Stripping of nitrogen-rich AGB ejecta from interacting dwarf irregular
  galaxies
  Dwarf irregular galaxies (dIrrs) including the Magellanic Clouds in the local
Universe, in many cases, exhibit an unusually low N/O abundance ratio (log N/O
~ -1.5) in H II regions as compared with the solar value (~-0.9). This ratio is
broadly equivalent to the average level of extremely metal-poor stars in the
Galactic halo, 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, while a clear signature of interactions is unseen
for dIrrs with high 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 SNe Ia 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 perform N-body + hydrodynamical simulations to trace the
fate of AGB ejecta inside a dIrr orbiting the Milky Way, and confirm that a
tidal interaction is responsible for the efficient stripping of AGB ejecta from
dIrrs.
