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Star factories: distant dwarf galaxies caught amid star-making frenzy

New research reveals that distant dwarf galaxies made stars at an astonishing rate. How did they do it? Scientists have theories, but none completely add up yet.

Dwarf irregular galaxy Holmberg II is dominated by huge bubbles of glowing gas, which are sites of ongoing star formation.


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When it comes to galaxies nurturing new stars in the early universe, the pint-sized can be surprisingly productive.

A team of astronomers has uncovered a population of dwarf galaxies some 9 billion to 10 billion light-years away that were producing stars at a furious pace.

The discovery presents a puzzle as scientists try to piece together the evolutionary history of dwarf galaxies, the most ubiquitous type of galaxy in the cosmos.

At the time these galaxies were blazing – some 3.7 billion to 4.7 billion years after the universe is estimated to have formed – dwarf galaxies represented about 2 percent of the star-formation activity underway in the universe, scientists believe. But they represented only about 0.1 percent of the overall amount mass available for star formation at the time.

The discrepancy implies an extraordinary level of activity.

At the rates these dwarf galaxies formed new stars, one of these would have doubled its population in about 10 million years, the wink of an eye on cosmic time scales. The Milky Way, by contrast, with about 100 times the mass of one of these dwarfs, would take about 10 billion years to double the number of stars it contains.

Typically, dwarf galaxies contain several billion stars, while the Milky Way is estimated to have between 200 billion to 400 billion. Many galaxies are even larger.

The startling star-formation rates stand in stark contrast to the production rates in dwarf galaxies near the Milky Way, which "are not really forming stars very fast, and neither is the Milky Way," says Steven Finkelstein, an astrophysicist at Texas A&M University in College Station, Texas, and a member of the US, British, and German team reporting the results in next week's issue of the Astrophysical Journal.

In one sense, that's no surprise. The universe ended its most-intense period of star formation about 6 billion years ago, researchers say. Indeed, when astronomers train their telescopes on the distant universe – literally looking back in time to a younger epoch – they see "galaxies are forming stars more rapidly than the Milky Way," Dr. Finkelstein says.


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