Variability-selected intermediate mass black hole candidates in dwarf
  galaxies from ZTF and WISE
  In this paper we present a sample of variability-selected AGN from a parent
sample of dwarf galaxies using optical photometry from the Zwicky Transient
Facility (ZTF) and forward-modeled mid-IR photometry of time-resolved
Wide-field Infrared Survey Explorer (WISE) coadded images. We found that 44 out
of 25,714 dwarf galaxies had optically variable AGN candidates, and 158 out of
79,879 dwarf galaxies had mid-IR variable AGN candidates, corresponding to
active fractions of $0.17\pm0.03$% and $0.20\pm0.02$% respectively. Only two
objects, NSA164884 and NSA451469, broad line AGN with virial masses
$M_{\text{BH}}=10^{6.9}M_\odot$ and $M_{\text{BH}}=10^{6.3}M_\odot$
respectively, were found in both optical and mid-IR searches. We find that
spectroscopic approaches to AGN identification would have missed 81% of our ZTF
IMBH candidates and 69% of our WISE IMBH candidates. Only $9$ candidates have
been detected previously in radio, X-ray, and variability searches for dwarf
galaxy AGN. The IMBHs with broad Balmer lines have virial masses of
$10^5M_\odot<M_{\text{BH}}<10^7M_\odot$ but for the rest of the sample, BH
masses predicted from host galaxy mass range between
$10^{4.8}M_\odot<M_{\text{BH}}<10^7M_\odot$. We found that only 5 of 152
previously reported variability-selected AGN candidates from the Palomar
Transient Factory in common with our parent sample were variable in ZTF. We
also determined a nuclear supernova fraction of $0.05\pm0.01$% year$^{-1}$ for
dwarf galaxies in ZTF. Our ZTF and WISE IMBH candidates show the promise of
variability searches for discovery of otherwise hidden low mass AGN in
preparation for the Legacy Survey of Space and Time (LSST) at Vera C. Rubin
Observatory.
