In the hearts of some massive galaxies lie strange objects known as quasars. These mysterious objects were named for their apparent similarity to stars (quasar is short for ‘quasi-stellar radio source’), but they're now understood to be the light from rapidly accreting, supermassive black holes. In addition to their prodigious light output, they often emit jets of charged particles from their poles at close to the speed of light.

Mysteriously, quasars come in a variety of seemingly random forms, leading scientists to search for the cause of their diversity. While there are trends in their variation, up until now, no definitive evidence has been found to confirm any of the models we had for their appearance. But a new study has found a clear relationship between quasar properties and how they look, suggesting an underlying mechanism.

The study, which made use of archival data from the Sloan Digital Sky Survey, analyzed the spectra of more than 20,000 quasars, the first time a study of this type has been achieved with a statistically significant sample size. Within that huge sample, a pattern began to emerge.

The main sequence

Every astronomy student is familiar with the H-R, or Hertzsprung-Russell, diagram. Even among those who don't recognize that name many have heard of that diagram’s most prominent feature: the main sequence. That’s because the diagram represents a monumentally simple, yet profound, relationship. The H-R diagram is just a graph of stars’ brightnesses versus their temperatures. When plotted together, they produce a clear, curved line, from top left to bottom right. That line was called the main sequence.

There’s no reason to assume that such a clear pattern would appear—from the varieties of stars observed in the sky, one might assume that they would fall in random locations across the graph. And yet they don’t. Instead, you get this:

The fact that such a relationship existed was a strong indicator of some underlying physical cause. It turned out that all the stars along the main sequence are in the hydrogen-burning phase of their evolution. That’s why the H-R diagram is so significant and why it’s worked its way into classrooms and even sci-fi novels. But what does any of this have to do with quasars?

Orientation & accretion

Theoretical models of quasar diversity thus far have predicted there will be trends found in their variations, and that those are driven by something called the Eddington ratio.

This is the ratio between a quasar’s luminosity and the luminosity of a black hole that has reached its maximum accretion rate, which is set by radiation pressure. The more matter falling into the black hole, the more radiation is produced. That radiation puts outward pressure on the infalling matter, and eventually this radiation pressure can balance gravity's pull, keeping the rate of infalling matter constant.

The new study confirms that the Eddington ratio is driving trends in the quasar variation, and it also finds that another variable is crucial in determining how they look from Earth: the orientation of the quasar’s accretion material. Another finding of the study, also implied by the quasars’ orientations, is that the gas and dust orbiting a quasar’s black hole is in the form of a flattened disk, like a pancake.

Measurements of a quasar’s orientation rely on the spectra of its emissions. There are two sources of these photons. One is from material that’s accreting onto the black hole, which emits light in a continuous spectrum. The second comes from clouds of gas orbiting the disk and the black hole that are ionized by radiation from the disk; these produce an emission spectrum that corresponds to the elements present in the gas. Those wavelengths—which appear as discrete lines on the otherwise blank spectrum—are shifted toward the red end of the spectrum as they move away from us, and shifted towards the blue as they move towards us.

Since the gas is orbiting the quasar, there’s always a portion moving towards us and a portion moving away. As a result, the line on the quasar’s emission spectrum appears wider. By measuring the width of the spectral lines, scientists can determine the disk’s orientation angle with respect to us.

The orientation of the quasar’s jets is also useful in determining the orientation of the quasar. The jet moving more or less towards us is blueshifted, while the one moving away is redshifted, allowing researchers to calculate the orientation of the accretion disk.

A Main Sequence of quasars

Using this information—the orientations and the Eddington ratios of all the quasars seen in the Sloan Digital Sky Survey—the team was able to construct a new graph, which appeared as Figure 1 in their paper. Astoundingly, all the quasars in the survey, all 20,000 of them, fit neatly into a pattern on the graph. Just as with stars, there's a main sequence of quasars. Only, unlike the main sequence of the H-R diagram, this one is more like a ‘main wedge’, as you can see.

The vertical axis, labeled ‘broad line width’, and also ‘orientation’, really represents the width of the lines from the quasar’s emission spectrum, which corresponds to its orientation angle with respect to Earth. The horizontal axis, meanwhile, labeled “FeII strength”, represents the Eddington ratio, which corresponds to the efficiency with which the black hole is taking in matter, a factor driven by the black hole’s mass.

“It is a reasonable comparison of Figure 1 to the H-R diagram,” Yue Shen, the paper’s lead author, told Ars, “although the full utility of Fig. 1 is still to be appreciated until we have a theoretical model to explain it.” This theoretical understanding would help us understand the physics behind the wedge pattern, much like the understanding of the phases of stellar evolution explains why stars fit into the main sequence on the H-R diagram.

While the comparison to the H-R diagram may be a good analogy in terms of significance, it only goes so far. “Please note that the physical meanings of the axes are completely different in Fig. 1 and in the HR diagram,” warns Shen.

Unification

Although a theoretical model is still lacking, finding a connection between those two quantities, as represented by the quasar main sequence, represents an important breakthrough. “The finding that the diversity of quasars can be unified by two simple quantities is very significant,” said Shen, “Because it means now we can build theoretical models to explain this main sequence of quasars, which in turn will help us understand the diversity in supermassive black hole accretion and feedback.”

Supermassive black holes are currently poorly understood. They seem to be at the center of every galaxy, but scientists still don’t know why that is—or how they could have formed at all, since they’re far too massive to have formed out of collapsing stars, like conventional black holes.

“Our findings have profound implications for quasar research,” Shen said in a press release. “This simple unification scheme presents a pathway to better understand how supermassive black holes accrete matter and interplay with their environments.”

“And better black hole mass measurements will benefit a variety of applications in understanding the cosmic growth of supermassive black holes and their place in galaxy formation,” added Luis Ho, the paper’s co-author, in the same press release.

Their role in galaxy formation is currently unknown, but it is expected that supermassive black holes do play some sort of role—whether they are a byproduct of the forming galaxy or an active contributor to its formation. To start building on this work, the next thing needed is “More theoretical modeling, to explain the ordered sequence,” explains Shen. “For example, [we should explore] how accretion efficiency changes the radiation field and how does that affects the production of these emission lines such as [iron(II)] and [doubly-ionized oxygen].”

Implication

The study also finds an interesting spatial relationship of the quasars, which confirms theoretical results. The most massive black holes, those that have the highest Eddington ratios and thus taking in the least matter, are clustered together on the large scale. That is, their host galaxies are relatively close together in the large scale structure of the Universe.

That means that quasars—which are microscopic compared to the galaxies that house them—are not only evolutionarily tied in to the formation of galaxies, but also into the properties of the Universe's large-scale structure, a structure relative to which galaxies themselves are tiny. That relationship is a mystery.

But that mystery highlights the importance of studying supermassive black holes. By studying them, we not only learn about one of the strangest objects in the Universe, but also about how the Universe itself works, on the grandest scales.

Nature, 2014. DOI: doi:10.1038/nature13712 (About DOIs).