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

Abstract

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.