ar X iv : a st ro - p h / 99 09 49 3 v 1 2 9 Se p 19 99 DDO 187 : do dwarf galaxies have extended , old halos ? 1 Antonio

If dwarf galaxies are primeval objects in the Universe, as hierarchical galaxy formation scenarios predict, they should show traces of their old stellar populations, perhaps distributed in extended, differentiated structures. The working hypothesis that such a structure could exist is tested for the case of DDO 187, a field dIrr galaxy showing a high gas fraction and low metallicity. For this purpose, the structure, star formation history (SFH) and other properties of the galaxy are analyzed using the spatial distribution of stars, the color–magnitude diagrams (CMDs) of about 1500 resolved stars and the fluxes of H ii regions, together with data about the gas distribution. From the I magnitude of the tip of the red giant branch (ITRGB), the distance of DDO 187 to the Milky Way is estimated to be 2.5± 0.2 Mpc. The distance to several neighbor galaxies and groups has been computed, showing that DDO 187 is probably an isolated, field galaxy. The distance of DDO 187 to the Milky Way is almost three times smaller than that obtained from Cepheid light-curves. Considering that this is the third case in which such a large disagreement is detected, it seems clear that Cepheid distance estimates based on a few stars, as usually happens in dwarf galaxies, must be accepted with caution. The star formation history of DDO 187 has been analyzed. The central region of DDO 187 shows an overall time decreasing star formation rate with a strong burst in its central region which happened between 20 and 100 Myrs ago and a present-day star formation activity three times smaller than the maximum one. Besides this, a spatially extended stellar component has been found that has no young stars and exceeds the size of the gas component. Based on observations made with the 2.5 m Nordic Optical Telescope operated on the island of La Palma by NOT S.A. in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrof́ısica de Canarias.