Discovery of new dwarf galaxy near the isolated spiral galaxy NGC 6503

Abstract

We report the discovery of a new dwarf galaxy (NGC 6503-d1) during the Subaru extended ultraviolet disk survey. It is a likely companion of the spiral galaxy NGC 6503. The resolved images, in the B, V, R, i, and Hα bands, show an irregular appearance due to bright stars with underlying, smooth and unresolved stellar emission. It is classified as the transition type (dwarf irregular (dIrr)/dwarf spheroidal (dSph)) between the dIrr and dSph types. Its structural properties are similar to those of the dwarfs in the Local Group, with an absolute magnitude MV ∼ - 10.5, half-light radius re ∼ 400 pc, and central surface brightness μ0, V ∼ 25.2. Despite the low stellar surface brightness environment, one H II region was detected, though its Hα luminosity is low, indicating an absence of any appreciable O-stars at the current epoch. The presence of multiple stellar populations is indicated by the color-magnitude diagram of ∼300 bright resolved stars and the total colors of the dwarf, with the majority of its total stellar mass ∼4 x 106 M⊙ in an old stellar population.