One of the biggest mysteries in astronomy is why some of the largest galaxies in the early universe stopped making stars surprisingly early in their lives.
Most galaxies, including our own Milky Way, continue forming stars for billions of years.
The Milky Way is nearly as old as the universe itself and still produces new stars today, although at a relatively slow pace.
Yet astronomers have found enormous galaxies that formed just a few billion years after the Big Bang and then quickly shut down star formation less than a billion years later.
A new study led by researchers at the University of São Paulo in Brazil may help explain why.
The researchers focused on two types of galaxies seen in the early universe. The first group consists of dusty star-forming galaxies, known as DSFGs.
These galaxies are cosmic star factories, producing stars at an astonishing rate. Some create as many as 500 stars’ worth of mass every year, compared with about one solar mass per year in the Milky Way.
Because these galaxies are wrapped in thick clouds of dust, they are difficult to see with ordinary optical telescopes.
However, they glow brightly at infrared and submillimeter wavelengths, allowing observatories such as the Atacama Large Millimeter/submillimeter Array and the James Webb Space Telescope to detect them.
The second group is known as massive quiescent galaxies, or MQs. These galaxies contain huge numbers of stars but have largely stopped producing new ones. Their existence has puzzled scientists because they seem to have formed and become inactive remarkably quickly.
To investigate the connection between the two groups, the team used computer models to trace the histories of galaxies when the universe was about three to four billion years old.
Their results suggest that between 86% and 96% of massive quiescent galaxies were once dusty star-forming galaxies. In other words, most of the giant “dead” galaxies seen in the early universe likely began as some of the most active star-producing galaxies ever observed.
The study proposes that the transformation was triggered by violent collisions between galaxies of similar size. When two large galaxies merged, enormous amounts of gas were driven into their centers. This fueled an intense burst of star formation while also feeding a rapidly growing supermassive black hole.
The starburst quickly consumed much of the cold gas needed to create new stars. At the same time, energy released by the active black hole heated the surrounding gas, preventing it from cooling and falling back into the galaxy. Without a fresh supply of gas, star formation came to an abrupt halt.
Not all dusty star-forming galaxies followed this path. Many grew more gradually and continued forming stars for much longer periods.
Although the new model provides a compelling explanation, some questions remain. Observations still reveal more dusty galaxies than current models predict.
Future instruments, including the giant Giant Magellan Telescope currently under construction in Chile, are expected to help astronomers solve these remaining puzzles and better understand how the universe’s largest galaxies lived fast and died young.
Source: KSR.
