A "Black Hole Star" Reveals the Remarkable Gas-Enshrouded Hearts of the Little Red Dots

Abstract

The physical processes that led to the formation of billion solar mass black holes within the first 700 million years of cosmic time remain a puzzle. Several theoretical scenarios have been proposed to seed and rapidly grow black holes, but direct observations of these mechanisms remain elusive. Here we present a source 660 million years after the Big Bang that displays singular properties: among the largest Hydrogen Balmer breaks reported at any redshift, broad multi-peaked H$β$ emission, and Balmer line absorption in multiple transitions. We model this source as a "black hole star" (BH*) where the Balmer break and absorption features are a result of extremely dense, turbulent gas forming a dust-free "atmosphere" around a supermassive black hole. This source may provide evidence of an early black hole embedded in dense gas -- a theoretical configuration proposed to rapidly grow black holes via super-Eddington accretion. Radiation from the BH* appears to dominate almost all observed light, leaving limited room for contribution from its host galaxy. We demonstrate that the recently discovered "Little Red Dots" (LRDs) with perplexing spectral energy distributions can be explained as BH*s embedded in relatively brighter host galaxies. This source provides evidence that black hole masses in the LRDs may be over-estimated by orders of magnitude -- the BH* is effectively dust-free contrary to the steep dust corrections applied while modeling LRDs, and the physics that gives rise to the complex line shapes and luminosities may deviate from assumptions underlying standard scaling relations.

Figures (9)

• Figure 1: JWST imaging and spectroscopy of MoM-BH*-1.Panel a.$3\times3"$ NIRCam and MIRI images of MoM-BH*-1 spanning 0.9-18 $\mu$m. The source is point-like and detected ($>3\sigma$) only in the F356W, F444W, and F770W bands, apparently disappearing in the bluer bands. Panel b. The NIRSpec prism spectrum (navy blue) shows the disappearance is due to an enormous Balmer break. Key spectral features such as the Balmer series are marked with dashed lines. Panel c. Inset $1"$ RGB image shows the almost identical slit positions with which the source was observed with the prism (panel b) and G395M grating (panel d). Panel d. Deep absorption features in H$\gamma$ and H$\beta$ are evident in the G395M grating spectra. The location of the central absorption is consistent across both H$\beta$, H$\gamma$ as well as across the prism and grating spectra. The systemic redshift is based on the [OIII]$4960, 5008$Å doublet. A representative draw from the emission line model posterior is plotted in dark orange (see Methods).
• Figure 2: The exceptional Balmer break strength of MoM-BH*-1.Panel a. Here we contrast MoM-BH*-1 against a quiescent galaxyWeibel24 and a Little Red DotFurtak24Furtak25Ji25BlackThunder that lie at a similar redshift ($z\approx7$) and display some of the strongest Balmer breaks reported yet ($\approx3$). The two wavelength windows we use to compute break strengths are highlighted in green -- these windows ([3620-3720]Å and [4000-4100]Å) are free of strong emission lines and are particularly suited for studying high redshift galaxiesWang24z8. The spectra shown here are normalized in the blue window -- flux in this window is detected at $>4.5\sigma$ for MoM-BH*-1. Panel b. We compare break strengths of quiescent galaxiesdegraaff24Weibel24QGStrait23, Little Red Dots with Balmer breaksFurtak24Kokorev24LRDbbreakLabbe24MonsterWang24z8, and stacks of star-forming galaxiesRoberts-Borsani24 at similar redshifts as MoM-BH*-1. The dashed line represents the maximum break strength expected for a dust-free stellar populationWang24z8 and a Chabrier initial mass functionChabrier03. MoM-BH*-1 displays the strongest Balmer break at these redshifts and lies well beyond this stellar population maximum.
• Figure 3: Comparison against a mock spectrum of a "black hole star" model. The schematic depicts an SMBH within a $\approx40$ AU "atmosphere" of dense gas -- the continuum is produced in hot regions close to the SMBH whereas absorption, scattering, and further emission occur in the dense gas atmosphere. The data (navy blue) are binned ($3\times$) to emphasize the continuum shape that our fiducial model (pink, with noise as per error spectrum) provides an excellent match to. The model is selected to reproduce the Balmer break strength and Balmer line EWs, while also matching the UV-faintness and MIRI long-wavelength data without having to invoke different mechanisms for lines and continuum. The narrow [OIII] emission and additional UV luminosity plausibly arise from the faint host galaxy, and are not captured by the BH* model (see Fig. 4). The excess flux around H$\infty$ is a Cloudy artifact due to modeling with a finite number of Hydrogen levelsJi25BlackThunder.
• Figure 4: Examples to illustrate Little Red Dots can be explained as BH*s embedded in comparably bright host galaxies. Panel a. MoM-BH*-1 (blue) lies close to a $M_{\rm{\star}}\approx10^{9.5} M_{\rm{\odot}}$ galaxy at the same redshift (silver). These sources are expected to merge in $\approx100$ MyrsPuskas25, and their superimposed spectrum (offset for clarity) and photometry ($1^{\prime\prime}\times1^{\prime\prime}$ NIRCam stamps) is shown in red. The combination bears a striking resemblance to the typical LRD -- a V-shaped SED, an inflection around $\rm{H}_{\infty}=3646\AA$, compactness in the rest-optical, a complex H$\beta$ profile with a broad component, and extended structure in the rest-UV. While the galaxy is dominant in the rest-UV, the BH* outshines it towards the rest-optical. Panel b. When the SED of a host galaxy is carefully subtracted from an LRD, the residual (orange) bears a striking resemblance to a BH*. Here, the host galaxy SED (silver) is a stack from the DAWN JWST archive selected to match the UV slope and weak [OIII] flux of this well-studied LRDFurtak24 (red). Interestingly, the stack resembles a "mini-quenched" galaxyStrait23Looser24Witten24, highlighting a possible connection between the evolution of supermassive black holes and their hosts. For the comparison in the right panel we similarly subtract a separate stack matched to MoM-BH*-1 (blue) to account for its very faint host galaxy.
• Figure Extended Data 5: Extended Data Figure 1 $|$ Simultaneous emission line fits to the prism (top) and grating (bottom) spectra. 100 draws from the posterior are shown in orange. The consistency of features across both modes inspires confidence in their reality. For example, the detailed structure of the H$\beta$ line -- a central absorber, extremely broad wings, and absorption even in the wings -- recurs in both panels. Similarly, the existence of narrow [OIII]5008Å emission would be difficult to discern in either mode by itself, but is recovered in the joint fit.