The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822

Abstract

To test the existence of a possible radial gradient in oxygen abundances within the Local Group dwarf irregular galaxy NGC 6822, we have obtained optical spectra of 19 nebulae with the EFOSC2 spectrograph on the 3.6 m telescope at ESO La Silla. The extent of the measured nebulae spans galactocentric radii in the range between 0.05 and 2 kpc (over 4 exponential scale lengths). In five H II regions (Hubble I, Hubble V, Kα, Kβ, KD 28e), the temperature-sensitive [O III] λ4363 emission line was detected, and direct oxygen abundances were derived. Oxygen abundances for the remaining H II regions were derived using bright-line methods. The oxygen abundances for three A-type supergiant stars are slightly higher than nebular values at comparable radii. Linear least-square fits to various subsets of abundance data were obtained. When all of the measured nebulae are included, no clear signature is found for an abundance gradient. A fit to only newly observed H II regions with [O III] λ4363 detections yields an oxygen abundance gradient of -0.14 ± 0.07 dex kpc-1. The gradient becomes slightly more significant (-0.16 ± 0.05 dex kpc-1) when three additional H II regions with [O III] λ4363 measurements from the literature are added. Assuming no abundance gradient, we derive a mean nebular oxygen abundance 12 + log(O/H) = 8.11 ± 0.10 from [O III] λ4363 detections in the five H II regions from our present data; this mean value corresponds to [O/H] = -0.55.