Astronomers are endlessly fascinated by the supermassive black holes that reside in the centers of most large galaxies. But how did they get so big? Using some of the largest optical telescopes on Earth, astronomers have caught the end of the raging adolescence of their growth.
Black holes that are a few times the mass of our sun are made in the explosive death of very large stars. However, it is not as easy to determine the formation of the supermassive black holes that live in the centers of most galaxies. This family of black hole heavyweights are typically millions, or even billions, of times more massive than our sun. Ill-fated gas and stars most likely travel to the center and fall into these supermassive black holes, thus adding to their bulk.
Galaxies go through cycles of such activity, but that doesn't explain how a black hole got there in the first place.
Astronomers Benny Trakhtenbrot, Hagai Netzer, Paulina Lira and Ohad Shemmer used the huge mirrors of the Gemini North Telescope, on Mauna Kea in Hawaii, and Very Large Telescope, on Cerro Paranal in Chile, to capture the light from active galaxies 12.4 billion light-years away. The black holes at their centers are observed to be actively collecting material. As the light has taken 12.4 billion years to reach us, this black hole activity actually occurred when the universe was just over a billion years old.
These black holes are on average smaller and more active (or brighter) than a similar population that was studied at an age of 2 billion years, and another at an age of almost 3 billion. In this way, the astronomers have tracked the evolution of these types of supermassive black holes, even estimating back to their formation near the beginning of the universe. Some may have originated from the "regular" black holes that we see forming today. Others may have originated as more mysterious and elusive intermediate-sized black holes.
I am impressed that this study has been able to collect a complete sample of such young active galaxies and their black holes. After all, it isn't wise to make general conclusions when just a handful of the brightest ones are present.
The ability of astronomers to look back in time by looking farther and farther out allows for direct observations of the evolution of our universe and will continue to drive the push for bigger and bigger telescopes. Here we are, on this little planet, asking questions about the biggest and grandest scale of the universe. And why not?
Image: Artist's conception of the disk around an active supermassive black hole, complete with jets of material escaping its pull. Credit: ESA/NASA/AVO/Paolo Padovani
With thanks to Ned Wright for his Cosmology Calculator.