Sumo Puff: Tidal Debris or Disturbed Ultra-Diffuse Galaxy?
  We report the discovery of a diffuse stellar cloud with an angular extent
$\gtrsim30^{\prime\prime}$, which we term "Sumo Puff", in data from the Hyper
Suprime-Cam Subaru Strategic Program (HSC-SSP). While we do not have a redshift
for this object, it is in close angular proximity to a post-merger galaxy at
redshift $z=0.0431$ and is projected within a few virial radii (assuming
similar redshifts) of two other ${\sim}L_\star$ galaxies, which we use to
bracket a potential redshift range of $0.0055 < z < 0.0431$. The object's light
distribution is flat, as characterized by a low Sersic index ($n\sim0.3$). It
has a low central $g$-band surface brightness of ${\sim}26.4$ mag
arcsec$^{-2}$, large effective radius of ${\sim}13^{\prime\prime}$ (${\sim}11$
kpc at $z=0.0431$ and ${\sim}1.5$ kpc at $z=0.0055$), and an elongated
morphology ($b/a\sim0.4$). Its red color ($g-i\sim1$) is consistent with a
passively evolving stellar population and similar to the nearby post-merger
galaxy, and we may see tidal material connecting Sumo Puff with this galaxy. We
offer two possible interpretations for the nature of this object: (1) it is an
extreme, galaxy-size tidal feature associated with a recent merger event, or
(2) it is a foreground dwarf galaxy with properties consistent with a quenched,
disturbed ultra-diffuse galaxy. We present a qualitative comparison with
simulations that demonstrates the feasibility of forming a structure similar to
this object in a merger event. Follow-up spectroscopy and/or deeper imaging to
confirm the presence of the bridge of tidal material will be necessary to
reveal the true nature of this object.
