| 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 that 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 [H(=107")] are found to be BT=13.99 mag, VT=13.58 mag, RT=13.19 mag, and IT=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". The central surface brightness is measured to be μB(0)=24.21 mag arcsec-2 and μV(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, BSG=19.89+/-0.13 mag, <(B-V)(3)>BSG=0.08+/-0.07 mag, RSG=20.39+/-0.10 mag, and <(B-V)(3)>RSG=1.29+/-0.12 mag. The corresponding absolute magnitudes are derived to be V(3)>BSG=-5.66 mag and V(3)>RSG=-5.16 mag, which are about 1 mag fainter than those expected from conventional correlations with galaxy luminosity. |
