The huge spectral diversity of quasars could be explained by taking two simple parameters into account – how quickly matter is falling into the black hole at the heart of the quasar, and the direction from which we observe it. This surprising conclusion comes from a duo of astronomers in the US and China, who have analysed the spectra of more than 20,000 quasars to create a basic interpretation of the diversity of these celestial powerhouses.

Regularly irregular

Quasars are hugely energetic and luminous supermassive black holes in the nuclei of distant galaxies, and are found in the furthest reaches of the observable universe. Although each quasar appears to have a distinct optical spectrum, they all seem to have similar physical properties. Indeed, these behemoths have a surprising amount of regularity in their more quantifiable physical properties, which follow well-defined trends (referred to as the “main sequence” of quasars) discovered more than 20 years ago.

The black holes nestled at the centres of quasars do not emit any light. Instead, most of the visible light that we detect from quasars is emitted as a continuous spectrum from matter in a hot accretion disc surrounding the black hole, and as a discrete set of emission lines from ionized gas clouds in the vicinity of the black hole.

The emission lines reveal key information about a quasar’s neighbourhood – indeed, the varying intensity of the line emission depends on the characteristics of the disc’s radiation field. Many of these spectral properties are thought to be systematically connected, suggesting that a common physical parameter could drive them. The Eddington luminosity or ratio – the maximum luminosity a stellar body can achieve when there is a balance between the outward force of radiation and the inward gravitational force – has long been suspected to be a driver of variation in quasar spectra.

Telltale emissions

Now, new work carried out by Yue Shen and Luis Ho from the Carnegie Observatories in the US and Peking University in China could show that most observed quasar phenomena can indeed be explained by the balance between gravity and luminosity, along with the viewing orientation of the astronomer. The duo looked at more than 20,000 quasars from the Sloan Digital Sky Survey, applying several new statistical tests to it. “Our findings have profound implications for quasar research. This simple unification scheme presents a pathway to better understand how supermassive black holes accrete matter and interplay with their environments,” says Shen.

The duo found that the orientation of the astronomer’s line-of-sight, as they peer into the quasars inner region, plays a significant role in the observed variations. This means that astronomers might have to pay close attention to the geometry of the line-emitting region closest to the black hole – Shen and Ho claim that the gas in this region is distributed in a flattened, pancake-like configuration. In turn, this refinement could help astronomers measure the masses of the black holes more accurately, because mass is calculated via the luminosity. “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,” says Ho.

The research is published in Nature.