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A Black Hole Star at Cosmic Noon: Extreme Balmer break, photospheric continuum, and broad absorption by thick winds in a Little Red Dot at z=1.7

Alberto Torralba, Jorryt Matthee, Andrea Weibel, Rohan P. Naidu, Yilun Ma, Aidan P. Cloonan, Aayush Desai, Anna de Graaff, Jenny E. Greene, Christian Kragh Jespersen, Ivan G. Kramarenko, Sara Mascia, Pascal A. Oesch, Wendy Q. Sun, Christina C. Williams

Abstract

Recent studies at high redshift have revealed an enigmatic class of Little Red Dots (LRDs) with extreme Balmer breaks, stronger than in any stellar atmosphere. However, it is unclear whether such objects exist at lower redshift, especially given the low number of LRDs reported at $z\lesssim 2$. Here we report the discovery of PAN-BH*-1, an LRD with an extreme Balmer break at $z=1.73$, identified from JWST/NIRCam pure-parallel imaging taken by the PANORAMIC survey, and confirmed by deep VLT/X-Shooter spectroscopy. The rest-optical to near-infrared spectral energy distribution of PAN-BH*-1 is consistent with a photospheric continuum with effective temperature $T_{\rm eff}\approx 4800$ K. The broad H$α$ emission line shows remarkably deep absorption, stronger than previously measured in any LRD. The absorption trough spans from $-520$ km/s to $+267$ km/s with respect to the systemic redshift. The presence of blue- and red-shifted absorption suggests complex dynamics of the obscuring gas along the line of sight. We speculate that the absorption trough can be produced by a thick wind launched from a thick, rotating photospheric disk, the latter being the source of the red optical continuum. While the source is unresolved in the rest-optical JWST data ($r_{\rm eff,UV}<47$ pc), the rest-NUV HST imaging shows an extended morphology with $r_{\rm eff,opt}=1.0^{+0.5}_{-0.3}$ kpc, that we interpret as a host galaxy with a stellar mass $\sim 10^8$ $M_\odot$, in line with the narrow H$α$ emission. The discovery of this object at cosmic noon highlights the feasibility of systematic searches for extreme LRDs with wide-area facilities such as Euclid and Roman.

A Black Hole Star at Cosmic Noon: Extreme Balmer break, photospheric continuum, and broad absorption by thick winds in a Little Red Dot at z=1.7

Abstract

Recent studies at high redshift have revealed an enigmatic class of Little Red Dots (LRDs) with extreme Balmer breaks, stronger than in any stellar atmosphere. However, it is unclear whether such objects exist at lower redshift, especially given the low number of LRDs reported at . Here we report the discovery of PAN-BH*-1, an LRD with an extreme Balmer break at , identified from JWST/NIRCam pure-parallel imaging taken by the PANORAMIC survey, and confirmed by deep VLT/X-Shooter spectroscopy. The rest-optical to near-infrared spectral energy distribution of PAN-BH*-1 is consistent with a photospheric continuum with effective temperature K. The broad H emission line shows remarkably deep absorption, stronger than previously measured in any LRD. The absorption trough spans from km/s to km/s with respect to the systemic redshift. The presence of blue- and red-shifted absorption suggests complex dynamics of the obscuring gas along the line of sight. We speculate that the absorption trough can be produced by a thick wind launched from a thick, rotating photospheric disk, the latter being the source of the red optical continuum. While the source is unresolved in the rest-optical JWST data ( pc), the rest-NUV HST imaging shows an extended morphology with kpc, that we interpret as a host galaxy with a stellar mass , in line with the narrow H emission. The discovery of this object at cosmic noon highlights the feasibility of systematic searches for extreme LRDs with wide-area facilities such as Euclid and Roman.

