Surface brightness fluctuation distances for nearby dwarf elliptical galaxies

We obtained deep B and R-band CCD images for the dwarf elliptical (dE) gala xies DDO 44, UGC 4998, KK98 77, DDO 71, DDO 113, and UGC 7356 at the Nordic Optical Telescope. Employing Fourier analysis technique we measure stellar R-band surface brightness fluctuations (SBFs) and magnitudes in 29 differe nt fields of the galaxies. Independent tip of the red giant branch distance s for DDO 44, KK98 77, DDO 71 are used to convert their set of apparent int o absolute SBF magnitudes. The results are combined with the corresponding local (B-R) colours and compared with the (B-R) (M) over bar (R) relation f or mainly old, metal-poor stellar populations as predicted by Worthey's pop ulation synthesis models using Padova isochrones. While the colour dependen cy of the theoretical relation is confirmed by the empirical data, we find a systematic zero point offset between observations and theory in the sense that models are too faint by 0.13 (+/-0.02) mag. Based on these findings w e establish a new semiempirical calibration of the SBF method as distance i ndicator for dE galaxies with an estimated uncertainty of approximate to 10 %. Taking first advantage of the improved calibration, we determine SBF dis tances for the other three early-type dwarfs UGC 4998, DDO 113, and UGC 735 6. Although found in the M 81 group region, previous velocity measurements suggested UGC 4998 is a background galaxy. This picture is confirmed by our SBF distance of 10.5 (+/-0.9) Mpc. We can further identify DDO 113 as a fa int stellar system at the near side of the Canes Venatici I (CVn I) cloud a t a distance of 3.1 (+/-0.3) Mpc. The second CVn I member in our sample, UG C 7356, lies at 6.7 (+/-0.6) Mpc and spatially close to NGC 4258 (M 106). W e derive BR surface brightness profiles and colour gradients for all dwarfs and determine photometric and Sersic parameters. Finally, we discuss two n on-stellar objects in DDO 71 and UGC 7356 which may resemble globular clust ers.