The young stellar population at the center of NGC 205

Context. NGC 205 is a peculiar dwarf elliptical galaxy hosting in its center a population of young blue stars. Their origin is still matter of debate, the central fresh star formation activity possibly being related to dynamical interactions between NGC 205 and M 31. Aims. The star formation history in the central 30´´ (~120 pc) around the NGC 205 central nucleus is investigated in order to obtain clues to the origin of the young stellar population. Methods. Deep HST/ACS CCD photometry is compared with theoretical isochrones and luminosity functions to characterize the stellar content of the region under study and compute the recent SF rate. Results. Our photometry reveals a previously undetected blue plume of young stars clearly distinguishable down to $I\simeq 26$. Our analysis suggests that $1.9 \times 10^5$ $M_\odot$ were produced between approximately 62 Myr and 335 Myr ago in the NGC 205 inner regions, with a latest minor episode occurring ~25 Myr ago. This implies a star formation rate of ~$7\times 10^{-4}$ $M_\odot$/yr over this period. Conclusions. The excellent fit of the observed luminosity function of young main sequence stars obtained with a model having a constant star formation rate argues against a tidally triggered star formation activity over the last ~300 Myr. Rather, a constant SF may be consistent with NGC 205 being on its first interaction with M 31.