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

Tak-Pong Woo

First listed 2025-12-31 | Last updated 2026-03-13

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.

Short digest

Proposes Little Red Dots as a brief, heavily obscured inflow phase inside deep fuzzy–dark‑matter soliton cores. Matching re ≈ 30–100 pc and Compton‑thick columns selects boson masses m ≈ few × 10^-22 eV (fiducial m22 = 2) with soliton masses Ms ~ 10^8–10^9 M☉; in this regime cooling/diffusion times undercut t_dyn, triggering an “Opacity Crisis” that forces rapid inflow or a radiation‑pressure–dominated envelope and naturally yields X‑ray‑faint, broad‑line systems. 512^3 Schrödinger–Poisson merger simulations demonstrate violent relaxation to compact, high‑density cores consistent with these scalings. A full demographic and spectral forecast awaits radiation‑hydrodynamic modeling.

Key figures to inspect

• Figure 1: Read off the allowed window in m22 by intersecting re ≈ 30–100 pc with the Compton‑thick requirement; check how the favored m22 ≈ 2 shifts under conservative baryon loading and if re truly traces the soliton core radius.
• Figure 2: Inspect where t_cool or t_diff < t_ff across Ms to see the instability band that defines the Opacity Crisis, and how it overlaps the columns needed for NH ≥ 10^24–10^25 cm^-2.
• Figure 3: Compare the simulated radial density profile to the analytic soliton curve to verify a compact, centrally peaked core; infer the core radius and map to physical 30–100 pc for m22 = 2.
• Figure 4: Cross‑check the illustrative inverse size–mass mapping by comparing BLR‑based MBH points to re; gauge scatter and whether re plausibly traces the soliton mass scale.