A panoramic view of the Local Group dwarf galaxy NGC 6822

TL;DR

This study provides a comprehensive deep imaging survey of NGC 6822, revealing its structure, lack of recent interactions, and orbital history, including a possible past close approach to the Milky Way.

Contribution

It offers the deepest and widest-area photometric analysis of NGC 6822, with new insights into its morphology, stellar populations, and orbital history based on Gaia data.

Findings

No stellar over-densities detected in outskirts

No evidence of dwarf satellites to M_V~-5

NGC 6822 likely passed near the Milky Way 3-4 Gyr ago

Abstract

We present a panoramic survey of the isolated Local Group dwarf irregular galaxy NGC 6822. 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 over-densities in the outskirts of NGC 6822 to $V\sim 30$ 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 $M_V\approx -5$. 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 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 $\sim3-4$ Gyr ago. This may explain the apparent flattening and twisting observed in the outskirts of its spheroid.