Dying Early Universe Galaxies Could Be Killed by Their Supermassive Black Holes

As far as galaxies go, the Milky Way isn't particularly active. Every year, it creates three to four Suns' worth of new stars over its spiral body, with stars of various ages strewn around.

There are certain galaxies that are much quieter - elliptical galaxies, for example, where most star production has stopped for a long time. There are no or very few stars younger than a particular age in these galaxies, implying that most star formation abruptly ended at some time, allowing the galaxy to slowly fade out over eons, star by star. 

It's unclear how star production stops in these smooth, practically featureless galaxies, but astronomers suspect it has something to do with the supermassive black holes that lurk at the heart of every galaxy. Now, an international group of astronomers led by Kei Ito of Japan's Graduate University for Advanced Studies, SOKENDAI, has looked back into the early Universe to see if this is true.

They gathered data in multiple wavelengths of light using some of the world's most powerful telescopes to identify galaxies whose light has traveled 9.5 billion to 12.5 billion years across the gulf of space-time – ancient galaxies like those elliptical galaxies closer to us in space and time, for whom star formation is on the verge of extinction.

The initial stage was to identify galaxies with ongoing star production and those with stopped star development using optical and infrared data.

The next stage was to look for supermassive black hole activity using X-ray and radio data. Astronomers believe this is the mechanism that prevents star formation. A supermassive black hole devours tremendous amounts of stuff from the space around it while it is active. This is a chaotic, violent process that results in what is referred to as "feedback."

We all know that nothing can escape a black hole's event horizon, but the space around it is a different story. Material flows around the black hole like water around a drain, generating powerful radiation that blazes across the Universe due to gravity and friction.

Jets bursting from the black hole's polar regions are another sort of feedback. Outside the event horizon, material is predicted to be propelled along the black hole's external magnetic field, and then released from the poles as intense, focussed plasma jets traveling at a considerable fraction of the speed of light.

Finally, active supermassive black holes produce powerful winds that spread over their galaxy. Radiation, jets, and winds are all assumed to heat and push away the cold molecular gas needed for the birth of young stars.

Galaxies are more difficult to discern at such huge distances; from our current vantage point, they are incredibly tiny and dim. As a result, the researchers had to'stack' the galaxies together to highlight the radio and X-ray photons that are the unmistakable indicators of an active supermassive black hole billions of years ago.

But it worked: in galaxies with little to no star formation, the scientists discovered a "excess" X-ray and radio signal that was too intense to be explained by stars alone. An active supermassive black hole is the most likely source of this radiation. Furthermore, in galaxies with active star formation, the signal was less significant.

The researchers found that an active supermassive black hole is quite likely to play a role in the sudden demise of these odd, ghostly galaxies.

Future studies, they suggested, might shed light on the mechanics of this cryptic process in greater detail.