COSMOS-3D: Dense Circumnuclear Gas across Black Hole Growth Phases at z ~ 3

Zi-Jian Li, Siwei Zou, Jianwei Lyu, Jaclyn B. Champagne, Jia-Sheng Huang, Cheng Cheng, Shuqi Fu, Zijian Zhang, Danyang Jiang, Khee-Gan Lee, Feige Wang, Xiaohui Fan, Jinyi Yang, Ruancun Li, Hollis B. Akins, Fuyan Bian, Y. Sophia Dai, Andreas L. Faisst, Luis C. Ho, Kohei Inayoshi, Linhua Jiang, Xiangyu Jin, Koki Kakiichi, Jeyhan S. Kartaltepe, Zihao Li, Weizhe Liu, Jan-Torge Schindler, Wei Leong Tee

First listed 2025-12-01 | Last updated 2026-03-20

Abstract

We report the discovery of two broad-line X-ray 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 HeI+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 dots (LRD)-like V-shape spectra energy distribution (SED) shape with a turnover near the Balmer 4000 Å break, and a narrow Ly$α$ line with $\log L_{\rm Lyα}=42.49\pm0.01~\mathrm{erg\ s^{-1}}$, with no additional metal lines detected. In contrast, cid_947 exhibits a higher HeI absorption column density, larger X-ray--inferred $N_{\rm H}$, lower intrinsic 2--10 keV luminosity, and strong blueshifted features in He I, Si IV, and C IV absorption with velocity offsets exceeding $5000~\mathrm{km\ s^{-1}}$. Photoionization modeling implies gas densities of $\sim10^{9-10} \mathrm{cm^{-3}}$ and sizes comparable to the broad-line region, consistent with dense gas envelopes predicted for LRDs. Together with previous detections of HeI absorption in compact little red dots, these results suggest that dense circumnuclear gas is likely prevalent at high redshift and may regulate obscuration and black hole--host co-evolution across AGN types.

Short digest

Using JWST/COSMOS-3D F444W grism spectroscopy, the authors identify two broad-line X-ray AGNs at z ~ 3 (cid_414, cid_947) with prominent He I 10830 + Paγ emission plus absorption and mid-IR detections signaling hot dust. The pair spans distinct regimes: cid_414 shows an LRD-like V-shaped SED with a Balmer-break turnover and a narrow Lyα line (log LLyα = 42.49 ± 0.01 erg s^-1) with no metal lines, while cid_947 has higher He I column and X-ray–inferred NH, lower intrinsic 2–10 keV luminosity, and >5000 km s^-1 blueshifted He I/Si IV/C IV absorption. Simple photoionization modeling favors very dense gas (n ~ 10^9–10^10 cm^-3) with BLR-scale extents, and SED/host decomposition indicates a more massive black hole but lower SFR in cid_947 than in cid_414. Together with prior He I detections in compact LRDs, the results argue that dense circumnuclear gas is prevalent at high redshift and may regulate obscuration and black hole–host co-evolution across AGN types.

Key figures to inspect

• Figure 1: Inspect the F444W grism He I 10830–Paγ complex and Gaussian decompositions; compare emission plus absorption profiles between cid_414 and cid_947 to see the deeper, blueshifted (~>5000 km s^-1) absorption in cid_947 and the relative broad/narrow components that set column and kinematics.
• Figure 2: Compare AGNfitter vs CIGALE SED fits and imaging; for cid_414, note the LRD-like V-shaped SED with a turnover near the Balmer break and the additional MIRI F2100W hot-dust detection, while the two-Sérsic host components contextualize the nuclear contribution; for cid_947, note weaker hot-dust and lower SFR i…
• Figure 3: Place each source on SFR–M* versus the Whitaker main sequence and on M*–MBH versus local relations; this shows cid_947 hosting the more massive BH but lower SFR than cid_414 and situates both relative to other JWST AGN/LRD samples and proposed BH–host growth tracks.
• Figure 4: Read off each AGN’s position in NH versus Eddington ratio against effective Eddington limits (with/without IR trapping) to assess whether radiation pressure can expel the dusty gas; use this to interpret cid_947’s heavier obscuration and whether either source is nearing a blowout phase.