We report the discovery of a diffuse stellar cloud with an angular extent \gtrsim 30 \arcsec , 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 Sérsic index ( n \sim 0.3 ) .
It has a low central g -band surface brightness of { \sim } 26.4 mag arcsec ^ { -2 } , large effective radius of { \sim } 13 \arcsec ( { \sim } 11 kpc at z = 0.0431 and { \sim } 1.5 kpc at z = 0.0055 ) , and an elongated morphology ( b / a \sim 0.4 ) .
Its red color ( g - i \sim 1 ) 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 .