A Constraint on the Formation of Dwarf Elliptical Galaxies in the Dense Coma Cluster Core

Abstract

Deep CCD photometry and positions for a new sample of $\simeq 250$ dwarf elliptical (dE) galaxies in the Coma cluster core are analyzed. A significant color gradient is detected in their projected radial distribution, given by $\Delta (B$--$R)/\Delta (\log R_{\rm cc}) = -0.08\pm0.02$ mag. Calibrating these dE galaxies against Galactic globular clusters yields a corresponding metallicity gradient which goes as $Z \propto R_{\rm cc}^{-0.29}$. Simulations reveal that this gradient in the projected radial distribution corresponds to a true (three-dimensional) radial color gradient which goes as $\Delta (B$--$R)/\Delta (\log r) \simeq -0.20$ mag, or $Z \propto r^{-0.69}$, over the same radial range. If this radial gradient is a primordial one, it is consistent with a model in which the intracluster gas exerted a significant confinement pressure, impeding the outflow of supernovae-driven metal-rich gas from the young dE galaxies.