Stellar populations and the local group membership of the dwarf galaxy DDO 210

We present deep BVI CCD photometry of the stars in the dwarf galaxy DDO 210. The color-magnitude diagrams of DDO 210 show a well-defined red giant branch (RGB) and a blue plume. The tip of the RGB is found to be at I_TRGB = 20.95 ± 0.10 mag. From this, the distance to DDO 210 is estimated to be d = 950 ± 50 kpc. The corresponding distance of DDO 210 to the center of the Local Group is 870 kpc, showing that it is a member of the Local Group. The mean metallicity of the RGB stars is estimated to be [Fe/H] = -1.9 ± 0.1 dex. Integrated magnitudes of DDO 210 within the Holmberg radius (r_H = 110″ = 505 pc) are derived to be M_B = -10.6 ± 0.1 and M_V = -10.9 ± 0.1 mag. B and V surface brightness profiles of DDO 210 are approximately consistent with an exponential law with scale lengths r_s(B) = 161 and r_s(V) = 175 pc. The brightest blue and red stars in DDO 210 (BSG and RSG) are found to be among the faintest in the nearby galaxies with young stellar populations: 〈M_V(3)〉_BSG = -3.47 ± 0.11 and 〈M_V(3)〉_RSG = -4.75 ± 0.13 mag. An enhancement of the star formation rate in the recent past (several hundred Myr) is observed in the central region of DDO 210. The opposite trend is observed in the outer region of the galaxy, suggesting a possible two-component structure of the disk/halo kind found in spiral galaxies. The real nature of this two-component structure must, however, be confirmed with more detailed observations.