THE DISTANCE TO NEARBY GALAXY NGC-3109 BASED ON THE TIP OF THE RED GIANT BRANCH

Deep VI CCD photometry of approximately 4200 stars in a 3.'9 x 3.'9 central field of NGC 3109, a bright Magellanic-type spiral galaxy (Sm IV), has been obtained. The tip of the red giant branch (TRGB) of low-mass stars is found to be at I(TRGB) = 21.55 +/- 0.1 mag and (V-I)TRGB = 1.7 +/- 0.2 mag. The mean metallicity of the red giant branch (RGB) stars has been measured using the color of the RGB stars at M(I) = -3.5 mag, (V-I)-3.5 = 1.41 +/- 0.07 mag: [Fe/H] = -1.6 +/- 0.2 dex. The distance modulus to NGC 3109 has been estimated using the I magnitude of the TRGB to be (m-M), = 25.45 +/- 0.15 mag for an adopted foreground extinction of A(I) = 0.08 mag, showing a very good agreement with the Cepheid distance modulus estimate based on very recent CCD photometry, (m-M)0 = 25.5 +/- 0.2 mag (Capaccioli, Piotto, & Bresolin).