The Milky Way is not very active, in relation to galaxies. Each year, it produces approximately three to four new stars in its entire spiral body, and stars of all ages can be found scattered throughout.
But there are some quieter galaxies – elliptical galaxies, most of which star formation has long ceased. In these galaxies, no or very few stars younger than a certain age can be found, suggesting that at some point most star formation stopped abruptly, leaving the galaxy slowly blinking over the ages, star after star.
The exact way star formation is halted in these smooth, almost featureless galaxies is a mystery, but astronomers believe it has something to do with the supermassive black holes at the center of each galaxy. Now an international team of astronomers led by Kei Ito of the Graduate University of Advanced Studies, SOKENDAI in Japan has peered once more into the early universe to see if that’s the case.
Using some of the world’s most powerful telescopes, they have collected data with multiple wavelengths of light to identify galaxies whose light has traveled from 9.5 billion to 12.5 billion years through the bay of spacetime – ancient galaxies like those elliptical galaxies closest to us in space and time, for which star formation was on about to collapse.
The first step was to use optical and infrared data to identify galaxies in which star formation continues, and those in which star formation has stopped.
The next step was to use X-ray and radio data to determine the activity of the supermassive black hole. This is the mechanism by which astronomers believe star formation can be quelled. When a supermassive black hole activates, it devours massive amounts of matter from the space around it. This process is chaotic and violent and produces what is known collectively as “feedback”.
We all know that nothing can appear from beyond the event horizon of a black hole, but the space around it is a different matter. Material revolves around the black hole, like water circulates around a drain; Gravity and friction generate intense radiation that ignites across the universe.
Another form of feedback takes the form of jets emanating from the black hole’s polar regions. Material outside the event horizon is thought to accelerate along the black hole’s external magnetic field, to be released from the poles in the form of powerful, focused plasma jets traveling at a large percentage of the speed of light.
Finally, active supermassive black holes generate strong winds that rush into their galaxies. All three forms of feedback — radiation, jets, and winds — are thought to heat and push the cold molecular gas required to form young stars.
Across such vast distances, galaxies are hard to see; It’s too small and too faint from our here and now. So the researchers had to “stack” galaxies together in order to confirm the radio light and X-rays that are telltale signs of a supermassive black hole that has been active for all those billions of years.
But she succeeded. The team found that the “excess” X-rays and radio signal are too powerful to be explained by stars alone in galaxies with little or no star formation. The best explanation for this signal is the presence of an active supermassive black hole. Moreover, the signal was not evident in galaxies with continuous star formation.
The researchers conclude that this indicates that it is very plausible that an active supermassive black hole played a role in the sudden death of these mysterious ghost galaxies.
They said future research may help shed light on the detailed physics of this mysterious process.
The search was published in Astrophysical Journal.
Cover photo credit: NASA, European Space Agency and Hubble Heritage Team/STScI/AURA; J. Blaxley/Washington State University.