Astronomers using the NASA/ESA/CSA James Webb Space Telescope have captured a spectacular image of I Zwicky 18 (I Zw 18 for short), the archetypal extremely metal-poor, star-forming, blue compact dwarf galaxy in the constellation of Ursa Major.
This Webb image shows I Zwicky 18, a blue compact dwarf galaxy some 59 million light-years away in the constellation of Ursa Major. A companion galaxy resides nearby to I Zwicky 18 can be seen at the bottom of the image. The companion may be interacting with the dwarf galaxy and may have triggered that galaxy’s recent star formation. Image credit: NASA / ESA / CSA / Webb / Hirschauer et al.
I Zw 18 is located approximately 59 million light-years away in the constellation of Ursa Major.
Also known as Mrk 116, LEDA 27182 or UGCA 166, the galaxy was discovered by the Swiss astronomer Fritz Zwicky in the 1930s.
It has a diameter of only 3,000 light-years and is much smaller than our own Milky Way Galaxy.
I Zw 18 has gone through several sudden bursts of star formation, and hosts two major starburst regions in its center.
The wispy brown filaments surrounding the central starburst region are bubbles of gas that have been heated by stellar winds and intense ultraviolet radiation unleashed by hot, young stars.
“Metal-poor star-forming dwarf galaxies in the local Universe represent accessible analogs to those at high redshift,” said Dr. Alec Hirschauer from the Space Telescope Science Institute and colleagues.
“As the enrichment history for a given system traces the build up of heavy elements from successive generations of stellar nucleosynthesis, a low-abundance galaxy mimics the astrophysical conditions common in the early Universe, including the universal epoch of peak star formation, at which point a majority of the Universe’s star formation and chemical enrichment is expected to have taken place.”
“At the very lowest metallicities, we may therefore approximate the star-forming environments of the time period shortly after the Big Bang.”
“I Zw 18 is among the most extremely metal-poor systems known, with a measured gas-phase oxygen abundance of only approximately 3% solar,” they added.
“At a distance of 59 million light-years and with global star-formation rate values measured at 0.13-0.17 solar masses per year, it is an ideal laboratory for study of both the young and evolved star demographics in an environment analogous to that found in the very early Universe.”
Dr. Hirschauer and co-authors used Webb to study the life cycle of dust in I Zw 18.
“Although previously believed to have only just recently begun forming its first generation of stars, the NASA/ESA Hubble Space Telescope found fainter, older red stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as 10 billion years ago,” they said.
“The galaxy, therefore, may have formed at the same time as most other galaxies.”
“The new observations from Webb reveal the detection of a set of candidate dusty evolved stars. It also provides details about Zw 18’s two dominant star-forming regions.”
“Webb’s new data suggest that the dominant bursts of star formation in these regions occurred at different times.”
“The strongest starburst activity is now believed to have happened more recently in the northwest lobe as compared to the galaxy’s southeast lobe.”
“This is based on the relative populations of younger versus older stars found in each of the lobes.”
The findings will be published in the Astronomical Journal.
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Alec S. Hirschauer et al. 2024. Imaging of I Zw 18 by JWST: I. Strategy and First Results of Dusty Stellar Populations. AJ, in press; arXiv: 2403.06980
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