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

TL;DR

This paper investigates how tidal interactions strip nitrogen-rich AGB star ejecta from dwarf irregular galaxies, explaining their low N/O ratios through combined chemical evolution models and hydrodynamical simulations.

Contribution

It introduces a new model showing that tidal interactions can strip AGB ejecta, accounting for low N/O ratios in dwarf irregular galaxies, supported by simulations.

Findings

Tidal interactions correlate with low N/O ratios in dIrrs.

Stripping of AGB ejecta explains observed chemical abundances.

Simulations confirm the efficiency of tidal stripping of AGB ejecta.

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.