As we celebrate the golden anniversary of quasars' discovery, some astronomers are pausing to question the direction of the field.
Fifty years ago on March 16, 1963, astronomer Maarten Schmidt described a peculiar star named 3C 273 in the journal Nature. This star was like none astronomy had ever seen — the strong radio source had an odd set of optical spectral lines, lines that could only be hydrogen Balmer lines, Schmidt wrote, but shifted redward by 16%. In other words, the star had to be speeding away from Earth at 16% of the speed of light. That left two options, both of them extraordinary: either the object was nearby but flying out of our galaxy at ridiculous speeds, or the star wasn’t a star at all, but rather the exceptionally brilliant nucleus of a galaxy 2.4 billion light-years away being carried away by the universe’s expansion.
“At the present time…the explanation in terms of an extragalactic origin seems most direct and least objectionable,” Schmidt wrote then.
And research aligned with his initial thoughts. Within two decades, astronomers from around the world had shown that the massive amount of energy released by faraway quasi-stellar objects, or quasars, likely came from a gaseous disk feeding a supermassive black hole millions or billions of times the mass of our Sun. As the gas spirals into the black hole, it releases energy in the form of radiation and heat, surrounding the black hole with a brilliant glow.
But despite initial advances, progress has slowed in recent years, Robert Antonucci (UC Santa Barbara) argues in a provocative editorial published in March 14th’s Nature. We still don’t know how black holes produce relativistic jets sometimes speeding millions of light-years into extragalactic space, or even what those jets are made of. We still don’t know what produces the X-rays seen from these sources. And even the gaseous accretion disk supposed to feed black holes has its share of theoretical problems.
“In my opinion, the greatest limiting factor in understanding quasars is not a lack of intelligence, effort or creativity, nor is it a dearth of fantastic new facilities,” Antonucci writes. “It is a wide-spread lack of critical thought among many researchers.”
Antonucci raises a number of criticisms, but chief among them is the lack of theory in the field.
“I see fewer and fewer serious theory papers on AGNs (active galactic nuclei), but there is a burgeoning effort to find ever more quasars in surveys. It is as if, having given up on understanding AGNs, the community now focuses on the more modest goal of counting them.”
This exact thought occurred to me more than once during my grad school/postdoc career in the quasar field. But counting AGNs has value too. Most “AGN census” surveys are designed to find those AGNs hidden behind a deep cover of gas and dust, rendering them invisible to most wavelengths. Finding these sources is critical to learning what kind of environment surrounds accreting black holes — especially if there are far more (or far fewer) of these “hidden” AGN than expected.
Antonucci also argues that X-ray astronomy, initially promising, has done little to enlighten the field, in part because of the lack of communication between astronomers studying different wavelengths.
Part of the problem, though Antonucci doesn’t cover this in the editorial, is the different nature of X-ray and optical/ultraviolet observing. Optical and ultraviolet spectra give a wealth of information about the accretion disk that feeds the black hole and the dusty wind that might arise from that disk.
But X-rays are a different story. As a favorite professor of mine used to say, X-ray photons are often so few in number, you can name them. As bright as AGNs are in the X-rays, telescopes such as the Chandra X-ray Observatory have to spend hours, sometimes even days, collecting enough X-ray photons to make a spectrum of lesser quality than a visible-wavelength telescope could collect in minutes. That means all the various processes that produce X-rays, or taken them away via absorption, are crammed into a single, hard-to-understand spectrum. Sometimes I feel it’s a wonder that progress has been made at all.
Some astronomers agree with Antonucci’s call to arms. Bradley Peterson (Ohio State University) adds, “certainly theory is lacking, but so are really meaningful observational constraints.” Other astronomers, who did not wish to be named, have less sympathy for the editorial’s argument. Quasars, as any other object in the universe, will take time to understand.
But regardless of whether the community accepts or rejects Antonucci’s directions for the future of the field, it’s hard to disagree with this: “Without predictive theories we have nothing — our best hope for understanding quasars is that extraterrestrials might drop in and explain them to us.”