A Decade-Long Increasing Mid-Infrared Luminosity in Galaxy NGC6447: a Turning-On Candidate of Active Galactic Nucleus

TL;DR

This study documents a decade-long increase in mid-infrared luminosity of galaxy NGC6447, indicating the galaxy's transition into an active galactic nucleus, supported by multi-wavelength observations and variability analysis.

Contribution

It provides evidence of a galaxy turning on as an AGN through long-term infrared monitoring, highlighting a new method to identify such events.

Findings

Mid-infrared luminosity increased over 14 years.

Galaxy transitioned into an AGN in 2018.

X-ray variability supports AGN activation.

Abstract

It is widely expected that the obscured accretion stage can be the initial turning-on stage of active galactic nuclei from quiescent galaxies. We present mid-infrared light curves of NGC 6447 in 3.5$\mu$m and 4.6 $\mu$m bands observed by WISE/NEOWISE, which show an almost monotonic increasing trend of 1.2 mag over 14 years. The optical light curve from ASAS-SN during the same period is consistent with a constant showing no variability. The mid-infrared color evolution shows that the galaxy transitioned into an active galactic nucleus (AGN) in 2018. The SPHEREx spectrum reveals an increasing continuum resembling warm to hot dust emission from an AGN. NuSTAR detected an X-ray source with a 2-30 keV luminosity of $8.4\times10^{41}$ ergs/s at the lower boundary of AGN X-ray emission range, and a factor of >7 variability in one year compared to the Swift upper limit. NGC 6447 was classified as a quiescent galaxy in the literature. The multi-wavelength timing and spectral properties of NGC 6447 are consistent with the expected AGN turning on event, where the obscuring material around the AGN central engine is gradually dispersed, revealing the central engine. This example shows that long-term infrared variability can be a powerful tool to find similar sources. Based on the sample selection statistics, we estimate the duration of the episodes of AGN accretion (duty cycle) signified by the turning-on event as $10^4$-$10^6$ yr.