Testing the Surface Brightness Fluctuations Method for Dwarf Elliptical Galaxies in the Centaurus A Group

We have obtained deep B and R-band CCD photometry for 5 dE galaxies that werepreviously identified on Schmidt films covering the Cen A group region. From aFourier analysis of the R-band CCD images we determined the surface brightnessfluctuation (SBF) magnitude for each stellar system. All magnitudes are similarand suggest that these low surface brightness galaxies lie approximately at thesame distance, regardless of the assumed SBF zero point. Long-slit spectra havebeen acquired to derive redshifts for two of the dwarfs, ESO269-066 andESO384-016. The velocities identify them unambiguously as Cen A group members.An age - metallicity analysis of the spectra reveals an underlying old andmetal-poor stellar population in both cases. Combining photometric andspectroscopic results we find strong evidence that indeed all dEs are Cen Agroup members. Based on accurate distances published for the two main Cen Agroup galaxies NGC5128 and NGC5253, we adopted a mean group distance of 3.96Mpc to calibrate the apparent fluctuation magnitudes. The resulting absoluteSBF magnitudes \bar{M}_R of the dEs correlate with the colours (B-R)_0 as goodagreement allows a calibration of the SBF method for dEs in the colour range0.8<(B-R)_0<1.5. One of the dwarfs, ESO219-010, is located slightly behind thecore of the Cen A group at 4.8 Mpc, while the remaining four recover the meangroup distance of 3.96 Mpc that was put into the calibration. The depth of thegroup is only 0.5 Mpc which identifies the Cen A group as a spatially wellisolated galaxy aggregate, in contrast to the nearby Sculptor group.Comment: 26 pages, 9 figures, to be published in The Astronomical Journal, January 2000 issu