mountain profile Institute for Astronomy University of Hawaii

Black Holes in the Universe Gain Weight and Light Up during Galaxy Collisions

Maintained by LG

For immediate release
March 25, 2010



Dr. Ezequiel Treister

Einstein Postdoctoral Fellow
Institute for Astronomy
University of Hawaii at Manoa
Honolulu, Hawaii 96822

Mrs. Karen Rehbock
Assistant to the Director
Institute for Astronomy
University of Hawaii at Manoa

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Science FIgure
Figure 1: Examples of strongly interacting/merging galaxies containing a heavily obscured growing supermassive black hole nearby (top panels) and in the early Universe (bottom panels), as observed by the Hubble Space Telescope.

Giant black holes in the centers of galaxies grow mainly as a result of intergalactic collisions, according to results presented by a group of astronomers led by Dr. Ezequiel Treister from the University of Hawaii, published in the March 25 issue of the international journal Science.

As gas clouds in galaxies are sucked into the central black hole, they emit vast amounts of radiation, giving rise to objects that astronomers call quasars. "We find that these growing black holes are originally hidden by large amounts of dust," Treister said, "but after 10-100 million years this dust is blown out by the strong radiation pressure, leaving a naked quasar, that is visible in optical wavelengths and keeps shining for another 100 million years."

For this study, the group 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 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 added. 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.

"We knew theoretically that the mergers of massive gas-rich galaxies were more frequent in the past; these observations fit very nicely within this scenario," added Prof. Priyamvada Natarajan of Yale University, the second author and theorist on the team. "We knew that this is definitely the case for nearby galaxies," said Prof. David Sanders from the University of Hawaii and participant in this investigation, "but this result shows that this happens across the Universe."

Researchers further analyzed images of these distant galaxies taken by the Hubble Space Telescope, using the new Wide Field Camera 3 installed 10 months ago during the last servicing mission. These images revealed obvious signatures of interactions and mergers, thus confirming the hypothesis of this group. Finally, using a simple theoretical prescription, the authors estimated that it takes about 100 million years for radiation from the growing black hole to wipe out the surrounding dust and gas and reveal the naked quasar.

Major galaxy mergers are important to trigger star formation episodes and modify galaxy morphologies. "This work confirms that mergers are also critical for the growth of the nuclear giant black hole," said Natarajan. Mergers are therefore essential for the evolution of a galaxy and also cause their central black hole to gain weight during both the obscured and unobscured phases.

Science Express will publish the paper presenting these results on March 25. Ezequiel Treister is an Einstein postdoctoral fellow at the University of Hawaii. Other coauthors of this work are Priyamvada Natarajan (Yale), David Sanders (University of Hawaii), Meg Urry and Kevin Schawinski (Yale) and former University of Hawaii's graduate student Jeyhan Kartaltepe (NOAO).

quasar stages

Figure 2: Artist's view of the quasar stages after a major galaxy merger by Karen Teramura.



Figure 1: Examples of strongly-interacting/merging galaxies containing a heavily obscured growing supermassive black hole observed by the Hubble Space Telescope. The top panels show nearby galaxies at ~500 million light-years from us, while the more distant galaxies in the bottom panels are ~6.5 billion light years away, when the Universe was half its current age. For the first 10-100 Myrs after the merger, the growing black hole remains highly obscured, after which it becomes an optically bright quasar that shines again for another 10-100 Myrs before it likely reaches its upper limit. Credit: Top panels: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University). Bottom panels: E. Treister and Karen Teramura (IfA, University of Hawaii)


Figure 2: 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 Myrs the strong pressure provided by the quasar emission is enough to blown out most of the surrounding gas, thus unveiling a "naked" quasar, which is clearly detectable at optical and UV wavelengths. These bright quasars shine for another 10-100 Myrs. Most of the black hole growth happens during the quasar stage. While the black hole before the quasar phase has a mass of about 1 million solar masses, the final mass can be up to 10,000 times larger.Credit: Artist's view: Karen Teramura (IfA, University of Hawaii). Galaxy image: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University).


Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.

Established in 1907 and fully accredited by the Western Association of Schools and Colleges, the University of Hawaii is the state's sole public system of higher education. The UH System provides an array of undergraduate, graduate, and professional degrees and community programs on 10 campuses and through educational, training, and research centers across the state. UH enrolls more than 50,000 students from Hawaii, the U.S. mainland, and around the world.