Hubble Space Telescope Wide Field Camera 3 Identifies an $r_p$ = 1 Kpc
  Dual Active Galactic Nucleus in the Minor Galaxy Merger SDSS J0924+0510 at z
  = 0.1495
  Kiloparsec-scale dual active galactic nuclei (AGNs) are active supermassive
black hole pairs co-rotating in galaxies with separations of less than a few
kpc. Expected to be a generic outcome of hierarchical galaxy formation, their
frequency and demographics remain uncertain. We have carried out an imaging
survey with the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) of AGNs
with double-peaked narrow [O III] emission lines. HST/WFC3 offers high image
quality in the near-infrared (NIR) to resolve the two stellar nuclei, and in
the optical to resolve [O III] from ionized gas in the narrow-line regions.
This combination has proven to be key in sorting out alternative scenarios.
With HST/WFC3 we are able to explore a new population of close dual AGNs at
more advanced merger stages than can be probed from the ground. Here we show
that the AGN SDSS J0924+0510, which had previously shown two stellar bulges,
contains two spatially distinct [O III] regions consistent with a dual AGN.
While we cannot completely exclude cross-ionization from a single central
engine, the nearly equal ratios of [O III] strongly suggest a dual AGN with a
projected angular separation of 0."4, corresponding to a projected physical
separation of $r_p$ = 1 kpc at redshift z = 0.1495. This serves as a proof of
principle for combining high-resolution NIR and optical imaging to identify
close dual AGNs. Our result suggests that studies based on low-resolution
and/or low-sensitivity observations may miss close dual AGNs and thereby may
underestimate their occurrence rate on $\lesssim$ kpc scales.
