Compact Core, Extended Reach: A Bipolar kpc-Scale Elongation in a Little Red Dot at $z \approx 5.5$

Zhiyuan Ji, Yang Sun, Mauro Giavalisco, Yongda Zhu, George H. Rieke, Christina C. Williams, Michael V. Maseda, Jianwei Lyu, Marcia Rieke, Sandro Tacchella

First listed 2026-06-18 | Last updated 2026-06-16

Abstract

Little Red Dots (LRDs) appear extremely compact at rest-frame optical wavelengths, yet many show extended rest-frame UV morphology revealing more complex internal structure. We present a combined analysis of VLT/MUSE rest-frame UV integral-field spectroscopy and continuum-subtracted [O III], H$β$, and H$α$+[N II] emission-line maps from JWST/NIRCam imaging at sub-kpc resolution for LRD-204851 at $z=5.482$ in GOODS-S. We find that LRD-204851 hosts a remarkably thin, bipolar, elongated structure passing through the optical continuum centroid and extending several kpc on either side, traced by both the UV continuum and the rest-frame optical emission lines, with a bright [O III] clump-like structure $\sim$2 kpc to the south-east of the centroid. The MUSE observations reveal a double-peaked Ly$α$ profile, with a broad and bright near-systemic red peak and a relatively faint peak blueshifted by $\sim$430 km s$^{-1}$, accompanied by a tentative N V $λ1238$ detection at similar velocity. In narrow-band imaging extracted from the MUSE IFU cube, both the blue Ly$α$ peak and the tentative N V emission lean toward this same south-eastern direction. Independently, radiative-transfer modeling of the integrated Ly$α$ profile favors a biconical low-column-density cavity in a dense, slowly expanding neutral envelope, in support of the bipolar geometry traced by the line maps. Together, these results suggest that the elongated emission of LRD-204851 is connected to radiation and/or gas flow from its central engine through a low-column-density channel with a small opening angle that may trace either a slow outflow or a quasi-static ionization cone. LRD-204851 is one of the first LRDs where the central engine's impact on its host galaxy is potentially directly observable on kpc scales.

Short digest

Using VLT/MUSE rest-UV IFU data together with sub-kpc JWST/NIRCam emission-line maps, this paper dissects the z=5.482 little red dot LRD-204851 and shows that its apparently compact optical core sits inside a much larger bipolar structure. The UV continuum and the continuum-subtracted [O III], Hβ, and Hα+[N II] emission all trace a strikingly thin elongation extending several kpc through the continuum centroid, with a bright [O III] clump about 2 kpc to the southeast. MUSE adds a double-peaked Lyα profile, a tentative N V λ1238 feature, and southeast-leaning blue Lyα emission, while Lyα radiative-transfer modeling favors a biconical low-column-density cavity embedded in a dense, slowly expanding neutral envelope. Taken together, LRD-204851 emerges as one of the clearest early cases where an LRD central engine may be directly imprinting itself on host-galaxy gas on kpc scales, likely through either a slow outflow or a quasi-static ionization cone.

Key figures to inspect

• Figure 1. Use this as the orientation figure because it establishes the core observational tension of the paper: LRD-204851 looks like a compact red point source in the rest-frame optical continuum, yet the rest-frame UV continuum and line-sensitive RGB views already reveal a southeast to northwest elongation and structured nebular emission. It also ties the morphology to the source’s LRD identity through the V-shaped continuum and previously confirmed broad Hα.
• Figure 2. This figure captures the spectral evidence that drives the physical interpretation. The double-peaked Lyα profile, with a bright near-systemic red peak plus a fainter blueshifted component, and the marked tentative N V λ1238 feature are the key rest-UV diagnostics that motivate the biconical escape-channel picture and connect directly to the radiative-transfer modeling.
• Figure 4. This is the central spatial comparison figure because it puts the JWST optical line maps and the MUSE Lyα and tentative N V contours on the same footing. Readers can directly see that the [O III], Hβ, and Hα+[N II] emission participate in the same elongated geometry that passes through the continuum centroid, while the southeastern side hosts the especially prominent [O III] structure emphasized in the abstract.
• Figure 5. Recommend this figure for the quantitative check that the extended structure is not just a PSF artifact. The radial profiles show that Hα+[N II] is broadly PSF-like while [O III] is measurably more extended, which strengthens the claim that at least part of the ionized gas distribution is genuinely resolved on galactic scales.
• Figure 9. This is the best synthesis figure for the paper’s bottom-line interpretation because it compares alternative Lyα radiative-transfer geometries and shows why the Bicone_X_Slab_Out model is preferred. It turns the morphological intuition from the imaging into a tested physical statement: the observed Lyα profile is most consistent with a low-column-density biconical cavity in a dense neutral medium.