Paper Structure

This paper contains 18 sections, 9 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations
  3. Photometry and source selection
  4. VLT/X-Shooter spectroscopy
  5. Properties of PAN-BH*-1
  6. Spectral shape: a photospheric continuum with strong $\text{H}{\alpha}$ emission
  7. $\text{H}{\alpha}$ and $\text{H}{\beta}$ emission lines
  8. Spatial morphology
  9. Absorber kinematics
  10. Unstable gas flows?
  11. The case for the disk wind hypothesis
  12. Implications of rotating winds for the emission lines of LRDs
  13. Implications for the galaxy and black hole masses
  14. Properties of the host galaxy
  15. Black hole mass from photosphere models
  16. ...and 3 more sections

Figures (9)

  • Figure 1: SED of PAN-BH*-1Top: Cutouts from all the HST and JWST images in which PAN-BH*-1 is covered. It shows a remarkably compact morphology in all the wavelengths, resolved only in the HST F606W and F814W bands (Sect. \ref{['sec:morph']}). Bottom: Photometry from JWST/NIRCam (blue squares), HST/ACS (purple pentagons), and Spitzer/IRAC+MIPS (red hexagons, and red triangle for the 5$\sigma$ upper limit). The empty square is the F200W flux after subtracting the $\text{H}{\alpha}$ flux measured from X-Shooter spectroscopy. We show the spectrum of The Cliff for comparison (gray line), shifted to $z=1.73$ and normalized to the F150W flux of PAN-BH*-1. We also show the best-fitting Blackbody spectrum (blue dashed line) and the best model from the synthetic LRD atmosphere models from Liu2026, shifted to $z=1.73$ (green line), under-sampled a factor 500 for clarity.
  • Figure 2: Spectroscopic sample of LRDs by redshift and Balmer break strength. We plot the redshift and Balmer break strength of PAN-BH*-1, and the JWST sample from degraaff20251 (purple diamonds), and three local LRDs in lin20252, for comparison. We also highlight three sources with a particularly strong Balmer break: The Cliffdegraaff20255, MoM-BH* naidu2025, and CAPERS-LRDz9 taylor2025. The Balmer break strength of the JWST spectroscopic sample is computed as $f_{\nu, 4000-4100}/f_{\nu, 3620-3720}$, whereas the value for PAN-BH*-1 is directly obtained from the F115W/F814W photometry.
  • Figure 3: X-Shooter spectrum of $\text{H}{\alpha}$ and $\text{H}{\beta}$ of PAN-BH*-1 (blue). We compare to the spectrum of The Cliff (gray; data from JWST DDT #9433), normalized in each panel to the flux of PAN-BH*-1 in the range $v\in (-3000, -2000)~{\rm km\,s^{-1}}$. Due to the low S/N, the $\text{H}{\beta}$ spectrum of PAN-BH*-1 is re-binned to a coarser grid by a factor 5, after masking the most relevant sky lines.
  • Figure 4: $\text{H}{\alpha}$ spectrum, and best fit to our fiducial model. We show the X-Shooter $R\sim 5600$ spectrum of the $\text{H}{\alpha}$ line of PAN-BH*-1, along with the best-fit to the model described in Sect. \ref{['sec:ha_fit']}; total model (red solid line) and individual components (discontinuous colour lines). The red wing of the line is severely affected by telluric absorption, thus the large uncertainties.
  • Figure 5: Geometric configurations for the absorber. We illustrate three scenarios that could give rise to the observed Balmer absorption in PAN-BH*-1. In scenario (a), the obscuring agent is a thick screen of gas with a certain bulk velocity, and turbulent motions produce the broadening of the absorption trough. In (b), there are two (or more) absorbers with opposite velocities in the line-of-sight. These first two scenarios are dynamically unstable, therefore variability is expected in the absorption. Lastly, in (c) we observed an extended source through a disk wind with a rotational component ($\mathbf{v}_\phi$) in addition to the poloidal (non-azimuthal) velocity ($\mathbf{v}_p$). In the last scenario, the redshifted absorption is produced by stream lines that oppose the observer when projected along the line-of-sight, despite the fact that the gas is outflowing from the central source.
  • ...and 4 more figures