Evidence for tidal interaction and a supergiant HI shell in the Local Group dwarf galaxy NGC 6822

Abstract

We present a wide-field, high spatial and velocity resolution map of the entire extended HI distribution of the Local Group dwarf galaxy NGC 6822. The observations were obtained with the Australia Telescope Compact Array in mosaicing mode. The interstellar medium of NGC 6822 is shaped by the presence of numerous H{\sc i} holes and shells, including a supergiant shell, and the effects of tidal interaction, in the form of a tidal arm and an infalling or interacting HI complex. The HI shell is situated outside the optical galaxy and occupies roughly a quarter of the area of the main HI disk. It measures 2.0 times 1.4 kpc, making it one of the largest supergiant HI shells ever found. The giant hole shows no signs of expansion and no obvious creation mechanism is evident from our data. If star formation was the cause, an energy equivalent of \~100 supernovae (10^53 erg) is needed to create the hole. We derive an upper limit for the age of order 100 Myr. The presence of a possible tidal arm indicates that NGC 6822 may recently have undergone some interaction. An HI complex located in the north-west of the galaxy may be the interaction partner. We argue that it is likely that these features were created about 100 Myr ago in an event that also enhanced the star formation rate