KDG 218, a Nearby Ultra-Diffuse Galaxy

Abstract: We present properties of the low-surface-brightness galaxy KDG 218 observed with the HST/ACS. The galaxy has a half-light (effective) diameter of $a_e =47^{\prime\prime}$ and a central surface brightness of $SB_V(0) = 24.4^m/sq^{\prime\prime}$. The galaxy remains unresolved with the HST/ACS, which implies its distance of {$D > 13.1$ Mpc} and linear effective diameter of $A_e >3.0$ kpc. We notice that KDG 218 is most likely associated with a galaxy group around the massive lenticular NGC 4958 galaxy at approximately $22$ Mpc, or with the Virgo Southern Extension filament at approximately $16.5$ Mpc. At these distances, the galaxy is classified as an ultra-diffuse galaxy (UDG) similar to those found in the Virgo, Fornax, and Coma clusters. We also present a sample of 15 UDG candidates in the Local Volume. These sample galaxies have the following mean parameters: $\langle D\rangle = 5.1$ Mpc, $\langle A_e\rangle = 4.8$ kpc, and $\langle SB_B (e) \rangle = 27.4^m/sq^{\prime\prime}$. All the local UDG candidates reside near massive galaxies located in the regions with the mean stellar mass density (within 1 Mpc) about 50 times greater than the average cosmic density. The local fraction of UDGs does not exceed 1.5% of the Local Volume population. We notice that the presented sample of local UDGs is a heterogeneous one containing irregular, transition, and tidal types, as well as objects consisting of an old stellar population.