Early galaxies were more efficient star factories

Early galaxies were more efficient star factories

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Billions of years ago, galaxies were more efficient at producing new stars. Given the same amount of gas – the material from which new stars are born – the rate of star formation was higher in the distant past than it is now, according to new measurements made by ALMA.

Most galaxies contain clouds of gas – the ‘building material’ of stars like our sun. Larger and more massive galaxies contain more gas than smaller galaxies. Little wonder then that hefty galaxies produce more stars than lightweight galaxies.

But in the distant past, some galaxies underwent real ‘birth explosions’ of new stars. In such starburst galaxies, the rate of star formation is much higher than you would expect on the basis of their mass. In some galaxies, no less than a few hundred new stars are born on average per year. For comparison: our own Milky Way galaxy produces just one new star per year or so.

Astronomers were curious about the underlying cause of these starbursts: Did they occur because there was much more gas available? Or were those galaxies just more efficient in turning the gas into stars?

The answer appears to be both. Yes, the distant starburst galaxies (which we observe as they were billions of years ago) contain large amounts of gas. But not enough to fully explain the high star formation rate. Apparently, if there’s enough gas available, the process of star formation becomes ever more efficient.

To arrive at these results, astronomers studied seven distant starburst galaxies. Infrared space telescopes were used to determine the star formation rate. Observations with ALMA revealed the amount of gas in the galaxies.

It’s not clear yet why the process of star formation in the early starburst galaxies was more efficient. The astronomers think that collisions and mergers of galaxies may somehow increase the star birth efficiency.

The seven starburst galaxies observed in this study were selected from Hubble Space Telescope catalogues. Using the Japanese Subaru Telescope on Mauna Kea, Hawaii, astronomers determined their distances: over 9 billion light-years, which means that we see those galaxies as they were some 4 billion years after the Big Bang. The American Spitzer Space Telescope and the European Herschel Space Observatory were used to measure the star formation rate in the seven starburst galaxies. Finally, using ALMA and a smaller submillimeter observatory in the French Alps (called IRAM), astronomers measured the amount of carbon monoxide gas in the seven galaxies – a good indicator of the total amount of gas.

This research was carried out by a large international team of astronomers led by John Silverman of the Kavli Institute for the Physics and Mathematics of the Universe at the University of Tokyo in Japan. John and his colleagues published their result in The Astrophysical Journal Letters on 12 October 2015.