COSMOS-3D: Dense Circumnuclear Gas Across Black Hole Growth Phases at z ∼ 3

Abstract We report the discovery of two broad-line X-ray active galactic nuclei (AGNs) ( cid_414 and cid_947 ) at z ∼ 3 identified in the JWST Cycle 3 COSMOS-3D program using NIRCam F444W grism spectroscopy. Both exhibit prominent He i λ 10830 +Pa γ emission and absorption, indicative of circumnuclear dense gas that is traced in these systems. Complementary UV and optical spectroscopy in the COSMOS field provides Ly α , Si iv , and C iv measurements. Both sources are detected in MIRI F1000W, and cid_414 is also detected in F2100W, indicating hot dust emission. The two AGNs show distinct black hole and obscuration properties. The source cid_414 displays little red dot (LRD)-like, V-shaped spectral energy distribution with a turnover near the Balmer 4000 Å break and a narrow Ly α line with log L Ly α = 42.49 ± 0.01 erg s − 1 , with no additional metal lines detected. In contrast, cid_947 exhibits a higher He i λ 10830 absorption column density, larger X-ray–inferred N H , lower intrinsic 2–10 keV luminosity, and strong blueshifted features in He i , Si iv , and C iv absorption with velocity offsets exceeding 5000 km s −1 . Photoionization modeling implies gas densities of ∼10 9−10 cm −3 and sizes comparable to the broad-line region, consistent with dense gas envelopes predicted for LRDs. Together with previous detections of He i λ 10830 absorption in compact LRDs, these results suggest that dense circumnuclear gas is likely prevalent at high redshift and may regulate obscuration and black hole–host coevolution across AGN types.