Quasars represent the physics of extremes: They're supermassive black holes affecting the matter around them so drastically they shine bright enough to see halfway across the galaxy. Differences between quasars have puzzled astronomers for decades, but a new theory suggests just two factors may explain almost all the variation we see.
The most important determinant of a quasar’s behavior is thought to be the Eddington Ratio. The larger a black hole’s mass, the more matter it could theoretically suck in per second. However, a shortage of suitable material around the hole may mean its giant appetite is not filled. The ratio measures the amount of mass a black hole is drawing in divided by the amount it potentially could.
The second factor Yue Shen and Luis Ho of Peking University included in their paper in Nature was the angle at which we are observing the quasar. Black holes are spherical, but the gasses they are capturing usually lie on a plane, so it can make a big difference whether we are looking at this plane face or edge on.
By combining these two factors, Shen and Ho have explained most of the differences we can see between quasars. "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," said Shen.
When observing quasars, two features of their light have been seen to be significant: the width of the hydrogen beta emission line and the strength of Fe-II emissions. The former indicates speed gas is orbiting the black hole, while the latter is a proxy for a number of other forms of quasar behavior.
As a result, Shen and Ho have been able to create a diagram with the two emission line measurements on the X and Y axes. From this, they create a main sequence, similar to that used to define hydrogen fusing stars on the Hertzsprung-Russell diagram.
HR diagrams allow us to determine the mass, size and, to a substantial extent, age of a star from its position on the diagram. Shen and Ho’s version, if confirmed, will allow astronomers to read off the Eddington ratio and angle of observation of a particular quasar based on the two emission lines. Using the quasar’s brightness and distance we can also get a good estimate of its intrinsic brightness, providing us with all the key features a quasar possesses.
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