The [NII] 205 μm line emission from high-z submillimeter galaxies and quasar host galaxies

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, I-40129 Bologna, Italy
² Indian Institute of Science Education and Research, Govt. ITI Building Engg. School Junction, Brahmapur, Odisha 760010, India
³ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38400 Saint-Martin-d’Hères, France
⁴ Sorbonne Université, CNRS UMR 7095, Institut d’Astrophysique de Paris, 98bis bvd Arago, 75014 Paris, France
⁵ Department of Physics, Winona State University, Winona, MN 55987, USA
⁶ Argelander-Institute for Astronomy, University of Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
⁸ Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333 CA Leiden, Netherlands
⁹ I. Physikalisches Institut, Universitát zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
¹⁰ Gemini Observatory, NSF’s NOIRLab, 670 N A’ohoku Place, Hilo, Hawai’i 96720, USA
¹¹ Max-Planck Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany

^⋆ Corresponding author; saimuralikolupuri@gmail.com

Received: 26 September 2024
Accepted: 31 January 2025

Abstract

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.