James Webb Space Telescope Feed Post

Imaging of I Zw 18 by JWST: I. Strategy and First Results of Dusty Stellar Populations

Three-color composite images of I Zw 18 in near- and mid-IR filters. A combination utilizing NIRCam F356W, F444W, and MIRI F770W (left) emphasizes the ISM structure of the galaxy in relation to its recent star formation. The longer-wavelength MIRI F1000W, F1500W, and F1800W filters (right) showcase bulk emission properties of the major SF regions. Image orientation is such that north is to the left, while east is down. Located at a distance of 18.2 Mpc, these imaging data present resolutions of 0.04 – 0.14" in NIRCam and 0.3 – 0.6" in MIRI. Abstract: We present a James Webb Space Telescope (JWST) imaging survey of I Zw 18, the archetypal extremely metal-poor, star-forming, blue compact dwarf galaxy. With an oxygen abundance of only ~3% Z?, it is among the lowest-metallicity systems known in the local universe, and is, therefore, an excellent accessible analog for the galactic building blocks which existed at early epochs of ionization and star formation. These JWST data provide a comprehensive infrared (IR) view of I Zw 18 with eight filters utilizing both NIRCam (F115W, F200W, F356W, and F444W) and MIRI (F770W, F1000W, F1500W, and F1800W) photometry, which we have used to identify key stellar populations that are bright in the near- and mid-IR. These data allow for a better understanding of the origins of dust and dust-production mechanisms in metal-poor environments by characterizing the population of massive, evolved stars in the red supergiant (RSG) and asymptotic giant branch (AGB) phases. In addition, it enables the identification of the brightest dust-enshrouded young stellar objects (YSOs), which provide insight into the formation of massive stars at extremely low metallicities typical of the very early universe. This paper provides an overview of the observational strategy and data processing, and presents first science results, including identifications of dusty AGB star, RSG, and bright YSO candidates. These first results assess the scientific quality of JWST data and provide a guide for obtaining and interpreting future observations of the dusty and evolved stars inhabiting compact dwarf star-forming galaxies in the local universe.