On the Origin of the Supergiant HI Shell and Putative Companion in NGC 6822

TL;DR

This study uses Hubble imaging to analyze the star formation history and structure of NGC 6822, suggesting the large HI hole is caused by stellar feedback and the extended HI structure is likely a warped disk, not a companion galaxy.

Contribution

It provides detailed star formation histories across NGC 6822 and challenges the idea of a companion galaxy causing the HI hole, proposing a warped disk as the explanation.

Findings

Star formation occurred mainly in the last 5 Gyr.

The HI hole can be formed by stellar feedback over >500 Myr.

No evidence supports the presence of a true companion galaxy.

Abstract

We present new Hubble Space Telescope Advanced Camera for Surveys imaging of six positions spanning 5.8 kpc of the HI major axis of the Local Group dIrr NGC 6822, including both the putative companion galaxy and the large HI hole. The resulting deep color magnitude diagrams show that NGC 6822 has formed >50% of its stars in the last ~5 Gyr. The star formation histories of all six positions are similar over the most recent 500 Myr, including low-level star formation throughout this interval and a weak increase in star formation rate during the most recent 50 Myr. Stellar feedback can create the giant HI hole, assuming that the lifetime of the structure is longer than 500 Myr; such long-lived structures have now been observed in multiple systems and may be the norm in galaxies with solid-body rotation. The old stellar populations (red giants and red clump stars) of the putative companion are consistent with those of the extended halo of NGC 6822; this argues against the interpretation of this structure as a bona fide interacting companion galaxy and against its being linked to the formation of the HI hole via an interaction. Since there is no evidence in the stellar population of a companion galaxy, the most likely explanation of the extended HI structure in NGC 6822 is a warped disk inclined to the line of sight.