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

First listed 2026-03-30 | Last updated 2026-04-02

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.

Short digest

Reports the discovery of PAN-BH*-1, a Little Red Dot at z=1.73 selected from JWST/NIRCam PANORAMIC imaging and confirmed with deep VLT/X-Shooter spectroscopy. Its rest-optical–NIR SED is consistent with a photospheric continuum (Teff ≈ 4800 K) and an extreme Balmer break; the Hα profile shows exceptionally deep, broad absorption spanning −520 to +267 km/s. The source is unresolved in JWST rest-optical (r_eff,UV < 47 pc) but HST rest-NUV reveals a host (r_eff,opt ≈ 1.0 kpc) with M★ ~ 10^8 M⊙ and narrow Hα consistent with the galaxy. The authors interpret the features as a thick wind from a thick, rotating photospheric disk, showing that extreme LRDs persist to cosmic noon and are accessible to Euclid/Roman searches.

Key figures to inspect

• Figure 1 (SED and cutouts): Check how the F115W/F814W photometry quantifies the extreme Balmer break, compare the Teff ≈ 4800 K blackbody and the synthetic LRD-atmosphere model, and note the compact morphology in JWST bands versus the resolved HST F606W/F814W.
• Figure 2 (z vs Balmer-break strength): See where PAN-BH*-1 sits relative to The Cliff, MoM-BH*, and CAPERS-LRDz9; this locates an extreme-break object at z ≈ 1.7 and illustrates continuity with the high‑z LRD population.
• Figure 3 (X-Shooter spectrum vs The Cliff): Inspect the Balmer-series behavior and continuum shape; verify the depth/width of the Hα absorption compared to The Cliff and note wavelength regions masked by skylines and low S/N rebinning.
• Figure 4 (Hα line modeling): Examine how the model decomposes broad emission, deep absorption, and the narrow host component; use the fitted velocities and trough depth to assess a thick, rotating wind, keeping in mind telluric-affected red wing uncertainties.