The Rotation of the halo of NGC 6822 from the radial velocities of carbon stars

TL;DR

This study measures the radial velocities of carbon stars in NGC 6822, revealing its rotation characteristics and suggesting it is a late-type galaxy with a rotation axis aligned with its HI gas disk.

Contribution

It provides the first detailed measurement of the rotation of the stellar halo in NGC 6822 using carbon star velocities.

Findings

NGC 6822's heliocentric radial velocity is -51±3 km/s.

The galaxy's stellar population rotates at about 11.2±2.1 km/s.

The rotation axis aligns with the HI gas disk, indicating a late-type galaxy.

Abstract

Using spectra taken with the AAOmega spectrograph, we measure the radial velocities of over 100 stars, many of which are intermediate age carbon stars, in the direction of the dwarf irregular galaxy NGC 6822. Kinematic analysis suggests that the carbon stars in the sample are associated with NGC 6822, and estimates of its radial velocity and galactic rotation are made from a star-by-star analysis of its carbon star population. We calculate a heliocentric radial velocity for NGC 6822 of $-51\pm3$ \kms\ and show that the population rotates with a mean rotation speed of $11.2\pm2.1$ \kms\ at a mean distance of 1.1 kpc from the galactic centre, about a rotation axis with a position angle of $26^\circ\pm13^\circ$, as projected on the sky. This is close to the rotation axis of the HI gas disk and suggests that NGC 6822 is not a polar ring galaxy, but is dynamically closer to a late type galaxy. However, the rotation axis is not aligned with the minor axis of the AGB isodensity profiles and this remains a mystery.