Oral Presentation
Search for fading AGNs at z < 0.4
Presenter: Janek Pflugradt (Tohoku University)
Active galactic nuclei (AGN) are a key population to understand the growth of supermassive black holes (SMBHs).
Recent observations have revealed an interesting AGN subclass that shows strong activity at large scales (~1 kpc) and weaker activity at small scales (< 10 pc), suggesting a strong change in the mass accretion rate of the central engine. These objects are known as fading AGNs. We systematically search for such declining or fading AGN by cross-matching the z < 0.4 ALPAKA SDSS type-1 AGN catalog, covering the [OIII] emission line which is a tracer for the narrow line region (NLR) emission (~1 kpc), with the WISE mid-infrared (MIR) catalog covering the emissions from the dusty tori (~10 pc). Out of the ~9,000 sources, we found 50 AGNs whose bolometric luminosities estimated from the WISE MIR band (L ~ 10^{44-46} erg/s) are at least one order of magnitude fainter than those estimated from the [OIII] emission line (L ~ 10^{45-47} erg/s). This declining population shows four important properties:
1) The past AGN activity estimated from the [OIII] line reaches around the Eddington limit, suggesting an experience of a past burst phase. This burst phase might be unstable and it does not last longer than the light travel time to the NLR from the center.
2) More than 30% of the fading AGN candidate population show a large absolute variability of ∆W1 > 0.45 mag in the NEOWISE 3.4 um band for the last ~10 yr.
3) The median log([NII]/ Hα) is -0.52 for this population and -0.24 for typical AGNs suggesting a lower metallicity or higher ionization rate for the selected AGNs.
4) The second epoch spectra of the population indicate a 31% rate of changing-look phenomena disappearing/appearing of the blue continuum and broad line region for this population.
This could indicate a possible impact from the past burst on the short-term variability for some AGNs.
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