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

TL;DR

This study identifies a population of AGN that experienced significant luminosity declines over the past 10,000 years, revealing insights into AGN evolution and variability through multi-wavelength analysis.

Contribution

It presents the first systematic search for declining AGN using SDSS and WISE data, uncovering properties of AGN with recent luminosity drops and spectral changes.

Findings

57 AGN candidates with luminosity decline over 10x from MIR and [OIII] estimates

Over 30% show large variability in WISE 3.4 μm band in last 10 years

15% exhibit spectral type changes in second epoch spectra

Abstract

Recent observations have revealed an interesting active galactic nuclei (AGN) subclass that shows strong activity at large scales ($\sim1$ kpc) but weaker at small scales ($<10$ pc), suggesting a strong change in the mass accretion rate of the central engine in the past $10^{3-4}$ yr. We systematically search for such 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 for the narrow-line region (NLR) emission, with the WISE mid-infrared (MIR) catalog covering the emissions from the dusty tori. Out of the 7,653 sources, we found 57 AGN whose bolometric luminosities estimated from the MIR band are at least one order of magnitude fainter than those estimated from the [OIII]$\lambda5007$ emission line. This luminosity declining AGN candidate population shows four important properties: 1) the past AGN activity estimated from the [OIII]$\lambda5007$ line reaches around the Eddington-limit, 2) more than 30% of the luminosity declining AGN candidates show a large absolute variability of $\Delta W1 > 0.45$ mag in the previous $\sim10$ yr at the WISE 3.4 $\mu$m band, 3) the median ratio of $\log$([NII]$\lambda6584/ \mathrm{H}\alpha\lambda6563)=-0.52$, suggesting a lower gas metallicity and/or higher ionization parameter compared to other AGN populations. 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 which started $\sim10^{3-4}$ yr ago and the decline in the recent $10$ yr.