I Zwicky 18, a dwarf irregular galaxy in Ursa Major

I Zwicky 18, a dwarf irregular galaxy in Ursa Major

I Zwicky 18 is a very luminous dwarf irregular galaxy, much smaller than our Milky Way, located about 59 million-light years away in the constellation Ursa Major. It is receding from us at a speed of 751 kilometers per second.

The galaxy has just a few old stars, but lots of very young stars and a few starburst regions. Spectroscopy shows that its stars are composed almost entirely of hydrogen and helium — the main ingredients created in the Big Bang — with heavier elements almost completely absent.

This youthful appearance resembles galaxies typically found only in the early Universe. The faint, older stars within this galaxy, however, suggest its star formation started at least one billion years ago and possibly as much as ten billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies in this region: more than 10 billions of years ago.

I Zwicky 18 may still be creating Population III stars — extremely massive and hot stars with virtually no surface metals, except for a small quantity of metals formed in the Big Bang, normally formed in the early Universe.

The concentrated bluish-white knots embedded in the heart of the galaxy are two major starburst regions where stars are forming at a furious rate. The wispy blue filaments surrounding the central starburst regions – which extend 16 times farther out from the center than the stars — are bubbles of gas that have been blown away by stellar winds and supernovae explosions from a previous generation of hot, young stars. This gas is now heated by intense ultraviolet radiation unleashed by a new generation of hot, young stars and emits at least 1/3 of the total light seen from the galaxy, and could account for as much as half the light emitted.

Besides the bluish-white young stars, white-reddish stars also are visible in I Zwicky 18. These stars may be as old as 10 billion years.

The distance of I Zwicky 18 has been measured by observing Cepheid variable stars within the galaxy. These massive flashing stars pulse in a regular rhythm. The timing of their pulsations is directly related to their brightness. Astronomers determined the observed brightness of three Cepheids and compared it with the actual brightness predicted by theoretical models. These models were calculated specifically for I Zwicky 18’s deficiency in heavy elements, indicating the galaxy’s stars formed before these elements were abundant in the Universe. The Cepheid distance also was validated through the observed brightness of the brightest red stars older than one billion years.

The galaxy’s larger-than-expected distance may explain why astronomers have had difficulty detecting older, fainter stars within the galaxy. The faint, old stars in I Zwicky 18 are almost at the limit of Hubble’s resolution and sensitivity.

The galaxy’s primordial makeup suggests that its rate of star formation has been much lower than that of other galaxies of similar age. However, it remains a mystery why I Zwicky 18 formed so few stars in the past, and why it is forming so many new stars right now. Possibly the trigger for this recent episode of bright star formation is the changing gravitational influence of I Zwicky 18’s smaller companion galaxy, not visible in this image.

The reddish extended objects surrounding I Zwicky 18 are ancient, fully formed galaxies of different shapes that are much farther away.

Image Credit: NASA, ESA, and A. Aloisi (Space Telescope Science Institute and ESA)

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