HST Photometry of Dwarf Elliptical Galaxies in Coma, and an Explanation for the Alleged Structural Dichotomy between Dwarf and Bright Elliptical Galaxies

As part of a research program exploring how and why dwarf elliptical (dE) galaxies depart from the fundamental plane defined by luminous elliptical (E) galaxies, we have analyzed archival Hubble Space Telescope F606W images of a sample of 18 dE galaxy candidates in the Coma Cluster. We model the full radial extent of their light profiles by simultaneously fitting a point-spread function (PSF) convolved Sérsic R1/n model and, when necessary, either a central point source or a central PSF-convolved Gaussian. Nucleation was detected in all but two of our final sample of 15 dE galaxies. When detected, the luminosities of the central component, Lnuc, scale with the host galaxy luminosity Lgal such that Lnuc = 104.76±0.10(Lgal/107)0.87±0.26. We confirm that the light profiles of the underlying host galaxies display systematic departures from an exponential model that are correlated with the model-independent host galaxy luminosity and are not due to biasing from the nuclear component. The Pearson correlation coefficient between log n and central galaxy surface brightness μ0 (excluding the flux from extraneous central components) is -0.83 at a significance level of 99.99%. Excluding one outlier, the Pearson correlation coefficient between the logarithm of the Sérsic index n and the host galaxy magnitude is -0.77 at a significance of 99.9%. We explain the observed relationship between dE galaxy luminosity and the inner logarithmic profile slope γ′ as a by-product of the correlation between luminosity and Sérsic index n. Including, from the literature, an additional 232 dE and E galaxies spanning 10 mag in absolute magnitude M, the dE galaxies are shown to display a continuous sequence with the brighter E galaxies, such that μ0 brightens linearly with M until core formation causes the most luminous (MB ≲ -20.5) E galaxies to deviate from this relation. The different behavior of dE and E galaxies in the M-⟨μ⟩e (and M-μe) diagram and the ⟨μ⟩e–log Re diagram has nothing to do with core formation and is in fact expected from the continuous and linear relation between M and μ0, and M and log n.