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The Rapid Formation of New Stars in Distant Galaxies

galaxy merger

A galaxy merger. Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University)

Galaxies forming stars at extreme rates nine billion years ago were more efficient than average galaxies today. So concluded a project led by John Silverman (Kavli Institute for the Physics and Mathematics of the Universe) that IfA astronomer David Sanders participated in. The researchers studied carbon monoxide (CO) gas content in seven distant starburst galaxies when the Universe was a young four billion years old.

In general, the larger a galaxy’s mass, the higher its rate of forming new stars—a relation called the star formation “main sequence.” However, every now and then, a galaxy displays a burst of newly formed stars that shine brighter than the rest. A collision between two large galaxies, in which the cold gas residing in the galaxies’ giant molecular clouds becomes the fuel for sustaining such high rates of star formation, is usually the cause of such starburst phases. The question astronomers have been asking is whether such starbursts in the early Universe were the result of having an overabundant gas supply, or whether some galaxies converted gas more efficiently.

This study found that the amount of CO-emitting gas was already diminished even though the galaxies continued to form stars at high rates. These observations are similar to those recorded for starburst galaxies in the vicinity of our Milky Way galaxy today, but the amount of gas depletion was not quite as rapid as expected. This led researchers to conclude there might be a continuous increase in the efficiency that depends on how high above the average rate a galaxy’s rate of star formation is.

The advent of the Atacama Large Millimeter/submillimeter Array (ALMA) made this study feasible. ALMA consists of 66 high-precision antennas located on the Chajnantor plateau at an altitude of 16,000 feet (5,000 meters) in northern Chile. These antennas work together to detect electromagnetic waves in the millimeter (radio) wavelength range that is essential for studying molecular gas, and together they have a sensitivity level that is far above that of previous millimeter/submillimeter arrays. ALMA is an international partnership of the European Southern Observatory (15 European countries and Brazil), the United States, Japan, Canada, Taiwan, and the Republic of Korea, in cooperation with the Republic of Chile.

This study also relied on a variety of powerful telescopes available through the Cosmic Evolution Survey (COSMOS). Observations made by the Spitzer and Herschel space observatories measured rates of star formation accurately, and the Subaru Telescope on Maunakea confirmed the nature and distance of these extreme galaxies using spectroscopy.