A direct black hole mass measurement in a Little Red Dot at the Epoch of Reionization (2508.21748v1)

Abstract: Recent discoveries of faint active galactic nuclei (AGN) at the redshift frontier have revealed a plethora of broad \Halpha emitters with optically red continua, named Little Red Dots (LRDs), which comprise 15-30\% of the high redshift broad line AGN population. Due to their peculiar spectral properties and X-ray weakness, modeling LRDs with standard AGN templates has proven challenging. In particular, the validity of single-epoch virial mass estimates in determining the black hole (BH) masses of LRDs has been called into question, with some models claiming that masses might be overestimated by up to 2 orders of magnitude, and other models claiming that LRDs may be entirely stellar in nature. We report the direct, dynamical BH mass measurement in a strongly lensed LRD at $z = 7.04$. The combination of lensing with deep spectroscopic data reveals a rotation curve that is inconsistent with a nuclear star cluster, yet can be well explained by Keplerian rotation around a point mass of 50 million Solar masses, consistent with virial BH mass estimates from the Balmer lines. The Keplerian rotation leaves little room for any stellar component in a host galaxy, as we conservatively infer $M_{\rm BH}/M_{*}>2$. Such a ''naked'' black hole, together with its near-pristine environment, indicates that this LRD is a massive black hole seed caught in its earliest accretion phase.

• The paper presents a direct dynamical measurement of a ~5×10^7 M⊙ black hole using the Keplerian rotation of narrow Hα emission in a high-redshift Little Red Dot.
• It employs high-resolution JWST/NIRSpec IFS data and spectroastrometry to robustly distinguish between point mass and extended mass models.
• The results reveal an extreme BH-to-stellar mass ratio in a chemically pristine environment, supporting heavy seed formation scenarios in the early universe.

Direct Dynamical Measurement of a Black Hole in a Little Red Dot at $z=7.04$

Introduction

This paper presents a direct dynamical measurement of the black hole (BH) mass in Abell2744-QSO1, a strongly lensed, high-redshift active galactic nucleus (AGN) classified as a "Little Red Dot" (LRD) at $z=7.04$. LRDs are characterized by optically red continua, blue UV slopes, and X-ray weakness, complicating standard AGN template fitting and challenging the reliability of single-epoch virial BH mass estimates. The authors utilize deep JWST/NIRSpec integral field spectroscopy (IFS) and gravitational lensing to resolve the kinematics of the narrow H$\alpha$ emission, enabling a robust dynamical mass measurement. The analysis demonstrates that the observed rotation curve is inconsistent with any plausible extended stellar or dark matter distribution, and is best explained by Keplerian rotation around a point mass, yielding a BH mass of $M_{\rm BH} \sim 5 \times 10^7~M_\odot$. The inferred BH-to-stellar mass ratio $M_{\rm BH}/M_* > 2$ is orders of magnitude above local scaling relations, indicating a "naked" black hole in a chemically pristine environment.

Observational Data and Kinematic Mapping

The paper leverages high-resolution NIRSpec IFS data, resampled to 0.02" pixel scale, focusing on the narrow H$\alpha$ emission after subtraction of broad line and continuum components. The spatial and kinematic maps reveal a velocity gradient of $\sim 10$ km s$^{-1}$ across $\sim 200$ pc, with the emission resolved into two beams due to lensing magnification and JWST's spatial resolution.

Figure 1: H$\alpha$ narrow line emission and kinematics maps of QSO1, showing surface flux, velocity field, and velocity dispersion $\sigma_v$.

The velocity field is aligned with the IFU slicer, ruling out calibration artifacts. The narrow line region is spatially extended, while the continuum remains unresolved, supporting the AGN classification.

Rotation Curve Construction and Spectroastrometry

The rotation curve is sampled by binning spaxels at 50, 100, and 150 pc from the dynamical center, with velocities corrected for dispersion support. To probe sub-beam scales, spectroastrometric techniques are employed, measuring centroid separations in velocity channels. The separation of $24.9 \pm 9.4$ pc between red and blue wings yields a spectroastrometric radius $r_{\rm spec} = 12.5 \pm 4.7$ pc and an enclosed mass lower limit $\log(M_{\rm spec}/M_\odot) = 6.90 \pm 0.23$ (uncorrected for inclination).

