Surface brightness fluctuation distances for dwarf elliptical galaxies in the Fornax cluster

We have obtained deep B and R-band CCD images of eight dwarf elliptical (dE) galaxies in the Fornax cluster using the FORS1 instrument at the VLT in service mode under excellent atmospheric conditions. A total of 92 fields distributed over the central regions of the galaxies have been analysed to measure local (B-R)₀ colours and R-band surface brightness fluctuation (SBF) magnitudes /line{m}_R. Within a galaxy the observed correlation of (B-R)₀ with /line{m}_R⁰ follows closely the predicted slope of the colour-fluctuation luminosity relation for composite single-burst, mainly old, metal-poor stellar populations. This allows to determine the distances of the dEs from simple offset measurements to a typically 9% accuracy. The distance distribution of these genuine cluster dwarfs centers at a mean distance of (m-M)₀=31.54+/-0.07 mag, or 20.3 +/- 0.7 Mpc, a value that is in best agreement with previous SBF work on Fornax early-type giants and thus represents a robust estimate of the distance to the Fornax cluster core. The application of the bootstrap resampling technique on the distance data further reveals a cluster depth of sigma_int = 1.4{+0.5 atop -0.8} Mpc. We take this preliminary result as a confirmation of the compact appearance of Fornax in the projection on the sky although the cluster might be slightly more elongated along the line of sight. Combining the newly derived cluster distance with the cosmological velocity of Fornax of 1324+/-41 km s^-1 gives a Hubble constant of H₀ = 65 +/- 4 km s^-1 Mpc^-1. This value is consistent at the 95% confidence level with both the most recent result from the team that favours a long distance scale and the final value adopted by the HST ``Key Project'' team in their work for the Hubble constant. Finally, we explore the possibility to determine rough metallicities of our cluster dEs from their (B-R)₀ colours via Worthey's stellar population synthesis models. The median metallicities are found in the range from -1.5 to -1 with a concentration around [Fe/H] =-1. A comparison with spectral line indices results available for three galaxies shows good agreement. Moreover, the derived metallicities place the bright Fornax dEs on the extension of the metallicity-luminosity relation defined by the low luminous Local Group dEs which provides additional support for the (B-R)₀ colour as a useful metallicity estimator. The data further suggest an age range between 10 and 12 Gyr for the Fornax dwarfs.

Based on observations collected at the European Southern Observatory (ESO 68.A-0176).