Born in the Dark: The Catastrophic Collapse of Fuzzy Dark Matter Solitons as the Origin of Little Red Dots

TL;DR

The paper addresses the origin of JWST-detected Little Red Dots (LRDs) as compact, Compton-thick sources at $z\ge 5$ and proposes that they trace a short-lived obscured phase inside fuzzy dark matter soliton cores. It develops an analytic framework tying soliton scaling to observed radii $r_e$ and column densities $N_H$, identifying a viable window around $m_{22} \approx 2$ for soliton masses $M_s \sim 10^8$–$10^9\ M_\odot$. A key contribution is the notion of an Opacity Crisis, where high $N_H$ within $r_c$ cannot be sustained as a long-lived hydrostatic atmosphere due to rapid radiative losses, favoring rapid inflow or radiation-pressure-driven evolution. The authors test core formation with 512^3 Schrödinger–Poisson simulations of idealized soliton mergers, showing robust formation of compact cores with $r_c\sim 50$ pc, and they outline concrete observational signatures (e.g., inverse size–mass relation, polarization, IR reprocessing) and call for future radiation-hydrodynamic modeling to predict demographics and spectra.

Abstract

JWST surveys have uncovered a population of compact, red sources ("Little Red Dots," LRDs) at $z \ge 5$ that exhibit broad Balmer emission yet remain X-ray faint, implying heavy obscuration with $N_H \ge 10^{24}$ cm$^{-2}$. We propose that LRDs may trace a short-lived, obscured phase associated with rapid baryonic inflow inside the deep solitonic cores of fuzzy dark matter (FDM) halos. Combining the soliton size scaling with (i) the observed compact radii ($r_e \sim 30-100$ pc) and (ii) the requirement that Compton-thick columns be achievable within a region of order the core radius, we find that particle masses $m$ few $\times 10^{-22}$ eV are plausible for soliton masses $M_s \sim 10^8 - 10^9 M_\odot$; we adopt $m_{22}=2$ as a fiducial choice. A conservative mass-budget estimate for the obscuring column, together with isothermal hydrostatic stratification, indicates that configurations reaching $N_H \ge 10^{24} - 10^{25}$ cm$^{-2}$ require densities for which radiative losses (cooling and/or diffusion) occur faster than the dynamical time, suggesting that a long-lived static hot atmosphere is unlikely (an "Opacity Crisis") and that rapid inflow or radiation-pressure-driven evolution is favored. Using $512^3$ pseudo-spectral Schrödinger-Poisson simulations of idealized soliton mergers, we illustrate that compact, high-density soliton cores can form via violent relaxation under representative scalings. We discuss observational implications and tests, and outline the need for future radiation-hydrodynamic modeling to predict demographics and detailed spectra.

Figures (4)

• Figure 1: Constraints on boson mass ($m_{22}$). The viable region is defined by the intersection of the observed LRD size range (green band, $r_e\sim 30$--$100$ pc) and the requirement that Compton-thick columns be achievable within a compact core. The red hatched region marks parameters for which even a maximally loaded core ($f_g=1, \xi=1$) fails to reach $N_{\rm H}=10^{24}\,{\rm cm^{-2}}$. The opacity constraint curves assume a soliton mass scale of $M_s \sim 10^9 M_{\odot}$ (typical of LRD hosts). We treat the observed $r_e$ as a proxy for the soliton core radius $r_c$; if $r_e$ traces a larger photosphere, the constraints would shift. To address uncertainties in baryon physics, the orange shaded area shows the sensitivity range: under conservative assumptions ($f_g=0.1, \xi=3$), the exclusion region expands. Under these working assumptions, a parameter window around $m_{22} \sim \text{few}$ is favored.
• Figure 2: The instability region. Comparison of free-fall ($t_{\rm ff}$) and cooling ($t_{\rm cool}$) timescales versus soliton mass for $m_{22}=2.0$. The shaded region indicates $t_{\rm cool}<t_{\rm ff}$, where a long-lived, optically-thin hot hydrostatic atmosphere cannot persist. The system is driven either toward rapid inflow/collapse (if radiative losses escape efficiently) or toward a radiation-pressure dominated, optically thick configuration (if radiation is trapped).
• Figure 3: Validation of a compact soliton core in an idealized merger. Radial density profile of the simulated core (red points) plotted against the analytic soliton solution (black line). The inner region is well described by the soliton profile.
• Figure 4: Size--mass illustration. Black points show SMBH masses from broad H$\alpha$ in spectroscopically confirmed LRDs Greene2024, plotted against characteristic effective radii ($r_e\sim 30$--$100$ pc; Kokorev2024). The red line indicates the soliton-inspired inverse scaling, shown here only as an illustrative mapping under the assumption that $M_{\rm BH}$ roughly traces the soliton core mass with large scatter.