NGC 6822 as a probe of dwarf galactic evolution

TL;DR

This study investigates the kinematic properties of NGC 6822, revealing a metallicity-dependent rotation pattern and insights into its formation history, using detailed stellar spectra analysis.

Contribution

It provides the first detailed kinematic analysis of RGB stars in NGC 6822, linking metallicity with rotation support and galaxy formation processes.

Findings

Rotation aligned with HI disk but opposite sense to the galaxy's major axis.

Higher metallicity stars show increased rotation support (v/σ).

Lower metallicity stars are found at larger radial distances, suggesting complex formation history.

Abstract

NGC 6822 is the closest isolated dwarf irregular galaxy to the Milky Way. Its proximity and stellar mass ($10^8 M_\odot$, large for a dwarf galaxy) allow for a detailed study of its kinematic properties. The red giant branch (RGB) stars at the galaxy's center are particularly interesting because they are aligned on an axis perpendicular to the galaxy's more extended HI disk. We detected a velocity gradient among the RGB population using spectra from Keck DEIMOS. This rotation is aligned with the HI disk, but the sense of rotation is about the major axis of the central RGB population. We measured the rotation velocity ($v$) and velocity dispersion ($\sigma$) of the RGB population in five metallicity bins. We found an increase of rotation support ($v/\sigma$) with increasing metallicity, driven primarily by decreasing dispersion. We also deduced an increasing radial distance for lower metallicity stars at $-0.5$~kpc/dex by relating the observed stellar kinematics to position via NGC 6822's HI velocity curve. While the inverted metallicity gradient-like could be interpreted as evidence for an outside-in formation scenario, it may instead indicate that stellar feedback disturbed a centrally star forming galaxy over time.