Stellar Populations of the Sagittarius Dwarf Irregular Galaxy

We present deep BV RI 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 ITRGB = 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 (< rH(= 107 arcsec)) are derived to be B T = 13.99 mag, V T = 13.58 mag, R = 13.19 mag, and I = 12.88 mag, and the corresponding absolute magnitudes are MB = −11.62 mag, MV = −11.97 mag, MR = −12.33 mag, and MI = −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(rt/rc) ≈ 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 μB(0) = 24.21 mag arcsec −2 and μV (0) = 23.91 mag arcsec . 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 < MV (3) >BSG= −5.66 mag and < MV (3) >RSG= −5.16 mag, which are about one magnitude fainter than those expected from conventional correlations with galaxy luminosity. [To appear in the Astronomical Journal in 2000] Subject headings: galaxy: evolution — galaxies: irregular — galaxies: individual (SagDIG) — galaxies: stellar content — galaxies: photometry — Distance scale