Optical broad and narrow band as well as NIR imaging data were presented and used to derive the structural parameters of the isolated dwarf galaxy UGC 685 which show very little signs of irregularities. UGC 685 is one of those dwarfs where the HI gas is reaching to far greater distances than the stars, and might be, like DDO 154, a good case for dark matter halo studies. In a separate paper, I will show that the stellar and ionized gas kinematics fit quite well to the HI rotation curve established by Hoffman et al. (1996). Total fluxes and colors, the color distribution, and the especially H flux and morphology point to a low star formation rate. Even for dwarf irregular galaxies, it is at the lower boundary of the observed values (Hunter & Gallagher 1985, 1986). The overall star formation history seems to be calm at a level of 0.003 yr-1 over the last yr, but this particular irregular dwarf was nevertheless able to establish a surface brightness at the upper boundary of dwarfs of this size (compare to Binggeli 1993, his Fig. 4, or Hopp 1994, Fig. 1). Over short intervals ( yr), the star formation activity appears strongly localized. Data with much better resolution are needed for a more detailed study and reconstruction.
A new distance estimate was established based on the resolved brightest supergiants. This new estimates confirms that UGC 685 belongs to the 10 Mpc sample of the very nearby universe and is indeed a very isolated galaxy.Acknowledgements
I would like to thank the Calar Alto staff for his kind support during the observations. Drs. Ralf Napiwotzki and Sabine Moehler helped with the electronic version of the Kurucz tables. I acknowledge many useful discussions with Drs. Bender, Greggio, Rosa, and Schulte-Ladbeck. Niv Drory did the Bruzual & Charlot model calculations. I was supported by the DFG (hopp/1801-1) and by the SFB 375.
Copyright The European Southern Observatory (ESO)