On the Origin of the Supergiant H I Shell and Putative Companion in NGC 6822
Abstract
We present new Hubble Space Telescope Advanced Camera for Surveys imaging of six positions spanning 5.8 kpc of the H I major axis of the Local Group dIrr NGC 6822, including both the putative companion galaxy and the large H I 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 H I 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 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 NGC 6822 is a warped disk inclined to the line of sight.
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.- Publication:
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The Astrophysical Journal
- Pub Date:
- March 2012
- DOI:
- 10.1088/0004-637X/747/2/122
- arXiv:
- arXiv:1201.3149
- Bibcode:
- 2012ApJ...747..122C
- Keywords:
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- galaxies: dwarf;
- galaxies: evolution;
- galaxies: individual: NGC 6822;
- galaxies: irregular;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- The Astrophysical Journal, in press. Full-resolution version available on request from the first author