Long-Term Decline of the Mid-Infrared Emission of Normal Galaxies: Dust
  Echo of Tidal Disruption Flare?
  We report the discovery of a sample of 19 low redshift (z<0.22)
spectroscopically non-Seyfert galaxies that show slow declining mid-infrared
(MIR) light-curves (LCs), similar to those of tidal disruption event (TDE)
candidates with extreme coronal lines. Two sources also showed a relatively
fast rising MIR LCs. They consist of 61% sample of the WISE MIR variable
non-Seyfert galaxies with SDSS spectra. In a comparison sample of optically
selected Seyfert galaxies, the fraction of sources with such a LC is only 15%.
After rejecting 5 plausible obscured Seyfert galaxies with red MIR colours,
remaining 14 objects are studied in detail in this paper. We fit the declining
part of LC with an exponential law, and the decay time is typically one year.
The observed peak MIR luminosities ($\nu L_\nu$) after subtracting host
galaxies are in the range of a few 10^42 to 10^44 erg~s^-1 with a median of
5x10^43 erg~s^-1 in the W2 band. The black hole masses distribute in a wide
range with more than half in between 10^7 to 10^8 ~M_sun, but significantly
different from that of optical/UV selected TDEs. Furthermore, MIR luminosities
are correlated with black hole masses, the stellar mass or luminosity of their
host bulges. Most galaxies in the sample are red and luminous with an absolute
magnitude of r between -20 to -23. We estimate the rate of event about 10^-4
gal^-1~yr^-1 among luminous red galaxies. We discuss several possibilities for
the variable infrared sources, and conclude that most likely, they are caused
by short sporadic fueling to the supermassive black holes via either the
instability of accretion flows or tidal disruption of stars.
