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Galaxy Collisions Create Large Black Holes and Quasars

quasar stages

Artistic representation of the quasar stages after a major galaxy merger. Initially, the growing black hole is completely enshrouded by large amounts of gas and dust. After about 100 million years, the strong pressure provided by the quasar emission is enough to blow out most of the surrounding gas, thus unveiling a "naked" quasar, which is clearly detectable at optical and ultraviolet wavelengths. Merging galaxies: NASA, STScI/AURA, ESA. Art by Karen Teramura.

Giant black holes at the centers of galaxies grow mainly as a result of intergalactic collisions, according to IfA astronomer Ezequiel Treister, leader of a study published in March. As gas clouds in galaxies are sucked into the central black hole, they emit vast amounts of radiation, giving rise to quasars, which are very luminous active galactic nuclei. "We find that these growing black holes are originally hidden by large amounts of dust, but after 10-100 million years this dust is blown out by the strong radiation pressure, leaving a naked quasar that is visible at optical wavelengths and keeps shining for another 100 million years," Treister said.

For this study, Treister and his colleagues combined data obtained with the Hubble, Chandra, and Spitzer space observatories to identify a large number of obscured, dust-enshrouded quasars at very large distances, up 11 billion light-years away, seen as they were when the Universe was still in its infancy. "For many years, astronomers believed that these sources were very rare. Now we are seeing them everywhere!" Treister said. Because most of the emission from these obscured quasars is hidden, astronomers looked at infrared wavelengths for signs of very hot dust, and in X-rays, which are less affected by obscuration. The investigators discovered that the number of obscured quasars relative to the unobscured ones was significantly larger in the early Universe than it is now.

Other team members are Priyamvada Natarajan, Meg Urry, and Kevin Schawinski (Yale), IfA astronomer David Sanders, and former IfA graduate student Jeyhan Kartaltepe, who is now with the National Optical Astronomy Observatories in Arizona.