Supermassive Stars Match the Spectral Signatures of JWST's Little Red Dots

TL;DR

This paper tests the hypothesis that JWST's Little Red Dots are the direct photospheric emission from primordial supermassive stars. Using a first-principles pipeline that maps a non-rotating, metal-free SMS of $M_\\ast=10^6\,M_\odot$ (from GENEC) to a synthetic spectrum, the authors reproduce the LRD signature: a deep Balmer break and broad Balmer lines with NLTE-driven line formation, without invoking obscured AGN components. They show that only the $10^6\,M_\odot$ model attains the required rest-frame luminosity at $4050$ Å, matching two observed LRDs after applying physically motivated wind and macroturbulent broadening; shorter luminous phases are predicted for higher-luminosity systems, yielding a luminosity-dependent duty cycle. The work connects LRDs to a concrete SMS phase preceding direct collapse into SMBH seeds, with implications for formation rates and the observational window for detecting such SMS-dominated epochs. Future improvements include full comoving-frame radiative transfer, a more detailed hydrogen atom treatment, and exploration of rotation and a broader SMS parameter space.

Abstract

The James Webb Space Telescope (JWST) has unveiled a population of enigmatic, compact sources at high redshift known as ``Little Red Dots'' (LRDs), whose physical nature remains a subject of intense debate. Concurrently, the rapid assembly of the first supermassive black holes (SMBHs) requires the formation of heavy seeds, for which supermassive stars (SMSs) are leading theoretical progenitors. In this work, we perform the first quantitative test of the hypothesis that LRDs are the direct observational manifestation of these primordial SMSs. We present a novel, first-principles pipeline generating synthetic spectra for a non-rotating, metal-free SMS up to $10^6 \, M_\odot$. We establish that its luminosity ($L_λ\approx 1.7 \times 10^{44} \, \text{erg} \, \text{s}^{-1} \, μ\text{m}^{-1}$ at 4050\,Å) provides a decisive constraint, matching prominent LRDs. Our model self-consistently reproduces their defining spectral features: the V-shaped Balmer break morphology is shown to be an intrinsic photospheric effect, while the complex line phenomenology, strong H$β$ in emission with other Balmer lines in absorption arises from non-LTE effects in a single stellar atmosphere. With wind and macroturbulent broadening, we match LRD spectra at $z=7.76$ and $z=3.55$, including the H$β$ width of MoM-BH*-1 to within 4\%. We predict a luminosity-dependent observability window, $\sim10^{4}$ yr for the most luminous systems and $10^{5}$--$10^{6}$ yr if $L_λ(4050\,\textÅ)$ is lower by 1--2 dex. These results provide a self-consistent alternative to multi-component obscured AGN scenarios and suggest JWST may be witnessing luminous stages of SMBH progenitors before collapse.

Figures (8)

• Figure 1: Evolution of a PopIII supermassive star accreting at 10$^3$ M$_\odot$ yr$^{-1}$ in the pre-main sequence.Left panel: An HR diagram depicting the evolution of luminosity versus the effective temperature. The colourbar represents the mass of the model in solar masses. Various markers in black showcase the mass of the model at different evolutionary stages.Right panel: A Kippenhahn diagram showcasing the internal structure of the model. The y-axis represents the radius in solar radii units and the x-axis depicts the age in years. The coral and purple zones represent convective and radiative regions respectively. The black lines, from bottom to top repsent the isomass lines starting from 1 M$_\odot$ and ending at 10$^6$ M$_\odot$.
• Figure 2: The intrinsic luminosity spectrum ($L_\lambda$) of PopIII supermassive stars (SMSs) as a function of stellar mass. The overall luminosity increases steeply with mass, as expected for stars shining near their Eddington limit. While lower-mass models ($10^4 M_\odot$, $10^5 M_\odot$, and $5 \times 10^5 M_\odot$) are orders of magnitude too faint to match the observed brightness of luminous LRDs, the $10^6 M_\odot$ model (blue curve) produces a luminosity that closely matches the observational requirements for both 'The Cliff' and 'MoM-BH*-1'.
• Figure 3: The intrinsic luminosity spectrum of the fiducial $10^6 \, M_\odot$ SMS model, plotted on a linear scale to illustrate the origin of the V-shaped continuum morphology. The spectrum is dominated by a strong Balmer Jump in emission at the 3646 Å series limit (red dashed line). The flux drops by a factor of $\approx 1.3$ across this break, creating a sharp 'vertex'. This physical discontinuity, embedded in a non-thermal continuum shaped by line-blanketing effects, is the primary feature responsible for the characteristic V-shape seen in LRD spectra. The intrinsic H$\beta$ emission spike and H$\gamma$/H$\delta$ absorption lines are also visible prior to any macroscopic broadening.
• Figure 4: The sources of continuum absorption opacity in the surface layers of the $10^6 M_\odot$ SMS model, plotted across the Balmer break. Longward of 3646 Å, the total absorption opacity (solid blue line) is low and determined by free-free (dotted green) and ground-state bound-free (dashed orange) processes. Shortward of the edge, a new, powerful opacity source from the photoionization of H(n=2) (the Balmer continuum, dash-dot purple line) becomes active. This process dominates all other sources of true absorption, causing the total absorption opacity to jump by a factor of four. This dramatic increase in opacity is the direct physical cause of the strong Balmer break observed in the LRD spectra.
• Figure 5: The line source function ($S_L$) versus the continuum source function ($S_C = B_\lambda(T)$) as a function of optical depth ($\tau_\lambda$) for the H$\beta$ (top) and H$\gamma$ (bottom) lines. The critical line-forming region, where photons escape the star, is near $\tau_\lambda = 1$ (shaded gray). Top Panel: For H$\beta$, the line source function is an order of magnitude greater than the continuum source function ($S_L > S_C$) in the line-forming region, a condition that produces a strong emission line. Bottom Panel: For H$\gamma$, the line source function is less than or equal to the continuum source function ($S_L \lesssim S_C$), resulting in an absorption line. This differential behavior explains the simultaneous presence of emission and absorption lines in the LRD spectra.