Deep Imaging of the HCG 95 Field. I. Ultra-diffuse Galaxies

We present a detection of 89 candidates of ultra-diffuse galaxies (UDGs) in a 4.9 degree² field centered on the Hickson Compact Group 95 (HCG 95) using deep g- and r-band images taken with the Chinese Near Object Survey Telescope. This field contains one rich galaxy cluster (Abell 2588 at z = 0.199) and two poor clusters (Pegasus I at z = 0.013 and Pegasus II at z = 0.040). The 89 candidates are likely associated with the two poor clusters, giving about 50–60 true UDGs with a half-light radius $r_e>1.5kpc$ and a central surface brightness $\mu (g,0)>24.0$ mag arcsec⁻². Deep $z^{\prime }$-band images are available for 84 of the 89 galaxies from the Dark Energy Camera Legacy Survey (DECaLS), confirming that these galaxies have an extremely low central surface brightness. Moreover, our UDG candidates are spread over a wide range in g − r color, and ∼26% are as blue as normal star-forming galaxies, which is suggestive of young UDGs that are still in formation. Interestingly, we find that one UDG linked with HCG 95 is a gas-rich galaxy with H i mass $1.1\times {10}^9$ M _⊙ detected by the Very Large Array, and has a stellar mass of $M_{\star }\sim 1.8\times {10}^8$ M _⊙. This indicates that UDGs at least partially overlap with the population of nearly dark galaxies found in deep H i surveys. Our results show that the high abundance of blue UDGs in the HCG 95 field is favored by the environment of poor galaxy clusters residing in H i-rich large-scale structures.