Local Active Galactic Nuclei with Large Broad-H{\alpha} Variability
  Reside in Red Galaxies
  Inspired by our serendipitous discovery of six AGNs with varying broad-Halpha
fluxes over years out of our searching for intermediate-mass black holes
(IMBHs), we conduct a systematic investigation of changing-look (CL) and
large-variability AGNs. We collect all the CL AGNs at z<0.15 and the
reverberation mapped AGNs with strongly variable broad Halpha, and perform
careful decomposition fittings to both their images and spectra. We find two
observational facts: (1) The host galaxies of local CL and large-variability
AGNs, mainly being Seyferts, are in the red (gas-poor) tail of the general
Seyfert galaxy population. (2) In contrast, there is a significant trend that
their more luminous counterparts namely CL and extremely variable quasars (CLQs
and EVQs) are different: CLQs are generally in blue galaxies; in terms of the
diagram of SFR and M* local CL Seyfert galaxies are located in the green
valley, whereas CLQ hosts are in the star-forming main sequence. We propose
explanations for those strongly variable Seyferts and quasars, respectively,
under the thought that accretion disks broadly depend on nuclear fueling modes.
Local large-variability and CL Seyferts are in nuclear famine mode, where
cold-gas clumps can be formed stochastically in the fueling flow, and their
episodic infall produces sharp peaks in the accretion-rate curve. CLQs and EVQs
are in feast fueling mode, which may account for both their preference to blue
galaxies and their variability pattern (high-amplitude tail of the continuous
distribution). Lastly, we propose a new thinking: to search for IMBHs by
optical variability in red galaxies.
