Astronomer Brings Coherence To Study Of Active Galactic Nuclei

March 09, 1999

At a critical juncture in astronomy's rapid maturation, Julian Krolik has stepped into the scholarly void to produce the first comprehensive textbook for researchers about the great powerhouses of the universe: active galactic nuclei.

Although the publication of a textbook is rarely a moment of startling significance, Krolik's signals a new moment of coherence in the field.

The book, "Active Galactic Nuclei: From Central Black Hole to the Galactic Environment," published by Princeton University Press, is being touted by one of the world's leading astronomers, Sir Martin Rees, as "an impressive book (that) fills a notable gap in the existing literature."

Krolik, a professor of theoretical astrophysics at The Johns Hopkins University, hopes the work will correct misconceptions and influence the next generation of astronomers.

For nearly a century, students of the cosmos have created, Krolik says, "an entire zoo of classifications" as they puzzled over the super-bright, super-massive enigmas that are now known as active galactic nuclei, or AGN. Indications of strange activity in the centers of galaxies accumulated gradually throughout this century, starting as early as 1918. As different examples were found, they were divided into an ever-increasing number of categories: Seyfert galaxies (which were further divided into two distinct types), radio galaxies, quasi-stellar radio sources, quasi-stellar objects, and even the fancifully named "blazars."

This untidy state of affairs could not be fully resolved until the 1990s, when scientists began to realize that their observations often simply amounted to different signatures of the same kinds of mighty objects. In some cases, objects simply looked different when viewed from different directions. For example, they would appear to change if viewed from the side rather than straight on.

"Astronomers often revel in the marvelous variety of the universe," Krolik observes, "and love to divide the objects they find into ever-finer classes. But we've now learned that there is a fundamental unity here, and much of the historic terminology is holding back progress more than promoting it."

In recent years, as the pace of startling discoveries finally slowed and long-standing conjectures -- most notably that AGN are powered by accretion onto supermassive black holes -- were bolstered by confirming evidence, Krolik seized the day. He decided to write the textbook, in part, to bring organization to a fragmented, but increasingly critical area of research.

"I want people to realize that many of the categorical distinctions we have made rest somewhere between arbitrary and misleading," he says. "Particularly for AGN, many astronomers have pretended there's nobody else studying the objects -- people have tended to observe them in their own favorite parts of the spectrum. Consequently, we have had optical astronomers and radio astronomers and X-ray astronomers and so on, each believing that the only really interesting action falls within their own bailiwick.

"It's silly--and it's unfortunate. Because what really distinguishes AGN is that they emit enormous amounts of power across the spectrum, all the way from radio wavelengths to X-rays, and sometimes even high-energy gamma-rays. What I have done is to finally write a book that encompasses the complete spectrum. We now have a text that can serve as a standard for everyone."

As the research has evolved and matured, interest in these exotic beasts has attracted the curiosity of increasing numbers of scientists. It is estimated today that as many as one in five of all research astronomers are making inquiries into AGN or some aspect of their awesome components.

With such a rising phalanx of support, and with the aid of new orbiting telescopes that have been able to detect the extremely broad range of wavelengths produced by AGN, it is now possible to produce a unified portrait that is easily available to researchers and advanced graduate students.

"When you're writing a technical book, you don't do it to get rich," Krolik says. "That's just not in the cards. You do it, first, to organize the subject in your own mind, and secondly, to influence others, especially those who are coming into the discipline fresh. I want to shape the way people think. There may be other ways to bend the twig, so to speak, but writing a good book can be particularly effective."
-end-


Johns Hopkins University

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