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We use multi-epoch 3.6 and 4.5 µm data from the Spitzer Extended Deep Survey (SEDS) to
probe the AGN population among galaxies to redshifts ∼3 via their mid-IR variability. About
1 per cent of all galaxies in our survey contain varying nuclei, 80 per cent of which are likely
to be AGN. Twenty-three per cent of mid-IR variables are also X-ray sources. The mid-IR
variables have a slightly greater fraction of weakly disturbed morphologies compared to a
control sample of normal galaxies. The increased fraction of weakly distorted hosts becomes
more significant when we remove the X-ray emitting AGN, while the frequency of strongly
disturbed hosts remains similar to the control galaxy sample. These results suggest that mid-IR
variability identifies a unique population of obscured, Compton-thick AGN revealing elevated
levels of weak distortion among their host galaxies.