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, based on imaging acquired with the Dark Energy Camera on the 4m Blanco telescope and Megacam on the 6.5m Magellan telescope. Our photometry reaches $\sim2-3$ magnitudes deeper than most previous studies and spans the widest area around the dwarf compared to any prior work. We observe no stellar streams or overdensities in the outskirts of NGC 6822 to a faint surface brightness limit $V\sim 30$ mag$\,$arcsec$^{-2}$ and a projected radius of $16.5$ kpc. This strongly indicates that NGC 6822 has not experienced any recent interaction or merger 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 $M_V\approx -5$. NGC 6822 contains a disk of HI gas and young stars, oriented at $\sim 60$ degrees to an extended spheroid composed of old stellar populations. We observe no correlation between dense clumps of young stars and/or HI gas, and the distribution of stars in the spheroid. Our imaging allows us to trace the spheroid to nearly $11$ kpc along its major axis -- commensurate with the extent of the globular cluster system in NGC 6822. We find that the spheroid becomes increasingly flattened at larger radii, and its position angle twists by up to $40$ degrees. We use astrometry from Gaia EDR3 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, falling inwards from the edge of the Local Group, we find that it likely passed within the virial radius of the Milky Way $\sim3-4$ Gyr ago. This may explain the apparent flattening and twisting observed in the outskirts of its spheroid.