SDSS IV MaNGA: Discovery of an $H_{\alpha}$ blob associated with a dry
  galaxy pair -- ejected gas or a `dark' galaxy candidate?
  We report the discovery of a mysterious giant $H_{\alpha}$ blob that is $\sim
8$ kpc away from the main MaNGA target 1-24145, one component of a dry galaxy
merger, identified in the first-year SDSS-IV MaNGA data. The size of the
$H_{\alpha}$ blob is $\sim$ 3-4 kpc in radius, and the $H_{\alpha}$
distribution is centrally concentrated. However, there is no optical continuum
counterpart in deep broadband images reaching $\sim$26.9 mag arcsec$^{-2}$ in
surface brightness. We estimate that the masses of ionized and cold gases are
$3.3 \times 10^{5}$ $\rm M_{\odot}$ and $< 1.3 \times 10^{9}$ $\rm M_{\odot}$,
respectively. The emission-line ratios indicate that the $H_{\alpha}$ blob is
photoionized by a combination of massive young stars and AGN. Furthermore, the
ionization line ratio decreases from MaNGA 1-24145 to the $H_{\alpha}$ blob,
suggesting that the primary ionizing source may come from MaNGA 1-24145, likely
a low-activity AGN. Possible explanations of this $H_{\alpha}$ blob include AGN
outflow, the gas remnant being tidally or ram-pressure stripped from MaNGA
1-24145, or an extremely low surface brightness (LSB) galaxy. However, the
stripping scenario is less favoured according to galaxy merger simulations and
the morphology of the $H_{\alpha}$ blob. With the current data, we can not
distinguish whether this $H_{\alpha}$ blob is ejected gas due to a past AGN
outburst, or a special category of `ultra-diffuse galaxy' (UDG) interacting
with MaNGA 1-24145 that further induces the gas inflow to fuel the AGN in MaNGA
1-24145.
