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Imaging of I Zw 18 by JWST. I. Strategy and First Results of Dusty Stellar Populations (2403.06980v2)

Published 11 Mar 2024 in astro-ph.GA and astro-ph.SR

Abstract: We present a JWST imaging survey of I Zw 18, the archetypal extremely metal-poor, star-forming (SF), blue compact dwarf galaxy. With an oxygen abundance of only $\sim$3% $Z_{\odot}$, 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 Near Infrared Camera (F115W, F200W, F356W, and F444W) and Mid-Infrared Instrument (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, 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 SF galaxies in the local Universe.

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References (3)

Summary

  • The paper demonstrates the effective use of JWST's infrared filters to distinguish and analyze dusty stellar populations in I Zw 18.
  • The paper employs custom data processing techniques to correct instrumental issues and align multi-band images for precise photometry.
  • The paper reveals that dusty evolved stars and YSO candidates in I Zw 18 offer new insights into dust production in low-metallicity environments.

Imaging of I Zw 18 by JWST: Detecting Dusty Stellar Populations

The paper by Hirschauer et al. presents an imaging survey of I Zw 18 using the James Webb Space Telescope (JWST). I Zw 18 is an archetypal extremely metal-poor, star-forming blue compact dwarf galaxy. This galaxy, with an oxygen abundance of approximately 3% of the solar metallicity, serves as an excellent analog for the building blocks of galaxies that existed in the early Universe. The paper reports on the utilization of JWST's Near Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) to capture I Zw 18 across eight filters and provides insights into the dusty stellar populations within this galaxy.

Observational Strategy and Data Processing

The authors employed NIRCam's filters (F115W, F200W, F356W, and F444W) and MIRI's filters (F770W, F1000W, F1500W, and F1800W) to gather high-resolution near- and mid-infrared observations of I Zw 18. This approach allowed them to distinguish different stellar populations and identify key features in the infrared spectrum of this dwarf galaxy. The data processing involved correcting for known instrumental issues and aligning images across all bands to facilitate source matching and photometric analysis. The team enhanced the reduction pipeline with custom corrections for instrumental noise and improved alignment accuracy.

Initial Scientific Results

The paper utilized color-magnitude diagrams (CMDs) to analyze the stellar populations of I Zw 18, revealing distributions of young, massive stars and evolved stars such as red supergiants (RSGs) and asymptotic giant branch (AGB) stars. The presence of dusty evolved stars and bright young stellar objects (YSOs) demonstrates the ongoing star formation in this low-metallicity environment. Of particular note are the identifications of many dust-enshrouded evolved stars and YSO candidates, where the authors employed longer-wavelength baselines to assess the impacts of dust within the stellar population.

Implications and Future Work

These observations by JWST significantly enhance our understanding of dust production and the lifecycle of stars in metal-poor environments analogous to the early Universe. The presence of dusty AGB stars suggests that such stars might have been significant contributors to the dust enrichment of early galaxies, contrary to initial expectations that low-metallicity environments would hinder substantial dust production. Moreover, the identification and analysis of YSOs provide insights into the mechanisms of star formation under primitive conditions.

Theoretical implications of this work include recalibrating expectations around dust evolution models in low-metallicity galaxies. This can influence simulations and models of galaxy formation and evolution in the high-redshift Universe. Practically, these findings can guide future JWST observations, suggesting a strategy for maximizing scientific output by focusing on specific infrared filters that enhance signal from dusty and evolved stars.

Overall, this paper exemplifies how JWST data can be leveraged to probe stellar populations in the context of galactic evolution, addressing key questions about the processes that dominated early cosmic epochs. Future exploration could involve spectral energy distribution modeling to further disentangle the contributions from different stellar and dust components in I Zw 18. As JWST continues to deliver unparalleled infrared data, its role in redefining our understanding of low-metallicity star formation and galactic evolution appears promising.

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