Surface brightness fluctuation distances for nearby dwarf elliptical galaxies

We obtained deep B and R-band CCD images for the dwarf elliptical (dE) galaxies 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 different fields of the galaxies. Independent tip of the red giant branch distances for DDO 44, KK98 77, DDO 71 are used to convert their set of apparent into absolute SBF magnitudes. The results are combined with the corresponding local $(B-R)$ colours and compared with the $(B-R)-\overline{M}_R$ relation for mainly old, metal-poor stellar populations as predicted by Worthey's population synthesis models using Padova isochrones. While the colour dependency 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 $0.13 (\pm0.02)$ mag. Based on these findings we establish a new semiempirical calibration of the SBF method as distance indicator for dE galaxies with an estimated uncertainty of ≈10% . Taking first advantage of the improved calibration, we determine SBF distances for the other three early-type dwarfs UGC 4998, DDO 113, and UGC 7356. 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 $10.5 (\pm0.9)$ Mpc. We can further identify DDO 113 as a faint stellar system at the near side of the Canes Venatici I (CVn I) cloud at a distance of $3.1 (\pm0.3)$ Mpc. The second CVn I member in our sample, UGC 7356, lies at $6.7 (\pm0.6)$ Mpc and spatially close to NGC 4258 (M 106). We derive BR surface brightness profiles and colour gradients for all dwarfs and determine photometric and Sérsic parameters. Finally, we discuss two non-stellar objects in DDO 71 and UGC 7356 which may resemble globular clusters