Zangetsu: A Candidate of Isolated, Quiescent, and Backsplash Ultra-Diffuse Galaxy in the COSMOS Field

TL;DR

The paper reports the discovery of Zangetsu, an extremely low surface brightness, isolated ultra-diffuse galaxy with unique features, challenging existing galaxy formation models and suggesting the existence of more such extreme systems.

Contribution

It presents the serendipitous discovery and detailed analysis of Zangetsu, a rare ultra-diffuse galaxy with unusual properties, highlighting potential gaps in current galaxy formation theories.

Findings

Zangetsu has an extremely low central surface brightness of 26.60 mag/arcsec^2.

It exhibits a very shallow surface brightness profile and a large size (~10.44 arcsec).

Zangetsu is an outlier in the luminosity-size relation of LSB galaxies.

Abstract

Deep imaging surveys have changed our view of the low surface brightness (LSB) Universe. The "renaissance" of the low surface brightness dwarf galaxy population, as the prime example of such recent development, continues to challenge our understanding of galaxy formation. Here, We report the serendipitous discovery of Zangetsu, an isolated, quiescent, and distorted ultra-diffuse galaxy (UDG) candidate in the COSMOS field, using images from the Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Zangetsu exhibits an extremely low central surface brightness ($\mathrm{\mu_{0,g}}=26.60\pm0.01$ mag arcsec$^{-2}$), a very shallow inner surface brightness profile ($\mathrm{n}_{\rm Sersic}=0.40\pm0.01$), and a large angular size ($\mathrm{R_e}\approx 10.44$ arcsec). Surprisingly, Zangetsu also has a quiescent stellar population ($\mathrm{g-i}=0.96$), an unusually elongated shape ($\mathrm{b/a}\sim 0.25$), and mild morphological asymmetry, making it a rare case among known UDGs. Surface brightness fluctuation analysis of HSC and Hubble Space Telescope (HST) images only provides a distance lower limit of $D>25.4$ Mpc (thus $\mathrm{R_e}>1.38$ kpc). However, Zangetsu remains an extreme outlier in the luminosity-size relation of known LSB galaxies, suggesting that it could be an exceptionally large and/or diffuse system. Classic internal or external UDG formation mechanisms alone struggle to explain such a system. A backsplash origin may account for its isolation and quiescent nature. This finding also raises the possibility that current works may overlook similarly extreme, elongated systems that could further our understanding of the LSB Universe.