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Quasars at 50: Luminous Cosmic Beacons Remain a Puzzle

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Quasar 3C 273 with jet

Quasar 3C 273 and its jet, as viewed in x-rays. Credit: NASA/CXC/SAO/H.Marshall et al.

Fifty years ago, in the journal Nature, astronomer Maarten Schmidt published a brief paper noting that a star-like object known as 3C 273 was simply too far away to be a star in the Milky Way. Schmidt, of the California Institute of Technology, concluded on the basis of spectroscopic observations that the object was most likely very distant (some two billion light-years away) and hence very, very bright. Shortly after Schmidt’s 1963 paper, 3C 273 and its ilk would become known as quasars, short for “quasi-stellar” objects.

On the fiftieth anniversary of the unmasking of quasars as distant beacons, their secrets have still yet to be fully unlocked, physicist Robert Antonucci of the University of California, Santa Barbara, notes in the March 14 issue of Nature. (Scientific American is part of Nature Publishing Group.) “Have we made good progress in understanding quasars in five decades?” Antonucci writes. “I do not think so.”

The general picture of quasars is now relatively uncontroversial—giant black holes at the centers of distant galaxies feed on gas and even nearby stars, glowing brightly across the electromagnetic spectrum as a result. These active galactic nuclei (AGNs), as they are known, can also launch jets of high-speed particles, as in the case of quasar 3C 273.

Antonucci’s complaint is that the general astrophysical understanding of quasars has not become more detailed. “Astronomers have had great fun detecting distant quasars, pushing ever closer to the big bang as technology has improved,” he writes. “But they have yet to understand the detailed physics of how quasars emit such enormous amounts of energy.” Among the outstanding questions: just what kinds of particles are in quasar jets? How much energy goes into the jets’ electric and magnetic fields, respectively? And what, exactly, happens in the accretion disk of material swirling around the black hole?

“I see fewer and fewer serious theory papers on AGNs, but there is a burgeoning effort to find ever more quasars in surveys,” he writes. “It is as if, having given up on understanding AGNs, the community now focuses on the more modest goal of counting them.”

The situation is nonetheless a bit better than it was in 1971, when Schmidt co-authored an article in Scientific American that still had to fight the notion that quasars are nearby exotic objects, rather than distant AGNs as originally proposed and now widely accepted. Still, some of the language is surprisingly similar to Antonucci’s comments today. “As for the ultimate source of the tremendous energy observed in quasars, there has been no lack of hypotheses,” Schmidt and Scientific American’s Francis Bello wrote. “There is also no agreement about the radiation mechanism….The solution of these problems constitutes one of the main challenges to present-day astronomy.”

The views expressed are those of the author and are not necessarily those of Scientific American.

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