Finding of a Population of Active Galactic Nuclei Showing a Significant Luminosity Decline in the Past ∼103–104 yr

Recent observations have revealed an interesting active galactic nucleus (AGN) subclass that shows strong activity at large scales (∼1 kpc) but weaker at small scales (3–104 yr. We systematically search for such declining or fading AGNs by cross-matching the Sloan Digital Sky Survey type 1 AGN catalog at z < 0.4, covering the [O iii] λ5007 emission line, which is a tracer for the narrow-line region emission, with the Wide-field Infrared Survey Explorer (WISE) mid-infrared (MIR) catalog covering the emissions from the dusty tori. Out of the 7653 sources, we found 57 AGNs whose bolometric luminosities estimated from the MIR band are at least one order of magnitude fainter than those estimated from the [O iii] λ5007 emission line. This luminosity-declining AGN candidate population shows four important properties: (1) the past AGN activity estimated from the [O iii] λ5007 line reaches approximately the Eddington limit; (2) more than 30% of the luminosity-declining AGN candidates show a large absolute variability of ΔW1 > 0.45 mag in the previous ∼10 yr at the WISE 3.4 μm band; (3) the median ratio of log([N ii] λ6584/Hα λ6563) = −0.52, suggesting a lower gas metallicity and/or higher ionization parameter compared to other AGN populations; and (4) the second-epoch spectra of the population indicate a spectral type change for 15% of the sources. This population provides insights on the possible connection between the luminosity decline that started ∼103–104 yr ago and the decline in the recent 10 yr.