On the origin of the supergiant HI shell and putative companion in NGC 6822

Abstract

We present new Hubble Space Telescope Advanced Camera for Surveys imaging of six positions spanning 5.8kpc of the H I major axis of the Local Group dIrr NGC6822, including both the putative companion galaxy and the large H I hole. The resulting deep color-magnitude diagrams show that NGC6822 has formed >50% of its stars in the last 5Gyr. The star formation histories of all six positions are similar over the most recent 500Myr, including low-level star formation throughout this interval and a weak increase in star formation rate during the most recent 50Myr. Stellar feedback can create the giant H I hole, assuming that the lifetime of the structure is longer than 500Myr; 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 NGC6822; 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 H I hole via an interaction. Since there is no evidence in the stellar population of a companion galaxy, the most likely explanation of the extended H I structure in NGC6822 is a warped disk inclined to the line of sight. © © 2012 The American Astronomical Society. All rights reserved.