Sumo Puff: Tidal Debris or Disturbed Ultra-Diffuse Galaxy?

Abstract

We report the discovery of a diffuse stellar cloud with an angular extent $\gtrsim 30^{\prime \prime }$, 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 ($n\sim 0.3$). It has a low central $g$-band surface brightness of $\sim 26.4$ mag arcsec${}^{-2}$, large effective radius of $\sim 13^{\prime \prime }$ ($\sim 11$ 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.

10 pages, 5 figures, submitted to PASJ for the HSC-SSP special issue