9 - The irregular dwarf galaxy NGC 6822

Summary

Introduction

NGC 6822 is a barred dwarf galaxy of type Ir IV–V that islocated at relatively low Galactic latitude. The first detailed study of this galaxy was by Hubble (1925c) who discovered 11 Cepheid variables. Hubble wrote “Cepheid variables, diffuse nebulae, dimensions, density, and distribution of stellar luminosities agree in defining the system as a curiously faithful copy of the [C]louds, but removed to a vastly greater distance. N.G.C. 6822 lies far outside the limits of the [G]alactic system, even as outlined by the globular clusters, and hence may serve as a stepping-stone for speculation concerning the habitants of space beyond.” From these observations Hubble (1936, p. 124) was able to establish that NGC 6822 is a relatively nearby member of the Local Group. Hubble also discovered nine nonstellar objects in this galaxy. Some of these are young star clusters embedded in nebulosity, whereas others appear to be old star clusters. Hubble's cluster VII has been studied by Cohen & Blakeslee (1998), who find that it is a typical metal-poor globular cluster with a metallicity [Fe/H]=-1.95±0.15 and an age of ∼11 Gyr. It would be interesting to determine the morphology of the horizontal branch of this cluster. Hodge (1977) gives V =16.28 for the cluster Hubble VII, from which M_V ≈-8.0. Cluster Hubble VI is a much younger object with an age of ∼2 Gyr and a metallicity of [Fe/H]≈-1.0, a value similar to the present abundance of the interstellar medium in this galaxy.