The Milky Way galaxy is not very active, as far as galaxies go. Each year it produces about three to four suns of new stars in its entire spiral body, and stars of all ages can be found dotted throughout.
But there are even quieter galaxies – elliptical galaxies, where most star formation has long ceased. In these galaxies, none or very few stars can be found younger than a certain age, suggesting that at some point most star formation abruptly ceased, leaving the galaxy to die out. slowly over the eons, star by star.
Exactly how star formation is turned off in these smooth, nearly featureless galaxies is a mystery, but astronomers believe it has something to do with the supermassive black holes found at the center of each galaxy. Now, an international team of astronomers led by Kei Ito of the Graduate University for Advanced Studies, SOKENDAI in Japan, has been peering into the early universe to find out if that’s the case.
Using some of the most powerful telescopes in the world, they collected data in multiple wavelengths of light to identify galaxies whose light has traveled 9.5 to 12.5 billion years through the Gulf of Spacetime – ancient galaxies like those elliptical galaxies closer to us in space. and time, for which star formation is about to be snuffed out.
The first step was to use optical and infrared data to identify the galaxies for which star formation is in progress and those for which star formation has ceased.
The next step was to use x-ray and x-ray data to identify supermassive black hole activity. This is the mechanism by which astronomers believe star formation can be extinguished. When a supermassive black hole is active, it devours large amounts of matter from the space around it. This process is messy and violent, producing what are collectively known as “feedbacks”.
We all know that nothing can emerge beyond the event horizon of a black hole, but the space around it is another matter. Matter swirls around the black hole, like water around a drain; gravity and friction generate intense radiation that passes through the Universe.
Another form of feedback takes the form of jets from the black hole’s polar regions. Matter outside the event horizon is thought to be accelerated along the outer magnetic field of the black hole, to be launched from the poles as powerful, focused plasma jets that move at a significant percentage of the speed of light.
Finally, active supermassive black holes generate intense winds that sweep through their galaxies. All three forms of feedback – radiation, jets and winds – are thought to heat and repel the cold molecular gas needed to form baby stars.
Over such vast distances, galaxies are much harder to see; they are very small and very weak, from our point of view here and now. The researchers therefore had to “stack” the galaxies together in order to showcase the radio and X-ray light that are the telltale signs of a supermassive black hole active billions of years ago.
But it worked; the team found an “excess” of X-rays and radio signal too strong to be explained by stars alone in galaxies with little or no star formation. The best explanation for this signal is an active supermassive black hole. Also, the signal was not as pronounced in galaxies where star formation was in progress.
This suggests, the researchers concluded, that it is very plausible that an active supermassive black hole played a role in the sudden demise of these mysterious ghostly galaxies.
Future research, they said, could help shed light on the detailed physics of this mysterious process.
The research has been published in The Astrophysical Journal.
Cover image credit: NASA, ESA and Hubble Heritage Team/STScI/AURA; J. Blakeslee/Washington State University.