Video: This video zooms in on the galaxy I Zwicky 18. It was made from a wide-field image of the constellation Ursa Major (Courtesy: ESA/Hubble/M Kornmesser/L L Christensen/A Fujii/Digitized Sky Survey 2)
A bizarre galaxy thought to have started forming stars billions of years after its peers is not such a late bloomer after all, new Hubble observations reveal. Nonetheless, its primordial composition 鈥� resembling the first galaxies in the universe 鈥� remains a mystery.
In 2004, observations by the Hubble Space Telescope鈥檚 Advanced Camera for Surveys (ACS) suggested that none of the stars in a small, nearby galaxy called I Zwicky 18 were more than 500 million years old (see Hubble watches baby galaxy in bloom). The galaxy was also made up almost entirely of hydrogen and helium 鈥� elements forged in the big bang.
That was surprising, since most galaxies in the universe formed their stars 10 billion years ago and have 100 times more heavy elements, or 鈥渕etals鈥�, such as carbon and oxygen, which are created in successive generations of stars.
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The observation suggested that I Zwicky 18 probably started out as a small gas cloud in a relatively empty region of space. Long starved of nearby gas from which it could form stars, its gestation period appeared to have been drawn out until relatively recently, making it a nearby example of the first galaxies to take shape in the universe.
But now, researchers led by Alessandra Aloisi at the Space Telescope Science Institute in Baltimore, Maryland, US, report that the galaxy probably started forming stars billions of years ago. The study is based on Hubble ACS observations made in 2005 and 2006 that lasted twice as long as the previous measurement.
Fainter stars
That deeper look allowed them to see fainter, older stars that were not detected before. It also revealed variable stars that helped pinpoint the galaxy鈥檚 distance at 59 million light years away, improving previous estimates that placed the galaxy just 45 million light years from Earth.
The newly observed faint stars are at least 1 billion years old, but 鈥渢hey can be as old as the first stars in the universe鈥�, Aloisi told New 杏吧原创. 鈥淲e think they have ages older than several billion years.鈥�
That suggests the galaxy was not a late bloomer. 鈥淚t started forming stars probably at the same time as the other [galactic] building blocks at the beginning of the universe,鈥� she says. 鈥淏ut then he didn鈥檛 have a lot of star formation because he was isolated, left alone. He had difficulty forming stars.鈥� Today, the galaxy is forming new stars 鈥� a result, perhaps, of a cloud of gas falling onto it, she says.
Alice Shapley, an astronomer at Princeton University in New Jersey, US, who is not on the team, says the new study suggests that I Zwicky 18 may not be an exact replica of the universe鈥檚 first galaxies.
Pristine gas
鈥淚t may not be purely as young and pristine as people thought. You need to be careful about making an analogy with galaxies forming in the very early universe,鈥� Shapley told New 杏吧原创.
Aloisi agrees, saying the earlier suggestion that all of the galaxy鈥檚 stars were young 鈥渨ould have been great news because we would have had an infant galaxy right around the corner鈥�. But she adds: 鈥淭his is still the closest we can get to primordial galaxies in the early universe, at least in terms of metallicity. It is interesting to understand how the physics works in an environment with such low metals.鈥�
Aloisi鈥檚 team will now perform computer simulations of different star birth models to try to match the Hubble observations. 鈥淲e will reconstruct the star formation history over the whole lifetime of the galaxy,鈥� she says.
She says one possible explanation for the galaxy鈥檚 pristine composition is that supernovae inside it created a galactic wind that pushed gas enriched with heavy elements out of the galaxy entirely.
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