Quasars Are Dramatically Changing The Color Of Galaxies

Artist’s impression of red and blue quasars. S. Munro

Quasars are the extremely luminous and active cores of galaxies. They are powered by supermassive black holes in a dramatic feeding frenzy and can easily outshine all the stars in a galaxy. Their lives continue to be full of mysteries but scientists have just gained some stunning new insights. They have gathered evidence to suggest that quasars change color.

When these objects are viewed through optical telescopes, the vast majority appear to have a clear blueish tint. The rest give off a red hue, which is due to the presence of cosmic dust within these particular objects. Astronomers were curious to uncover the origins of these two populations. One possibility was that their orientation was causing the difference; they might be angled in such a way that more of their light passes through the clouds of dust.

But in this new study, published in the Monthly Notices of the Royal Astronomical Society, researchers argue that the difference is the result of evolution rather than orientation. The team studied 10,000 red and blue quasars as they would have looked between 7 and 11 billion years ago and found that orientation didn't seem to come into play.

They believe all red quasars are destined to become blue. The activation of a supermassive black hole is a violent affair with a lot of energy funneled into the galaxy and complex winds of material leaving the nucleus. This phase leads, over time, to the dissipation of dust clouds. Once the dust is blown away the remaining quasar appears blue.

“How quasars develop has been the cause of significant uncertainty. What our results suggest is that quasars undergo a brief transition phase, changing color from red to blue, when they emerge from the deep shroud of dust and gas surrounding them,” lead author Lizelke Klindt, a graduate researcher at Durham University, said in a statement. "What we believe we are seeing is a rare but important step in the life of these galactic beasts during galaxy evolution when their black holes are starting to shape their environments."

The color change is not just an aesthetic feature of the galaxy. It has profound consequences. The winds generated by the black hole warm and spread out the interstellar gas. Without that gas, new stars cannot form. Ironically, redder stars are longer-lived than the white-blue ones, so over time, the galaxy will go back to looking red.

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