KK258, a new transition dwarf galaxy neighbouring the Local Group

Here we present observations with the Advanced Camera for Surveys on the Hubble Space Telescope of the nearby, transition-type dwarf galaxy KK258 = ESO468-020. We measure a distance of 2.23$\pm$0.05 Mpc using the Tip of Red Giant Branch method. We also detect H$\alpha$ emission from this gas-poor dwarf transition galaxy at the velocity $V_h$ = 92$\pm$5 km s$^{-1}$ or $V_{LG}$ = 150 km s$^{-1}$. With this distance and velocity, KK258 lies near the local Hubble flow locus with a peculiar velocity $\sim$3 km s$^{-1}$. We discuss the star formation history of KK258 derived from its colour-magnitude diagram. The specific star formation rate is estimated to be log[sSFR] = $-2.64$ and $-2.84$ (Gyr$^{-1}$) from the FUV-flux and H$\alpha$-flux, respectively. KK258 has the absolute magnitude $M_B = -10.3$ mag, the average surface brightness of 26.0 mag arcsec$^{-2}$ and the hydrogen mass ${\rm log}(M_{HI}) < 5.75 M_\odot$. We compare KK258 with 29 other dTr- galaxies situated within 5 Mpc from us, and conclude that its properties are typical for transition dwarfs. However, KK258 resides 0.8 Mpc away from its significant neighbour, the Sdm galaxy NGC 55, and such a spatial isolation is unusual for the local transition dwarfs.