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 0.05 Mpc using the Tip of Red Giant Branch method. We also detect H emission from this gas-poor dwarf transition galaxy at the velocity V h = 92 5 or V LG = 150 km s -1 . With this distance and velocity, KK258 lies near the local Hubble flow locus with a peculiar velocity 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 logsSFR = -2.64 and -2.84 (Gyr -1 ) from the FUV-flux and H-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 log(M H I ) < 5.75 M . 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.