Compact Core, Extended Reach: A Bipolar Kiloparsec-scale Elongation in a Little Red Dot at z ≈ 5.5

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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 the Very Large Telescope/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 subkiloparsec 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 kiloparsecs on either side, traced by both the UV continuum and the rest-frame optical emission lines, with a bright [O iii ] clump-like structure ∼2 kpc to the southeast 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 ∼430 km s −1 , accompanied by a tentative N v λ 1238 detection at similar velocity. In narrowband imaging extracted from the MUSE integral-unit-field cube, both the blue Ly α peak and the tentative N v emission lean toward this same southeastern 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 kiloparsec scales.
Funding Information
  • NASA ∣ NASA Headquarters (NAS5-02015)

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