Optical and mid-infrared line emission in nearby Seyfert galaxies

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129 Bologna, Italy
e-mail: anna.feltre@inaf.it, feltre.anna@gmail.com
² Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch, 7701 South Africa
³ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁴ South African Astronomical Observatory, PO Box 9, Observatory 7935, Cape Town, South Africa
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ Dipartimento di fisica e astronomia, Universitá di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁷ INAF – Osservatorio astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
⁹ Sub-department of Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH UK
¹⁰ Southern African Large Telescope, PO Box 9, Observatory 7935, Cape Town, South Africa
¹¹ Centre for Astrophysics Research, Department of Physics, Astronomy & Mathematics, University of Hertfordshire, Hatfield, AL10 9AB UK
¹² INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121, Milano, Italy & Via Bianchi 46, 23807 Merate, Italy
¹³ UNIVAP – Universidade do Vale do Paraíba. Av. Shishima Hifumi, 2911, CEP: 12244-000, São José dos Campos, SP, Brazil
¹⁴ Institut de Physique, Laboratoire d’astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, Chemin Pegasi 51, 1290 Versoix, Switzerland
¹⁵ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
¹⁶ Department of Physics & Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC, 29634 USA
¹⁷ Steward Observatory, University of Arizona, 933 N Cherry Ave., Tucson, AZ, 85721 USA
¹⁸ Dipartimento di Fisica e Astronomia, Universitá degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
¹⁹ The Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch, 7701 South Africa
²⁰ The Inter-University Institute for Data Intensive Astronomy, Department of Physics & Astronomy, University of the Western Cape, 7535 Bellville, Cape Town, South Africa
²¹ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
²² Observatorio Astronómico Nacional, C/ Alfonso XII 3, 28014 Madrid, Spain
²³ École Normale Supérieure, CNRS, UMR 8023, Université PSL, Sorbonne Université, Université de Paris, 75005 Paris, France

Received: 21 November 2022
Accepted: 3 May 2023

Abstract

Line ratio diagnostics provide valuable clues as to the source of ionizing radiation in galaxies with intense black hole accretion and starbursting events, such as local Seyfert galaxies or galaxies at the peak of their star formation history. We aim to provide a reference joint optical and mid-IR line ratio analysis for studying active galactic nucleus (AGN) identification via line-ratio diagnostics and testing predictions from photoionization models. We first obtained homogenous optical spectra with the Southern Africa Large Telescope for 42 Seyfert galaxies with available Spitzer/IRS spectroscopy, along with X-ray to mid-IR multiband data. After confirming the power of the main optical ([O III]λ5007) and mid-IR ([Ne V]14.3 μm, [O IV]25.9 μm, [Ne III]15.7 μm) emission lines in tracing AGN activity, we explored diagrams based on ratios of optical and mid-IR lines by exploiting photoionization models of different ionizing sources (AGN, star formation, and shocks). We find that pure AGN photoionization models are good at reproducing observations of Seyfert galaxies with an AGN fractional contribution to the mid-IR (5 − 40 μm) continuum emission larger than 50 per cent. For targets with a lower AGN contribution, even assuming a hard ionizing field from the central accretion disk (F_ν ∝ ν^α, with α ≈ −0.9), these same models do not fully reproduce the observed mid-IR line ratios. Mid-IR line ratios such as [Ne V]14.3 μm/[Ne II]12.8 μm, [O IV]25.9 μm/[Ne II]12.8 μm, and [Ne III]15.7 μm/[Ne II]12.8 μm show a dependence on the AGN fractional contribution to the mid-IR, unlike optical line ratios. An additional source of ionization, either from star formation or radiative shocks, can help explain the observations in the mid-IR. While mid-IR line ratios are good tracers of the AGN activity versus star formation, among the combinations of optical and mid-IR diagnostics in line-ratio diagrams, only those involving the [O I]/Hα ratio are promising diagnostics for simultaneously unraveling the relative roles of AGN, star formation, and shocks. A proper identification of the dominant source of ionizing photons would require the exploitation of analysis tools based on advanced statistical techniques as well as spatially resolved data.