Deep Wide-Field BVI CCD Photometry of the Sextans Dwarf Spheroidal Galaxy

We present deep wide-field VI CCD photometry of the Sextans dwarf spheroidal galaxy (dSph) in the Local Group, covering a field of 42′ × 28′ located at the center of the galaxy (supplemented by short B photometry). The limiting magnitudes with 50% completeness are V = 24.4 mag and I = 23.6 mag. Color-magnitude diagrams of the Sextans dSph show a well-defined red giant branch (RGB), blue horizontal branch (BHB), prominent red horizontal branch (RHB), and asymptotic giant branch (AGB), as well as ∼120 variable star candidates including RR Lyrae stars and anomalous Cepheids, ∼230 blue stragglers (BSs), and main-sequence (MS) stars. The main-sequence turnoff (MSTO) of the old population is found to be located at V ≈ 23.7 mag and V-I ≈ 0.56. The distance to the galaxy is derived using the I-band magnitude of the tip of the RGB at ITRGB = 15.95 ± 0.04: (m - M)0 = 19.90 ± 0.06, for an adopted reddening of E(B-V) = 0.01. This estimate agrees well with the distance estimate based on the mean V-band magnitude of the HB at V(HB) = 20.37 ± 0.04. The mean metallicity of the RGB is estimated from the V-I color: [Fe/H] = -2.1 ± 0.1 (statistical error) ±0.2 (standard calibration error) dex, with a dispersion of σ[Fe/H] = 0.2 dex. The age of the MSTO of the main old population is estimated to be similar to that of the metal-poor Galactic globular cluster M92, and some stellar populations with younger age are seen. There is found to be one RGB bump at V = 19.95 ± 0.05 mag (MV = 0.03 mag) and a weak brighter bump at V = 19.35 ± 0.05 mag (MV = -0.58 mag), which is probably an AGB bump. The V-band luminosity function of the RGB and MS stars is in general similar to that of the globular cluster M92, with a slight excess of stars in the magnitude range brighter than the MSTO with respect to that of M92. The bright BSs are more centrally concentrated than the faint BSs. The V-band luminosity function of the BSs in the inner region is found to extend to a brighter magnitude and to have a flatter slope compared with that of the BSs in the outer region. Significant radial gradients are seen for several kinds of populations: the RHB, the red RGB, the red subgiant branch (SGB), and the bright BSs are more centrally concentrated toward the center of the galaxy compared with the BHB, the blue RGB, the blue SGB, and the faint BSs, respectively.