The Quasar Main Sequence

A new diagram suggests two physical properties — how efficiently the black hole feeds and the system's orientation — lie behind quasars' diversity.

Astronomers are edging closer to understanding why some quasars look different from others. Quasars are the most powerful active galactic nuclei, blazing beacons in distant galaxies’ centers powered by supermassive black holes chowing down gas. Their visible-light emission comes from two main sources: the hot accretion disk around the black hole, and gas clouds orbiting nearby that are ionized by the radiation coming from the disk.


No one's ever seen a quasar up close, so artists can only guess at what we'd see. Though we know a brilliant accretion disk feeds the supermassive black hole at its center, astronomers aren't even sure how big the disk would be.
Credit: ESO / M. Kornmesser

The spectral lines emitted by the clouds often look different from quasar to quasar. These lines’ appearance is connected to what’s going on in the physical environment around the black hole. But astronomers are still working to unite what they observe with whatever physical causes underlie what they see.

Essentially, what quasar researchers want is the equivalent of the Hertzsprung-Russell diagram. The HR diagram plots stars by their color and luminosity, and in doing so reveals stars’ age, temperature, mass, and size. Stars fusing hydrogen in their cores lie along a curving line called the main sequence. The diagram is the cornerstone of modern stellar astronomy. (As one of my professors intoned ad infinitum, “You must be one with the HR diagram.”)

Building off decades of work in the astronomical community, Yue Shen (Carnegie Observatories and Peking University, Beijing) and Luis Ho (Peking University) have now taken a step toward identifying a quasar main sequence — although, as quipped by astronomer Michael Brotherton (University of Wyoming), their main sequence looks more like a “main wedge.”

The duo looked at about 20,000 quasars from the Sloan Digital Sky Survey. They divided the sample based on two observed characteristics, the width of an emission line called hydrogen beta (Hβ) and the strength of emission from singly ionized iron (Fe II) atoms relative to the strength of Hβ.

Hβ’s width relates to the orbital velocity of gas along our line of sight. Fe II’s strength is a proxy for something called Eigenvector 1.

Eigenvector 1 is not a single property; rather, it’s a set of correlations between several observed properties that all seem to vary together. (Think of it like a box of if-this-then-thats all tied together.). According to the Eigenvector 1 trend, first identified by Todd Boroson and Richard Green in 1992, quasars with strong optical Fe II emission have weak emission from doubly ionized, rarefied oxygen ([O III]). These properties in turn connect to changes in the width of the Hβ line, the excess of soft X-rays, and so forth.

The systematic correlations suggest that a single physical parameter drives Eigenvector 1. Astronomers strongly suspect it’s the Eddington ratio, the ratio of how fast the black hole is accreting mass to the maximum rate it could theoretically achieve. Another way to think of it is the luminosity divided by the mass.

Drawing the Main Sequence

Shen and Ho’s work has two main parts. First, they plotted all the quasars on a grid, with the relative Fe II strength on the x-axis and the width of Hβ on the y-axis. The dots drew a solid wedge on the graph.

Second, they set about confirming what Hβ and Fe II actually tell us about quasars. After various crosschecks, they conclude that Hβ’s width reveals the orientation of the quasar’s disk to our line of sight, with a wider line corresponding to a more edge-on disk. They also confirmed that the Eddington ratio lies behind Eigenvector 1 (and its Fe II proxy).

If these connections to the Hβ and Fe II emission hold up, then the orderly diagram confirms that a quasar’s Eddington ratio and orientation determine most of the observed variation in these objects’ emission. But more fundamentally, the diagram reveals what a quasar’s Eddington ratio and orientation are, based on where the quasar lies in the plot — in other words, observe the Hβ and Fe II lines, and you’ll know something about the quasar’s orientation and accretion. Add in the quasar’s luminosity (figured out from its spectrum), and you can also estimate the black hole’s mass.

“This paper presents the most compelling picture I have seen so far that makes sense of what has been measured,” Boroson says. But he cautions that there are still many unanswered questions, plus additional tests to be done. For example, the authors looked at radio-loud quasars to confirm that a more edge-on quasar has broader Hβ lines. Astronomers need to look at other types of quasars and check that the interpretation holds up. “But these authors have certainly clarified the way forward,” he concludes.

Brotherton has a similar assessment in his perspective piece in the September 11th Nature, in which Shen and Ho’s article appears. “Clearly, the main sequence of quasars needs further testing, and only time will tell whether its utility is equal to that of the stellar main sequence,” he writes. But if it does, it would be “an invaluable tool.”


Reference: Y. Shen and L. C. Ho. "The Diversity of Quasars Unified by Accretion and Orientation." Nature. September 11, 2014.

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