A Local Group Polar Ring Galaxy: NGC 6822

Abstract

Star counts obtained from a 2° × 2° area centered on NGC 6822 have revealed an optical image of this galaxy composed of two components: in addition to the well-known H I disk with its young stellar component, there is a spheroidal stellar structure as extensive as its H I disk, but with its major axis at roughly right angles to it, that we traced to at least 36′. Radial velocities of over 100 intermediate-age carbon stars found within this structure display kinematics contrasting strongly with those of the H I disk. These C stars belong to the spheroid. Although devoid of gas, the spheroid rotation is consistent with the I-band Tully-Fisher relation. The orientation of the rotation axis that minimizes the stellar velocity dispersion coincides with the minor axis of the stellar population ellipsoid, lying very nearly in the plane of the H I disk. We conclude that the H I disk is a polar ring and that the spheroidal component is an erstwhile disk, a fossil remainder of a past close encounter episode. © 2006. The American Astronomical Society. All rights reserved.