Stellar Populations of the Dwarf Galaxy UKS 2323--326 in the Sculptor Group

Abstract

We present deep BVRI CCD photometry of the stars in the dwarf irregular galaxy UKS 2323-326 in the Sculptor Group. The color-magnitude diagrams of the measured stars in UKS 2323-326 show a blue plume which consists mostly of young stellar populations, and a well-defined red giant branch (RGB). The tip of the RGB is found to be at I_TRGB = 22.65 +/- 0.10 mag. From this the distance to this galaxy is estimated to be d = 2.08 +/- 0.12 Mpc. The corresponding distance of this galaxy from the center of the Local Group is 1.92 Mpc, showing that it is located outside the Local Group. The large distance, combined with its velocity information, indicates that it is very likely to be a member of the Sculptor Group. The mean metallicity of the red giant branch is estimated to be [Fe/H] = -1.98 +/- 0.17 dex. Total magnitudes of UKS 2323-326 (< r_H = 70 arcsec) are derived to be B^T=14.07 mag, V^T=13.50 mag, R^T=13.18 mag, and I^T=12.83 mag, and the corresponding absolute magnitudes are M_B=-12.58 mag, M_V=-13.14 mag, M_R=-13.45 mag, and M_I=-13.79 mag. Surface brightness profiles of the central part of UKS 2323-326 are approximately consistent with a King model with a core concentration parameter c = log (r_t / r_c) ~= 1.0, and those of the outer part follow an exponential law with a scale length of 21 arcsec. The magnitudes and colors of the brightest blue and red stars in UKS 2323-326 (BSG and RSG) are measured to be, respectively, _{BSG} = 20.33 +/- 0.25 mag, <(B-V)(3)>_{BSG} = 0.14 +/- 0.07 mag, _{RSG} = 20.74 +/- 0.18 mag, and <(B-V)(3)>_{RSG} = 1.35 +/- 0.08 mag. The corresponding absolute magnitudes are derived to be _{BSG} = -6.31 mag and _{RSG} = -5.91 mag, which are about half magnitude fainter than those expected from conventional correlations with galaxy luminosity.