Stripping of nitrogen-rich AGB ejecta from interacting dwarf irregular galaxies

Many dwarf irregular galaxies (dIrrs) exhibit an unusually low N/O ratio in H II regions. This ratio is broadly equivalent to the average level of Galactic extremely metal-poor stars, suggesting that N released from asymptotic giant branch (AGB) stars is missing in the present-day interstellar matter of these dIrrs. We find evidence for past tidal interactions in the properties of individual dIrrs exhibiting low N/O ratios. Accordingly, we propose that the ejecta of massive AGB stars that correspond to a major production site of N can be stripped from dIrrs that have undergone a strong interaction with a luminous galaxy. The physical process of its stripping is made up of two stages: (i) the ejecta of massive AGB stars in a dIrr are first merged with those of the bursting prompt Type Ia supernovae and pushed up together to the galaxy halo and (ii) subsequently through tidal interactions with a luminous galaxy, these ejecta are stripped from a dwarf galaxy's potential well. Our new chemical evolution models with stripping of AGB ejecta succeed in reproducing the observed low N/O ratio. Furthermore, we confirm that a tidal interaction is responsible for the efficient stripping of AGB ejecta from dIrrs by N-body+hydrodynamical simulations.