Near-infrared emission line diagnostics for AGN from the local Universe to z ∼ 3^⋆

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
e-mail: antonello.calabro@inaf.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ Aix-Marseille Univ., CNRS, CNES, LAM Marseille, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
⁶ Université Paris-Saclay, Université Paris-Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁷ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raul Bitrán 1305, La Serena 2204000, Chile
⁸ Departamento de Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La Serena 1720236, Chile
⁹ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712-1205, USA
¹⁰ Dipartimento di Fisica e Astronomia “G. Galilei”, Universitá di Padova, Via Marzolo 8, 35131 Padova, Italy
¹¹ Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹² George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A& M University, College Station, TX 77843-4242, USA
¹³ University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA 01003-9305, USA
¹⁴ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
¹⁵ Institute of Physics, Laboratory of Galaxy Evolution, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
¹⁶ Laboratory for Multiwavelength Astrophysics, School of Physics and Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Drive, Rochester, NY 14623, USA
¹⁷ Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹⁸ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹⁹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
²⁰ ESA/AURA Space Telescope Science Institute, Baltimore, USA
²¹ Department of Physics, University of Connecticut, 196 Auditorium Road, Unit 3046, Storrs, CT 06269, USA
²² Department of Physics and Astronomy, University of California, 900 University Ave, Riverside, CA 92521, USA
²³ Astronomy Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK
²⁴ Institute of Space Sciences and Astronomy, University of Malta, Msida 2080, Malta
²⁵ Astrophysics Science Division, NASA Goddard Space Flight Center, 8800 Greenbelt Rd, Greenbelt, MD 20771, USA
²⁶ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla, 4059 Valparaíso, Chile

Received: 14 June 2023
Accepted: 5 September 2023

Abstract

Optical rest-frame spectroscopic diagnostics are usually employed to distinguish between star formation and active galactic nucleus (AGN) powered emission. However, this method is biased against dusty sources, hampering a complete census of the AGN population across cosmic epochs. To mitigate this effect, it is crucial to observe at longer wavelengths in the rest-frame near-infrared (near-IR), which is less affected by dust attenuation and can thus provide a better description of the intrinsic properties of galaxies. AGN diagnostics in this regime have not been fully exploited so far, due to the scarcity of near-IR observations of both AGN and star-forming galaxies, especially at redshifts higher than 0.5. Using Cloudy photoionization models, we identified new AGN – star formation diagnostics based on the ratio of bright near-IR emission lines, namely [SIII] 9530 Å, [CI] 9850 Å, [PII] 1.188 μm, [FeII] 1.257 μm, and [FeII] 1.64 μm to Paschen lines (either Paγ or Paβ), providing simple, analytical classification criteria. We applied these diagnostics to a sample of 64 star-forming galaxies and AGN at 0 ≤ z ≤ 1, and 65 sources at 1 ≤ z ≤ 3 recently observed with JWST-NIRSpec in CEERS. We find that the classification inferred from the near-IR is broadly consistent with the optical one based on the BPT and the [SII]/Hα ratio. However, in the near-IR, we find ∼60% more AGN than in the optical (13 instead of eight), with five sources classified as “hidden” AGN, showing a larger AGN contribution at longer wavelengths, possibly due to the presence of optically thick dust. The diagnostics we present provide a promising tool to find and characterize AGN from z = 0 to z ≃ 3 with low- and medium-resolution near-IR spectrographs in future surveys.