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Recent observations have revealed an interesting active galactic nuclei (AGN)subclass that shows strong activity at large scales ($\sim1$ kpc) but weaker atsmall scales ($<10$ pc), suggesting a strong change in the mass accretion rateof the central engine in the past $10^{3-4}$ yr. We systematically search forsuch declining or fading AGN by cross-matching the SDSS type-1 AGN catalog at$z<0.4$, covering the [OIII]$\lambda5007$ emission line which is a tracer forthe narrow-line region (NLR) emission, with the WISE mid-infrared (MIR) catalogcovering the emissions from the dusty tori. Out of the 7,653 sources, we found57 AGN whose bolometric luminosities estimated from the MIR band are at leastone order of magnitude fainter than those estimated from the[OIII]$\lambda5007$ emission line. This luminosity declining AGN candidatepopulation shows four important properties: 1) the past AGN activity estimatedfrom the [OIII]$\lambda5007$ line reaches around the Eddington-limit, 2) morethan 30% of the luminosity declining AGN candidates show a large absolutevariability of $\Delta W1 > 0.45$ mag in the previous $\sim10$ yr at the WISE3.4 $\mu$m band, 3) the median ratio of $\log$([NII]$\lambda6584/\mathrm{H}\alpha\lambda6563)=-0.52$, suggesting a lower gas metallicity and/orhigher ionization parameter compared to other AGN populations. 4) the secondepoch spectra of the population indicate a spectral type change for 15% of thesources. This population provides insights on the possible connection betweenthe luminosity decline which started $\sim10^{3-4}$ yr ago and the decline inthe recent $10$ yr.

Finding of a population of active galactic nuclei showing a significant luminosity decline in the past $\sim 10^{3-4}$ yrs