Starbursting Nuclei in Old Dwarf Galaxies

Nuclei of early-type dwarf galaxies (dEs) are usually younger than the galaxy main body, and this discrepancy in age has been a puzzle. To explore the origin of young nuclei in dEs, we study a sample of dEs having compact star-forming blobs that are visually similar to dEs' nuclei but by far bluer. We find that (1) the compact star-forming blobs have a typical stellar mass of one percent of the host galaxy stellar mass; (2) some of the blobs are positioned slightly off from the center of the galaxies; (3) the Hα equivalent width measured from the publicly available Sloan Digital Sky Survey fiber spectroscopy shows their formation ages being an order of few megayear; (4) their emission line metallicities, 12 + log(O/H), are as high as the solar value, while the underlying galaxies have the typical stellar populations of dEs, i.e., logZ/Z ∼ -0.8. Based on the results, we argue that the central star-forming blobs can provide a caught-in-the-act view of nuclei formation in dEs, and discuss possible formation mechanisms of young nuclei in old dEs. We particularly propose that these off-centered compact star-forming regions may act as seeds of nuclei as proposed in the "wet migration"scenario of Guillard et al.