2025A&A...695A.201K

We present [NII] 205 μm fine structure line observations of three submillimeter galaxies (SMGs) and three quasar host galaxies (QSOs) at 4 ≲ z ≲ 6 using the Institut de radioastronomie millimétrique (IRAM) interferometer. The [NII] emission is detected in three sources, and we report detections of the underlying dust continuum emission in all sources. The observed [NII]-to-infrared luminosity ratio spans at least 0.5 dex for our sources. Comparing our estimates with sources detected in the [NII] 205 μm at similar redshifts shows that the overall [NII]-to-IR luminosity ratio spans over 1 dex in magnitude from L_[NII]/L_IR ∼ 10^–4-10^–5 and follows the trend of the so-called [NII] fine structure line deficit observed in (ultra)-luminous IR galaxies in the local Universe. The [CII]-to-[NII] luminosity ratio is greater than ten for most of our sources, indicating that the bulk of the [CII] 158 μm line emission (f([CII]^PDR) > 75%) arises from the neutral medium. From our analysis, we do not find significant differences in the [NII] 205 μm emission and the respective ratios between SMGs and QSOs, suggesting a negligible contribution to the boosting of [NII] 205 μm emission due to the active galactic nucleus (AGN) photoionization. Future investigations involving other fine structure lines and optical diagnostics will provide further insight into a suite of ionized medium properties and reveal the diversity between AGN and non-AGN environments.