Little Red Dots and Supermassive Black Hole Seed Formation in Ultralight Dark Matter Halos

Dongsu Bak, Jae-Weon Lee

First listed 2026-01-29 | Last updated 2026-03-08

Abstract

We investigate how supermassive black hole (SMBH) seeds form in the early Universe at the centers of ultralight dark matter (ULDM) halos. Focusing on the ULDM Jeans scale, we identify the critical conditions under which high-redshift baryonic gas, strongly confined by central solitonic cores of the halos, undergoes direct and monolithic collapse. The solitonic potential naturally drives rapid inflow and shock heating, allowing the gas temperature to exceed the critical atomic-cooling threshold of $\sim 10^4 \,{\rm K}$ required for fragmentation suppression without invoking an external UV background. We derive semi-analytic relations for the halo mass, soliton mass, baryonic core radius, and thermodynamic state of the gas, including the effects of baryonic contraction. These relations simultaneously determine the minimum and maximum SMBH seed masses as functions of redshift. In this framework, pristine gas clouds that satisfy the temperature threshold collapse without fragmentation, forming SMBH seeds with characteristic masses of order $ 10^5M_\odot$, while systems below the threshold are expected to form compact star clusters instead. Our model also implies an upper limit on the attainable SMBH mass, predicting a maximum mass scale of order $10^{10}M_\odot$, consistent with the most massive quasars observed to date.The ULDM particle mass required to reproduce the inferred seed mass scale, $m \simeq 10^{-22}{\rm eV}$, coincides with the value favored by galactic-scale observations, providing a unified explanation for the characteristic masses of both galactic cores and early SMBH seeds. Our model predicts efficient SMBH seed formation at redshifts $z \gtrsim 10$ and offers a natural interpretation of recently observed little red dots as SMBHs embedded in compact, hot, ionized gas clouds.

Short digest

Proposes a ULDM-assisted direct-collapse route where baryons confined by central solitonic cores shock-heat above ~10^4 K, suppressing fragmentation without an external UV background. Semi-analytic scalings link halo mass, soliton mass, baryonic core size, and gas thermodynamics (including baryonic contraction) to predict a characteristic SMBH seed mass ~1e5 Msun and an upper BH mass scale ~1e10 Msun. The preferred ULDM particle mass m ≈ 10^-22 eV reproduces the seed scale and aligns with galactic-core inferences, with efficient seeding at z ≳ 10. The framework naturally interprets JWST little red dots as SMBHs embedded in compact, hot, ionized gas clouds.

Key figures to inspect

• Figure 1: Compare CDM vs ULDM HMFs to see the small-scale cutoff and how it limits low-mass halos at z ≳ 10, setting the host-halo window where soliton-confined baryons can meet the collapse criteria.
• Figure 2: Read off the minimum and maximum ULDM halo masses vs redshift (for m = 10^-22 eV) and translate them via the core–halo relation to soliton masses and baryonic-core radii; note how the allowed collapse window narrows with increasing z.
• Figure 3: Inspect the predicted SMBH mass bands (seed line from Eq. III.8, stability ceiling from Eq. III.9, gray non-fragmenting region from Eq. III.17) against observed quasars and LRD candidates to see that seeds cluster near ~1e5 Msun and that the model caps at ~1e10 Msun.