Sumo Puff: Tidal debris or disturbed ultra-diffuse galaxy?

Abstract

We report the discovery of a diffuse stellar cloud with an angular extent greater than or similar to 30”, 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 similar to L-* 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 Se ‘ rsic index (n similar to 0.3). It has a low central g-band surface brightness of similar to 26.4mag arcsec-2, large effective radius of similar to 13”(similar to 11 kpc at z = 0.0431 and similar to 1.5 kpc at z = 0.0055), and an elongated morphology (b/a similar to 0.4). Its red color (g - i similar to 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-sized 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 tidalmaterial will be necessary to reveal the true nature of this object.