Radio Telescopes Reveal Unseen Galactic Cannibalism

June 24, 2008

Radio-telescope images have revealed previously-unseen galactic cannibalism -- a triggering event that leads to feeding frenzies by gigantic black holes at the cores of galaxies. Astronomers have long suspected that the extra-bright cores of spiral galaxies called Seyfert galaxies are powered by supermassive black holes consuming material. However, they could not see how the material is started on its journey toward the black hole.

One leading theory said that Seyfert galaxies have been disturbed by close encounters with neighboring galaxies, thus stirring up their gas and bringing more of it within the gravitational reach of the black hole. However, when astronomers looked at Seyferts with visible-light telescopes, only a small fraction showed any evidence of such an encounter. Now, new images of hydrogen gas in Seyferts made using the National Science Foundation's Very Large Array (VLA) radio telescope show the majority of them are, in fact, disturbed by ongoing encounters with neighbor galaxies.

"The VLA lifted the veil on what's really happening with these galaxies," said Cheng-Yu Kuo, a graduate student at the University of Virginia. "Looking at the gas in these galaxies clearly showed that they are snacking on their neighbors. This is a dramatic contrast with their appearance in visible starlight," he added.

The effect of the galactic encounters is to send gas and dust toward the black hole and produce energy as the material ultimately is consumed. Black holes, concentrations of matter so dense that not even light can escape their gravitational pull, reside at the cores of many galaxies. Depending on how rapidly the black hole is eating, the galaxy can show a wide range of energetic activity. Seyfert galaxies have the mildest version of this activity, while quasars and blazars are hundreds of times more powerful.

The astronomers picked a number of relatively nearby Seyfert galaxies that had previously been observed with visible-light telescopes. They then carefully studied the Seyferts with the VLA, specifically looking for radio waves emitted by hydrogen atoms. The VLA images showed the vast majority of the Seyferts were disturbed by encounters with neighbor galaxies.

By comparison, similar VLA images of inactive galaxies showed that very few were disturbed. "This comparison clearly shows a connection between close galactic encounters and the black-hole-powered activity in the cores," said Ya-Wen Tang, who began this work at the Institute of Astronomy & Astrophysics, Academia Sinica (ASIAA), in Taiwan and now is a graduate student at the National Taiwan University.

"This is the best evidence yet for the fueling of Seyfert galaxies. Other mechanisms have been proposed, but they have shown little if any difference between Seyferts and inactive galaxies," Tang added.

"Our results show that images of the hydrogen gas are a powerful tool for revealing otherwise-invisible gravitational interactions among galaxies," said Jeremy Lim, also of ASIAA. "This is a welcome advance in our understanding of these objects, made possible by the best and most extensive survey ever made of hydrogen in Seyferts," Lim said.

Kuo, Tang and Lim worked with Paul Ho, of ASIAA and the Harvard-Smithsonian Center for Astrophysics. The scientists reported their findings in the Astrophysical Journal.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

