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

Devesh Nandal, Abraham Loeb

First listed 2025-07-16 | Last updated 2025-12-30

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.

Short digest

First-principles synthetic spectra for non-rotating, metal-free supermassive stars (up to 10^6 M⊙) are used to test whether LRDs are direct photospheres of SMSs. The model’s luminosity at 4050 Å (Lλ ≈ 1.7×10^44 erg s^-1 μm^-1) and NLTE physics reproduce hallmark LRD traits—a V-shaped Balmer break and strong Hβ emission with other Balmer lines in absorption—with wind/macroturbulent broadening matching observed profiles. Quantitative fits to sources at z=7.76 and z=3.55 include an Hβ width match for MoM‑BH*-1 to within 4%, offering a self-consistent alternative to multi-component obscured AGN. A luminosity-dependent visibility window of ~10^4 yr for the brightest systems and 10^5–10^6 yr for objects 1–2 dex fainter implies LRDs may capture brief, pre-collapse stages of SMBH progenitors.

Key figures to inspect

• Figure 2: Check that only the ~10^6 M⊙ model reaches the required continuum level for bright LRDs and that an intrinsic Hβ emission spike with neighboring Balmer absorption is already present before any macroscopic broadening; note the tie to The Cliff and MoM‑BH*-1.
• Figure 3: Inspect the opacity budget across 3646 Å to see the H(n=2) Balmer continuum dominate shortward of the edge, explaining the steep, V-shaped Balmer break central to LRD spectra.
• Figure 4: Compare S_line versus S_cont with optical depth for Hβ and Hγ to understand why Hβ emerges in emission while higher Balmer lines stay in absorption—the differential NLTE behavior that matches LRD line phenomenology.
• Figure 1: Use the HRD and Kippenhahn diagrams for an accreting Pop III SMS at Ṁ≈10^3 M⊙ yr^-1 to verify the cool, extended, near-Eddington phase and to gauge the brief timescales as mass approaches 10^6 M⊙, consistent with the predicted rarity/visibility window.