Stellar Populations of the Sagittarius Dwarf Irregular Galaxy

Abstract

We present deep BVRI CCD photometry of the stars in the dwarf irregular galaxy SagDIG. The color-magnitude diagrams of the measured stars in SagDIG show a blue plume which consists mostly of young stellar populations, and a well-defined red giant branch (RGB). The foreground reddening of SagDIG is estimated to be E(B-V)=0.06. The tip of the RGB is found to be at I_(TRGB)=21.55 +/- 0.10 mag. From this the distance to this galaxy is estimated to be d = 1.18 +/- 0.10 Mpc. This result, combined with its velocity information, shows that it is a member of the Local Group. The mean metallicity of the red giant branch is estimated to be [Fe/H] < -2.2 dex. This shows that SagDIG is one of the most metal-poor galaxies. Total magnitudes of SagDIG (< r_H (= 107 arcsec)) are derived to be B^T=13.99 mag, V^T=13.58 mag, R^T=13.19 mag, and I^T=12.88 mag, and the corresponding absolute magnitudes are M_B=-11.62 mag, M_V=-11.97 mag, M_R=-12.33 mag, and M_I=-12.60 mag. Surface brightness profiles of the central part of SagDIG are approximately fit by a King model with a core concentration parameter c = log (r_t / r_c) ~ 0.6, and those of the outer part follow an exponential law with a scale length of 37 arcsec. The central surface brightness is measured to be mu_B (0) = 24.21 mag arcsec^(-2) and mu_V (0) =23.91 mag arcsec^(-2). The magnitudes and colors of the brightest blue and red stars in SagDIG (BSG and RSG) are measured to be, respectively, <V(3)>_BSG = 19.89 +/- 0.13 mag, <(B-V)(3)>_BSG = 0.08 +/- 0.07 mag, <V(3)>_RSG = 20.39 +/- 0.10 mag, and <(B-V)(3)>_RSG = 1.29 +/- 0.12 mag. The corresponding absolute magnitudes are derived to be <M_V(3)>_BSG = -5.66 mag and <M_V(3)>_RSG = -5.16 mag, which are about one magnitude fainter than those expected from conventional correlations with galaxy luminosity.