The Little Red Dots Are Direct Collapse Black Holes

TL;DR

JWST observations of compact, red sources (LRDs) pose a challenge to standard stellar interpretations. The authors demonstrate that these spectra are naturally produced by accreting Direct Collapse Black Holes (DCBHs), using radiation-hydrodynamic simulations of a $M_ullet=10^5\,M_\odot$ seed in an atomic-cooling halo, with a dense, Compton-thick inflow that reprocesses emission and generates Balmer absorption without a stellar continuum. Post-processing with CLOUDY yields a full $\text{IR}$–$\text{X-ray}$ spectrum, where UV/optical light emerges from reprocessed radiation and X-rays are suppressed by a large column density ($N_H\gtrsim10^{24}\, rm cm^{-2}$), while dust reddening matches the observed SED. Fitting to JWST LRD data at $z\sim4$–9 supports a heavy-seed, high-redshift scenario: the LRDs are likely witnessing the widespread formation of heavy black hole seeds in the early Universe, with implied short- to intermediate-term variability set by radiation feedback and long-lived, parsec-scale accretion structures.

Abstract

The discovery by JWST of a substantial population of compact "Little Red Dots" (LRDs) presents a major puzzle: their observed spectra defy standard astrophysical interpretations. Here, we show that LRD spectra are naturally reproduced by emission from an accreting Direct Collapse Black Hole (DCBH). Using radiation-hydrodynamic simulations, we follow the growth of the DCBH seed via a dense, compressionally heated, collisionally ionized accretion flow. The model self-consistently reproduces the screen responsible for the observed Balmer absorption, while allowing UV/optical emission to partially escape, along with reprocessed infrared radiation. Crucially, this structure is not a blackbody and requires no stellar contribution: the UV continuum originates entirely from reprocessed DCBH radiation, attenuated only by a small amount of dust with an extinction curve consistent with high-redshift galaxies. This single framework simultaneously explains the key observational puzzles of LRDs: (a) weak X-ray emission, (b) metal and high-ionization lines alongside absent star-formation features, (c) overmassive black holes, (d) compact morphology, (e) abundance and redshift evolution -- linking them directly to pristine atomic-cooling halos, (f) long-lived ($>100$ Myr), slowly variable phases driven by radiation pressure. Our findings indicate that JWST is witnessing the widespread formation of heavy black hole seeds in the early Universe.

Figures (4)

• Figure 1: Radial profiles of gas density, cumulative hydrogen column density, velocity, and optical depth at $75$ Myr after DCBH seeding, corresponding to a representative, intermediate evolutionary stage of the RHD simulation Pacucci_2015. These profiles are used to generate the DCBH spectrum in Fig. \ref{['fig:spectrum']}.
• Figure 2: Artist's illustration of our DCBH model for Little Red Dots.
• Figure 3: Comparison between the attenuated DCBH spectrum (red) and the spectrum of a prototypical LRD at $z = 5.28$ (blue), RUBIES-EGS-42046 Kocevski_2024. The unattenuated DCBH spectrum is also shown (orange), together with the dust attenuation law $\tau_d(\lambda)$. The bottom panel displays the residuals between the attenuated DCBH spectrum and the LRD spectrum. The DCBH spectrum is shown at the intermediate accretion stage of $\sim 75$ Myr, when it reached a mass of $\sim 10^6 \,{\rm M_\odot}$Pacucci_2015.
• Figure 4: Reference DCBH synthetic spectrum, from the far-infrared ($120 \, \rm \mu m$) to the hard X-rays ($100$ keV). Line smoothing was applied to improve the clarity of the continuum visualization. The inset shows a zoom-in of the wavelength range highlighted in Fig. \ref{['fig:spectrum']}. In this particular run, as the column density is $>10^{25} \, \rm cm^{-2}$, the X-ray emission is faint. This fact does not imply that all DCBH accretion phases are X-ray weak to this extent; on the contrary, in the final phases, when most of the gas is depleted, DCBH accretion is X-ray bright PFVD_2015.