Core Discovery of Galaxy Evolution

The core of the circled red galaxy was fully formed 1.5 billion years after the Big Bang.

National Astronomical Observatory of Japan

An estimated range of 100 to 200 billion galaxies, large and small, populate the visible universe today. Astrophysicists believe clumps of stars began to condense about thirteen billion years ago from diffuse clouds of expanding gas that filled space a few hundred million years after the Big Bang. Those stars formed the first massive galaxies, whose strong gravity drew in more material to form additional stars at a frenetic pace for one or two billion years, until that star formation was “quenched” by the consequent depletion of gas. Telescope data from the most distant “quenching” galaxy known to date has allowed researchers to understand how such galaxies continue to grow.

An international research team, led by Masayuki Tanaka of the National Astronomical Observatory of Japan, used giant telescopes at the W. M. Keck Observatory in Hawaii to obtain an infrared spectrum of a massive distant galaxy that had previously been identified as “quenching.” The infrared spectrum data allowed the researchers to calculate that the mass of this distant galaxy was similar to the mass of the cores of giant galaxies much nearer to our own, despite appearing to be only a tenth the size of modern, closer galaxies. Thus, the distant “quenching” galaxy—which will continue to grow in diameter until it reaches a comparable size to modern galaxies—already had a fully formed, mature core when the universe was relatively young. This supports the hypothesis that “quenching” galaxies grow when a single galaxy consolidates around its mature core and gradually consumes smaller galaxies and adds stars to its outer edges, rather than by merging an immature core with those of other massive galaxies.

The researchers also confirmed that the light of the distant galaxy was emitted just 1.5 billion years after the Big Bang. “The previous measurement of this kind was made when the universe was 2.5 billion years old,” Tanaka commented. “We pushed the record up to 1.5 billion years and found, to our surprise, that the core was already pretty mature.” This implies that the cores of massive galaxies were fully formed in the early universe and that further growth of these giant stellar metropolises occurs by annexing their smaller suburban neighbors, not by collisions of two massive cores. (The Astrophysical Journal Letters)