A Decade-scale Ionization Echo of Black Hole Accretion in Galactic Nuclei

Abstract Circumnuclear structures in active galactic nuclei (AGN) respond to accretion variability over different spatial and temporal scales, but whether compact narrow-line gas responds on decade timescales has not been systematically established. We use repeat Sloan Digital Sky Survey and Dark Energy Spectroscopic Instrument spectroscopy of 473 variability-selected galactic nuclei with evidence for recent nuclear brightening. We identify 19 sources whose narrow [O III ] λ 5007 flux increased by more than a factor of 2 over rest-frame baselines of 12.0–20.4 yr, which we term narrow-line ionization echoes (NIEs). In equal-weight stacked spectra, [O III ] increased by 229% ± 22%, while lower-ionization lines remained nearly unchanged or declined, indicating an ionization-dependent response rather than uniform spectral rescaling. The NIE hosts also moved coherently toward higher excitation in optical diagnostic diagrams, with mean increases of 0.46 ± 0.18 dex in log ( [ O III ] / H β ) and 0.58 ± 0.13 dex in log ( [ O III ] / [ O II ] ) . Photoionization models reproduce this evolution with a higher ionization parameter and a harder AGN-like ionizing spectrum at fixed gas abundance. For 13 sources with well-defined mid-infrared (MIR) outbursts, the integrated W1+W2 excess energy correlates strongly with the absolute [O III ] luminosity change ( r = 0.87), linking the dust and narrow-line responses to the same nuclear event. The MIR timescale and spectroscopic baseline constrain the variable [O III ]-emitting gas to characteristic scales of 1.5–5.2 pc. Compact narrow-line gas therefore appears to form an intermediate, decade-scale response layer between the infrared dust echo and the long-lived extended narrow-line region.

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Journal
The Astrophysical Journal
Published
2026-11-04
DOI
https://doi.org/10.3847/1538-4357/aeae2b
Primary Topic
Galaxies: Formation, Evolution, Phenomena
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article
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article

A Decade-scale Ionization Echo of Black Hole Accretion in Galactic Nuclei

Leah K. Morabito, Matthew J. Temple, Victoria A. Fawcett, Claire Greenwell et al.
The Astrophysical Journal
Galaxies: Formation, Evolution, Phenomena
article

A Decade-scale Ionization Echo of Black Hole Accretion in Galactic Nuclei

Leah K. Morabito, Matthew J. Temple, Victoria A. Fawcett, Claire Greenwell, Bin Luo, Swayamtrupta Panda, Marconi Alessandro, Shengxiu Sun, Pu Du, Shuo Zhai, David M. Alexander, Yan-Rong Li, Chen Hu, Feige Wang, Wei-Jian Guo, Cheng Cheng, Jian-Min Wang, Zhi-Qiang Chen, Jinyi Yang
article en

Abstract

Abstract Circumnuclear structures in active galactic nuclei (AGN) respond to accretion variability over different spatial and temporal scales, but whether compact narrow-line gas responds on decade timescales has not been systematically established. We use repeat Sloan Digital Sky Survey and Dark Energy Spectroscopic Instrument spectroscopy of 473 variability-selected galactic nuclei with evidence for recent nuclear brightening. We identify 19 sources whose narrow [O III ] λ 5007 flux increased by more than a factor of 2 over rest-frame baselines of 12.0–20.4 yr, which we term narrow-line ionization echoes (NIEs). In equal-weight stacked spectra, [O III ] increased by 229% ± 22%, while lower-ionization lines remained nearly unchanged or declined, indicating an ionization-dependent response rather than uniform spectral rescaling. The NIE hosts also moved coherently toward higher excitation in optical diagnostic diagrams, with mean increases of 0.46 ± 0.18 dex in log ( [ O III ] / H β ) and 0.58 ± 0.13 dex in log ( [ O III ] / [ O II ] ) . Photoionization models reproduce this evolution with a higher ionization parameter and a harder AGN-like ionizing spectrum at fixed gas abundance. For 13 sources with well-defined mid-infrared (MIR) outbursts, the integrated W1+W2 excess energy correlates strongly with the absolute [O III ] luminosity change ( r = 0.87), linking the dust and narrow-line responses to the same nuclear event. The MIR timescale and spectroscopic baseline constrain the variable [O III ]-emitting gas to characteristic scales of 1.5–5.2 pc. Compact narrow-line gas therefore appears to form an intermediate, decade-scale response layer between the infrared dust echo and the long-lived extended narrow-line region.

The Astrophysical JournalVol. 1011(2)
European Southern Observatory (DE), Nanjing Normal University (CN), Durham University (GB), Chinese Academy of Sciences (CN), Peking University (CN), University of Michigan (US), Arcetri Astrophysical Observatory (IT), European Southern Observatory (CL), NSF NOIRLab (US), University of Chinese Academy of Sciences (CN), International Gemini Observatory (US), University of Florence (IT), National Institute for Astrophysics (IT), Nanjing University (CN)
National Science Foundation, U.S. Department of Energy, Gordon and Betty Moore Foundation, UK Research and Innovation, Ministerio de Ciencia, Innovación y Universidades, National Natural Science Foundation of China, Commissariat à l'Énergie Atomique et aux Énergies Alternatives, Eötvös Loránd Tudományegyetem, Office of Science, Science Mission Directorate, Science and Technology Facilities Council, Agencia Estatal de Investigación, Division of Astronomical Sciences, Planetary Science Division
Openalex Percentile: Top 24%
Galaxies: Formation, Evolution, Phenomena
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