| Abstract | The Automatic Plate Measuring facility at Cambridge was used to measure CFHT and CTIO 4-m prime-focus plates of NGC 3109. Identification of bright blue and red stars on the CMD, together with periods for five Cepheid variables, enable a distance modulus of 25.98, or 1.57 Mpc, to be determined. The galaxy extends to a diameter of 30 arcmin along the major axis, twice as large as previous optical estimates. At a distance of 1.57 Mpc, this implies that NGC 3109 is one of the largest Magellanic dwarfs close to the Local Group, having a major-axis diameter of 14 kpc. The galaxy also exhibits clear spiral structure in the outer regions, with somewhat more irregular features toward the center. Bright blue stars are found everywhere in NGC 3109, but particularly along the spiral arms, indicating that star formation is taking place on a galactic scale at the present epoch. |
