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

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Abstract
We have obtained deep B and R-band CCD photometry for five dwarf elliptical galaxies that were previously identified on Schmidt films covering the region of the Centaurus A (Cen A) group. From a Fourier analysis of the R-band CCD images we determined the surface brightness fluctuation (SBF) magnitude R for each stellar system. All magnitudes are similar, and, given the small color spread, they suggest that these low surface brightness galaxies lie approximately at the same distance, regardless of the assumed SBF zero point. Long-slit spectra have been acquired to derive redshifts for two of the dwarfs, ESO 269-066 and ESO 384-016. The velocities, v⊙ = 784 km s-1 and v⊙ = 561 km s-1, respectively, identify them unambiguously as Cen A group members. An age (Hδ)-metallicity (C2 λ4668) analysis of the spectra reveals an underlying old and metal-poor stellar population in both cases. Combining photometric and spectroscopic results we find strong evidence that indeed all dwarf galaxies are Cen A group members. Based on Cepheid, TRGB, and PNLF distances published for the two main Cen A group galaxies, NGC 5128 and NGC 5253, we adopted a mean group distance of 3.96 Mpc to calibrate the apparent fluctuation magnitudes. The resulting absolute SBF magnitudes R of the dEs correlate with the dereddened colors (B-R)0 as predicted by Worthey's stellar synthesis models, using the theoretical isochrones of Bertelli and collaborators. This good agreement allows a calibration of the SBF method for dwarf ellipticals in the color range 0.8 < (B-R)0 < 1.5. However, two branches of stellar populations appear in the R-color plane, and care has to be taken to decide which branch applies to a given observed dwarf. For dwarfs with (B-R)0 < 1 there is very little color dependence (R ≈ -1.2), in accord with our previous SBF analysis of faint, blue Sculptor group dEs. For red dwarfs, (B-R)0 > 1.2, the R-color relation is steep, and accurate colors are needed to achieve SBF distances with an uncertainty of only 10%. One of the dwarfs, ESO 219-010, is located slightly behind the core of the Cen A group at about 4.8 Mpc, while the remaining four recover the mean group distance of 3.96 Mpc that was put into the calibration. The depth of the group is only 0.5 Mpc, which identifies the Cen A group as a spatially well-isolated galaxy aggregate, in contrast to the nearby Sculptor group.
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