A new study published in Nature Astronomy describes the most luminous object ever observed by astronomers. It is a black hole with a mass of 17 billion suns, which swallows a greater amount of mass every day than the sun.
It has been known for decades, but because it is so bright, astronomers assumed it must be a nearby star. Only recent observations revealed its extreme distance and brightness.
The object has been given the name J0529-4351. This name simply refers to the coordinates on the celestial sphere – a way of projecting the objects in the sky onto the inside of a sphere. It is a type of object called a quasar.
The physical nature of quasars was initially unknown. But in 1963, the visible light from a quasar called 3C 273 was split into all its wavelengths (known as its spectrum). This showed that it was located at a distance of almost 2 billion light years.
Given how bright 3C 273 appears to us and how far away it is, it must be extremely luminous – a term in astronomy that refers to the amount of light an object emits in a unit of time. The only known power source for such extreme brightness was material falling into a supermassive black hole. Quasars are therefore the most actively growing black holes in the universe.
Power source
Supermassive black holes are often located at the centers of galaxies. Like all quasars, J0529-4351 is powered by material, mainly superheated hydrogen and helium gas, that falls into its black hole from the surrounding galaxy.
Every day, about one time the mass of the Sun falls into this black hole. How exactly so much gas can be channeled into the centers of galaxies to increase the mass of black holes is an unanswered question in astrophysics.
In the center of the galaxy, the gas forms a thin disk shape. The properties of viscosity (resistance to the flow of matter in space) and friction in the thin disk help heat the gas to tens of thousands of degrees Celsius. This is hot enough to glow when viewed at ultraviolet and visible light wavelengths. It is that glow that we can see from Earth.
With a mass of about 17 billion suns, J0529-4351 is not the most massive known black hole. One object, at the center of the star cluster Abell 1201, is equivalent to 30 billion suns. However, we must remember that because of the time it takes for light to travel the enormous distance between this object and Earth, we are witnessing it when the universe was only 1.5 billion years old. It is now about 13.7 billion years old.
So this black hole must have been growing or expanding at this rate for a significant portion of the age of the universe by the time it was observed. The authors believe that the gas accretion through the black hole occurs close to the limit of the laws of physics. Faster accretion creates a brighter disk of gas around the black hole, which in turn can prevent more material from falling in.
Story of the discovery
J0529-4351 has been known for decades, but despite having an accretion disk of gas 15,000 times larger than our solar system and occupying its own galaxy – which is probably close to the size of the Milky Way – it is so far away that it appears as a single point of light in our telescopes.
This means it is difficult to distinguish from the billions of stars in our own galaxy. Discovering that it is in fact a distant, powerful, supermassive black hole required some more complex techniques. First, astronomers collected light from the center of the infrared waveband (light with much longer wavelengths than we can see).
Stars and quasars look very different at those wavelengths. To confirm the observation, a spectrum was recorded (as with quasar 3C 273), using the Australian National University’s 2.3 meter telescope at Siding Spring Observatory, New South Wales.
And as with 3C 273, the spectrum revealed both the nature of the object and how far away it was: 12 billion light-years. This emphasized how extreme its brightness must be.
Detailed checks
Despite these measurements, a number of checks had to be made to confirm the true brightness of the quasar. First, astronomers had to be sure that the light had not been magnified by a source in the sky closer to Earth. Like lenses used in glasses or binoculars, galaxies can act as lenses. They are so compact that they can bend and magnify light from more distant sources that are perfectly behind them.
Data from the European Space Agency’s Gaia satellite, which has highly accurate measurements of J0529-4351’s position, was used to determine that J0529-4351 is truly a single light source without a lens in the sky. This is supported by more detailed spectra recorded at the European Southern Observatory’s Very Large Telescope (VLT) facility in Chile.
J0529-4351 is likely to become a very important tool for the future study of quasars and black hole growth. The mass of black holes is a fundamental property, but it is very difficult to measure directly because there are no standard scales for such absurdly large, mysterious objects.
One technique is to measure the effect the black hole has on more diffuse gas orbiting it in large clouds, called the “broad line region.” This gas is revealed in the spectrum through broad ’emission lines’, which are caused by electrons jumping between specific energy levels in the ionized gas.
The width of these lines is directly related to the mass of the black hole, but the calibration of this relationship has been tested very poorly for the most luminous objects such as J0529-4351. However, because it is physically so large and so luminous, J0529-4351 will be observable by a new instrument installed on the VLT called Gravity+.
This instrument will make a direct measurement of the black hole’s mass and calibrate the relationships used to estimate the masses of other high-luminosity objects.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Philip Wiseman works at the University of Southampton and is funded by the Science and Technology Facilities Council.