A panoramic view of the Local Group dwarf galaxy NGC 6822

We present a panoramic survey of the isolated Local Group dwarf irregular galaxy NGC 6822. Our photometry reaches similar to 2-3 mag deeper than most previous studies and spans the widest area around the dwarf compared to any prior work. We observe no stellar overdensities in the outskirts of NGC 6822 to V similar to 30 mag arcsec(-2) and a projected radius of 16.5 kpc. This indicates that NGC 6822 has not experienced any recent interaction with a companion galaxy, despite previous suggestions to the contrary. Similarly, we find no evidence for any dwarf satellites of NGC 6822 to a limiting luminosity M-V approximate to -5. NGC 6822 contains a disc of H I gas and young stars, oriented at similar to 60 degrees to an extended spheroid composed of old stellar populations. We observe no correlation between the distribution of young stars and spheroid members. Our imaging allows us to trace the spheroid to nearly 11 kpc along its major axis, commensurate with the extent of the NGC 6822 globular cluster system. We find that the spheroid becomes increasingly flattened at larger radii, and its position angle twists by up to 40 degrees. We use Gaia EDR3 astrometry to measure a proper motion for NGC 6822, and then sample its orbital parameter space. While this galaxy has spent the majority of its life in isolation, we find that it likely passed within the virial radius of the Milky Way similar to 3-4 Gyr ago. This may explain the apparent flattening and twisting observed in the outskirts of its spheroid.

Automated summary

This study presents a panoramic survey of the NGC 6822 dwarf irregular galaxy, revealing no recent interactions with a companion galaxy or any dwarf satellites around it. NGC 6822 contains a disc of H I gas and young stars, with an extended spheroid of old stellar populations. The spheroid becomes increasingly flattened and twisted at larger radii, possibly due to a past interaction with the Milky Way.