National Radio Astronomy Observatory

---

---

	Galaxies by Seymour Simon (Author) This close-up look at our own Milky Way and other enormous clusters of stars describes the many different types of galaxies, how they were formed, and how they got their different shapes. "A dazzling photo-essay."--School Library Journal.
	Galaxies, Galaxies! by Gail Gibbons (Author) Planet Earth is in the Milky Way Galaxy, the cloudy band of light that stretches clear across the night sky. How many galaxies are there in the universe? For years astronomers thought that the Milky Way was the universe. Now we know that there are billions of them. Gail Gibbons takes the reader on a journey light-years away.
	Galaxies by Timothy Ferris (Author) Beyond the Milky Way and toward infinity, Timothy Ferris navigates an exploration of the universe, guided by stars and galaxies that have been photographed through telescopes at the greatest observatories in the world. The book celebrates the wonder and the power of the cosmos in these photographs, which take you through silvery star clusters and vast red, exploding nebulae toward blue quasars 15 billion light years away. Revealing not only the ingredients of our own galaxy, the Milky Way, Ferris' vivid text also visits local galaxies, such as the Magellanic Clouds, the Andromeda, and the Sculptor and Fornax dwarves. The author then proceeds to discuss the elliptical, spiral and even violent aspects of galaxies, concluding with an examination of cosmology, which adresses such subjects as...
	Galaxy Formation and Evolution by Houjun Mo (Author), Frank van den Bosch (Author), Simon White (Author) The rapidly expanding field of galaxy formation lies at the interface between astronomy, particle physics, and cosmology. Covering diverse topics from these disciplines, all of which are needed to understand how galaxies form and evolve, this book is ideal for researchers entering the field. Individual chapters explore the evolution of the Universe as a whole and its particle and radiation content; linear and nonlinear growth of cosmic structure; processes affecting the gaseous and dark matter components of galaxies and their stellar populations; the formation of spiral and elliptical galaxies; central supermassive black holes and the activity associated with them; galaxy interactions; and the intergalactic medium. Emphasizing both observational and theoretical aspects, this book provides...
	Galaxies: A Very Short Introduction by John Gribbin (Author) In this fascinating Very Short Introduction, popular science writer John Gribben tells the story of our growing understanding of galaxies, from the days before Galileo to our present-day observations of our many hundreds of millions of galactic neighbors. Not only are galaxies fascinating astronomical structures in themselves, but their study has revealed much of what we know today about the cosmos, providing a window on the Big Bang and the origins of the Universe. Gribben looks at our own "Milky Way" Galaxy in detail, from the different kinds of stars that are born within it, to the origins of its magnificent spiral structure. Perhaps most interesting, Gribben describes the many exciting discoveries have been made about our own galaxy and about those beyond: how a supermassive black...
	Universe: Stars and Galaxies by Roger A. Freedman (Author), William J. Kaufmann (Author) What astronomers know about our universe changes every day. For the past twenty years, Universe has been the most thrilling way to communicate the excitement of discovery in astronomy, to students of all scientific backgrounds. This edition of Universe which contains the full text of chapters 1-8 and 18-30 gives students the means to explore the stars and galaxies like never before. In addition to covering all the top discoveries, the text and accompanying CD contains spectacular images, captivating writing, and guides students on a fascinating journey around the cosmos and back again.
	Cosmic Collisions: The Hubble Atlas of Merging Galaxies by Lars Lindberg Christensen (Author), Davide de Martin (Author), Raquel Yumi Shida (Author) Like no other telescope ever invented, the NASA/ESA Hubble Space Telescope has given us magnificent high resolution views of the gigantic cosmic collisions between galaxies. Hubble's images are snapshots in time and catch the colliding galaxies in different stages of collision. Thanks to a new and amazing set of 60 Hubble images, for the first time these different stages can be put together to form a still-frame movielike montage showing the incredible processes taking place as galaxies collide and merge. The significance of these cosmic encounters reaches far beyond aesthetics. Galaxy mergers may, in fact, be some of the most important processes that shape our universe. Colliding galaxies very likely, hold some of the most important clues to our cosmic past and to our destiny. It now...
	Stars and Galaxies by Dana Backman (Author), Michael A. Seeds (Author) Addressing the newest developments and latest discoveries in the exciting study of astronomy, this revised seventh edition of STARS AND GALAXIES helps students use astronomy to understand science and use science to understand what we are. Fascinating and engaging, STARS AND GALAXIES illustrates the scientific method and guides students to answer these fundamental questions: What are we? How do we know? In discussing the interplay between evidence and hypothesis, author Michael Seeds provides not only facts but also a conceptual framework for understanding the logic of science. Coverage includes observations of the shapes of stars; evidence of dark energy, quasars, and galaxy collisions; and a look inside supernova explosions. And whether you choose to assign homework in an online...
	Galaxies in the Universe: An Introduction by Linda S. Sparke (Author), John S. Gallagher III (Author) This extensively illustrated book presents the astrophysics of galaxies since their beginnings in the early Universe. It has been thoroughly revised to take into account the most recent observational data, and recent discoveries such as dark energy. There are new sections on galaxy clusters, gamma ray bursts and supermassive black holes. The authors explore the basic properties of stars and the Milky Way before working out towards nearby galaxies and the distant Universe. They discuss the structures of galaxies and how galaxies have developed, and relate this to the evolution of the Universe. The book also examines ways of observing galaxies across the whole electromagnetic spectrum, and explores dark matter and its gravitational pull on matter and light. This book is self-contained and...
	The Ultimate Hitchhiker's Guide to the Galaxy by Douglas Adams (Author), Neil Gaiman (Introduction) At last in paperback in one complete volume, here are the five classic novels from Douglas Adams’s beloved Hitchiker series. The Hitchhiker’s Guide to the Galaxy Seconds before the Earth is demolished for a galactic freeway, Arthur Dent is saved by Ford Prefect, a researcher for the revised Guide. Together they stick out their thumbs to the stars and begin a wild journey through time and space. The Restaurant at the End of the Universe Facing annihilation at the hands of warmongers is a curious time to crave tea. It could only happen to the cosmically displaced Arthur Dent and his comrades as they hurtle across the galaxy in a desperate search for a place to eat. Life, the Universe and Everything The unhappy inhabitants of planet Krikkit are sick of...