The Rotation of the Halo of NGC 6822 Determined From the Radial Velocities of Carbon Stars

Abstract

The kinematics of stars in galaxies is becoming a key means to understand the dynamics of galaxies, their formation and merger history. For many years, kinematics has been measured using integrated field techniques in the optical, infra-red and radio wavebands, but modern CCD multi-channel cameras are now making it possible to measure the spectra of individual stars in galaxies and thus enabling more detailed analysis of galaxy kinematics. Using spectra taken with the AAOmega spectrograph, I 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. I calculate a heliocentric radial velocity for NGC 6822 of -51 _ 3 km s¯1 and show that the population rotates with a mean rotation speed of 11:3 _ 2:1 km s¯1 at a mean distance of 1.1 kpc from the galactic centre, about a rotation axis with a position angle of 25_ _ 14_, 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. Finally, further work is suggested which would examine stars in the galaxy outside the central sample for which I had data, to confirm my findings and to attempt to understand the misalignment of the rotation axis and isophotes. In addition, I suggest further work to constrain the dynamical mass of the galaxy, which up to now is not well determined.