Figure 2: Spectroastrometry on the narrow line, showing centroid locations and the extracted line profile.

Dynamical Modeling: Point Mass vs. Extended Distributions

The authors fit the rotation curve with both a point mass (Keplerian) and extended mass models (Milky Way-like nuclear star cluster, Plummer sphere, NFW dark matter profile). The point mass model yields $\log(M_{\rm BH}/M_\odot) = 6.94 \pm 0.15$ with $\chi^2_R = 0.8$, while the NSC model is strongly disfavored ($\chi^2_R = 3.8$) and collapses to a point mass when the scale radius is left free. The Plummer sphere and NFW models similarly reduce to unphysically compact configurations, indicating that any extended mass component is subdominant at $<200$ pc scales.

Figure 3: Direct dynamical measurements of the black hole mass, showing the rotation curve and residuals for point mass and NSC models.

Figure 4: Comparison between QSO1 and star cluster observations on the mass-radius plane, demonstrating the implausibility of a NSC in QSO1.

Robustness: 3D Kinematic Modeling and Systematics

To account for instrumental effects, the MOKA3D framework is used for 3D kinematic modeling, incorporating PSF, inclination, and flux distribution. The best-fit model is Keplerian rotation around a point mass with $\log(M_{\rm BH}/M_\odot) = 7.7 \pm 0.3$ and inclination $52^\circ \pm 2^\circ$, consistent with spectroastrometric lower limits. Non-parametric disk models yield consistent inclination estimates across independent rings.

Figure 5: MOKA3D inclination constraints, showing agreement between parametric and non-parametric disk models.

Residuals for extended mass models remain systematically high, and biconical outflow models fail to reproduce the observed kinematics ($\chi^2_R = 9.6$), confirming the dominance of the central point mass.

Figure 6: Residuals of a fit assuming a biconical outflow, demonstrating poor agreement compared to rotating disk models.

Comparison with Virial and Alternative BH Mass Estimates

The direct dynamical BH mass is compared with single-epoch virial estimates and alternative broadening scenarios (electron scattering, Balmer scattering). The dynamical and virial masses are consistent, validating the use of virial relations for LRDs at $z=7$. Electron scattering models, while fitting the line profile, yield BH masses nearly 2 dex lower, violating physical constraints.

Figure 7: Summary of BH mass estimates for QSO1, comparing direct, MOKA3D, virial, and electron scattering scenarios.

Figure 8: Fits to the line assuming electron scattering, showing good spectral fit but a BH mass 2 dex below dynamical measurements.

Host Galaxy Properties and Stellar Mass Constraints

Optical and UV continuum analysis, combined with dynamical modeling, constrains the stellar mass to $M_* < 2 \times 10^7~M_\odot$, yielding $M_{\rm BH}/M_* > 2$. This ratio is three orders of magnitude above local scaling relations, placing QSO1 at the extreme tail of overmassive BHs identified by JWST.

Figure 9: Location of QSO1 on the $M_{\rm BH}$ vs. $M_*$ plane, highlighting its extreme overmassiveness relative to local relations.

Implications for Black Hole Seeding and Early Growth

The observed properties of QSO1—high BH mass, negligible stellar component, and near-pristine environment—are incompatible with standard stellar or extended mass scenarios. The only viable formation channels are "heavy seed" models: direct collapse black holes (DCBHs), primordial black holes (PBHs), or intermediate "Not-Quite-Primordial Black Holes" (NQPBHs). However, DCBH models typically predict $M_{\rm BH}/M_{\rm dyn} < 0.1$, more than 1 dex below the observed lower limit. PBH scenarios require significant accretion or merging to reach the observed mass, and the system's low metallicity provides some support for this channel.

Conclusion

This work provides the first direct dynamical measurement of a supermassive black hole in a Little Red Dot at the Epoch of Reionization, robustly ruling out alternative explanations based on extended stellar or dark matter distributions. The consistency between dynamical and virial mass estimates validates the use of virial relations for LRDs at $z=7$. The extreme $M_{\rm BH}/M_*$ ratio and chemically pristine environment indicate that QSO1 is a massive black hole seed in its earliest accretion phase, with implications for models of early black hole formation and growth. Future high-resolution spectroscopic surveys of lensed high-$z$ AGN will be essential to further constrain the demographics and formation channels of the first supermassive black holes.