KK258, a new transition dwarf galaxy neighbouring the Local Group

Abstract

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 Hα 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 log[sSFR] = -2.64 and -2.84 (Gyr^-1) from the FUV-flux and Hα-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.