population, not just the cMBH, thereby avoiding the overmassive cMBH problem implied by standard $R-L$ estimates. Fitting to JWST/LRD data with components for AMS, MBH, and NSB (plus envelope absorption) reproduces the observed diversity of LRD SEDs, predicts minimal X-ray emission, and suggests evolutionary paths toward quasars via inward migration of sMBHs and mergers, with potential gravitational-wave signatures detectable by future facilities.

Abstract

As an unprecedented large population in the early universe, the JWST-discovered little red dots (LRDs) have garnered much attention for formation of massive black holes and galaxies, but their nature remains a mystery. The LRDs appearing as ``Chimeras" like both active galactic nuclei (AGNs) and galaxies have stimulated renewed interest in the roadmap of central massive black hole (cMBH) formation in AGNs. In this paper, we suggest that the LRDs contain $M_{\bullet}\lesssim 10^6\,M_{\odot}$ cMBHs as demonstrated by the Sołtan argument and there is a large population of stellar-mass black holes (sMBHs with total mass of $\mathscr{M}_{m_{\bullet}}$) embedded inside cMBH accretion disks (cMBH-disk) as motivated by anomalous reverberations of broad H$β$ line in local AGNs. This embryo structure of LRDs ($M_{\bullet}<\mathscr{M}_{m_{\bullet}}$) is formed as a consequence of gravitational collapse of primordial clouds. In this Chimera, accretion onto sMBHs powers the rest-frame optical continuum of the LRDs but the UV continuum is jointly contributed by slim parts of the cMBH-disks and nuclear starbursts in the core of collapsing clouds governing the appearance of the observed V-shaped spectral energy distributions (SEDs). Outflowing clumped-envelopes are unavoidably formed by radiation pressure leading to absorption features of the Balmer lines. The present model works very well for LRDs' SEDs and avoids the issues of overly massive cMBHs. Evolution of LRDs is briefly discussed including gravitational waves.

Figures (6)

• Figure 1: Structures of the LRDs formed during the collapse of primordial cloud proposed in this paper. The clumped-envelope covers the system composed of the BLR and the cMBH-disk system contributing the V-shaped SEDs of the LRDs. The cMBH-disk consists of two parts: 1) the slim disk of the central massive black hole (between $R_{\rm in}$ and $R_{\rm out}$); and 2) spt@ cMBH-disk represented by the part between ${\cal{R}}_{\rm in}$ and ${\cal{R}}_{\rm out}$ embedding a population of stellar-mass black holes. Since the cMBHs are not significantly smaller than normal AGNs, the spt@ cMBH-disk system refers to an embryo of AGNs. Given the current mass density of a collapsing primordial clouds ($\rho_{\rm c}\sim 10^{-19}\,{\rm g\,cm^{-3}}$), an outflowing clumped-envelope is formed with a typical velocity of $\sim 100\,\rm km~s^{-1}$ by the radiation pressure, and has an optical depth of $\tau_{\rm abs}\approx 0.1-0.2$, explaining the absorption features when the clumps are on the line-of-sight.
• Figure 2: Spectral energy distributions of the represent LRDs with three necessary components. NSB component composes of only single stellar population as the necessary for the part around the Balmer break.
• Figure 3: Spectral energy distributions of the represent LRDs with two necessary components contributed by slim disks and spt@ cMBH-disk. NSB components are not necessary for them.
• Figure 4: Spectral energy distributions of the third type of LRDs in which multiple NSBs are necessary between the Balmer break wavelength and $\sim 0.2\,\mu$m. Sometimes slim disks are not necessary, implying the cMBH masses are quite light (much less than $10^6\,M_{\odot}$). We also note that RUBIES-EGS 42046-prisim shows P Cyg profile of the Balmer lines Hviding2025.
• Figure 5: Radial profiles of the disk model parameters and their dependence on $\mathscr{M}_{m_\bullet}$, $\dot{\mathscr{M}}$, and $\beta_\gamma$. We fix the central massive black hole mass of $M_{\bullet}=10^5\,M_\odot$ for all the cases. Here $R_{\rm g}=GM_{\bullet}/c^2$ is the gravitational radius of the cMBH. The panels in the first row are for different $\mathscr{M}_{m_\bullet}$ and fixing $\dot{\mathscr{M}}=100$, $\beta_\gamma=1$, $\mathcal{R}_{\rm in}=10^4\,R_{\rm g}$, and $\mathcal{R}_{\rm out}=10^6\,R_{\rm g}$. The panels in the second row are for dependence of structures on different $\dot{\mathscr{M}}$. The panels in the third row are for dependence on $\beta_{\gamma}$.