Early-type Dwarf Galaxies in the Local Universe. Evidence of Ex-situ Growth

We report the discovery of a rare early-type dwarf galaxy (dE), SDSS J125651.47+163024.2 (hereafter dE1256), possessing a tidal feature that was likely built up by accretion of an even smaller dwarf galaxy. dE1256 is located in a nearly isolated environment, at the outskirt of the Virgo cluster. A detailed morphological examination reveals that the accreted stellar population is mainly deposited in the outer part of dE1256, where the tidal tail is most prominent. The inner part of dE1256 is perfectly modeled with a simple S\'ersic function of index n = 0.63 and half-light radius R$_{h}$ = 0.6 kpc, but in contrast, the entire galaxy has a size of R$_{h}$ = 1.2 kpc. The mass ratio between the host and the putative accreted dwarf galaxy is calculated to be 5:1, assuming that the observed two components, inner S\'ersic, and outer tidal tail residual, represent the host's and accreted galaxy's stellar populations, respectively. We suggest that while the accretion contributes only 20% of the overall stellar population, the size of dE1256 grew by a factor of two via the accretion event. Our results provide, for the first time, strong observational evidence that a dE is undergoing a two-phase growth, a common phenomenon for massive galaxies.
Comment: Accepted for publication in MNRAS