Stellar Populations of the Sagittarius Dwarf Irregular Galaxy

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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 [<rH(=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, 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 1 mag fainter than those expected from conventional correlations with galaxy luminosity.