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Monday 3 August 2015

The Impossible Quasar at the Dawn of the Universe

The recent extraordinary discovery of the biggest and brightest quasar of the early universe has intrigued astronomers worldwide. The reason behind this? The quasar - SDSS J010013.02+280225.8 (affectionately nick-named J0100+2802), is far larger than current black hole theories predict it should be[1].

Artist’s impression of quasar J0100+2802.
Among the oldest and brightest entities in the universe, quasars eject jets of very bright light that can be seen from lightyears away. It was initially believed that different events were being seen when quasars were observed, but it was later established that our line of sight affected the appearance of the quasar, for example a blazar is a quasar with jets that are pointing towards Earth. Image credit: ESO/M. Kornmesser

What is a quasar?

It is generally accepted that at the centre of most galaxies resides a supermassive black hole, whose size is related to that of its host galaxy. A supermassive black hole, like any black hole, is a region of space from which no light can escape. Supermassive black holes have a far higher mass than standard black holes, resulting in a much higher schwarzschild radius.

Model of a gravitational well with event horizon
The hole in the center is within the schwarzschild radius - the “event horizon” beyond which light cannot escape. The larger the schwarzschild radius the lower the density of the black hole. Image: public domain

The gravitational pull of a supermassive black hole causes any nearby matter, such as gas, dust and general debris, to orbit around it in the form of a flattened ring known as an accretion disk. The gravity then accelerates and compresses the matter within the disk, raising the temperature and hence emitting immense amounts of electromagnetic radiation. This extremely bright region, surrounding the central black hole, is known as a quasar. For this reason, quasars are among the most luminous entities in the universe, and can be hundreds of times brighter than the galaxy in which they reside.

Over time, the matter making up the quasar loses angular momentum, and falls into the black hole. In our own Milky Way, the central supermassive black hole releases no observable source of radiation. This is due to a lack of surrounding material, leaving the black hole with no energy source. It will lie ‘dormant’, as it were, until more spiralling matter falls into its grasp, acting as fuel for a quasar to form once again. Scientists are yet to understand the process by which the matter in the accretion disk loses angular momentum; there are speculations that the process could be a product of the magnetic fields, torques, and radiation pressure, or that possibly incoming matter pushes the matter closest to the black hole past the event horizon – the point past which even light cannot escape[2].

Supercomputer simulation exploring the inner zone of the accretion disk. The gas closest to the event horizon is extremely hot and approaches the speed of light before it falls into the black hole.

The Impossible Quasar

The quasar in question is observed to be 429 trillion times brighter than our Sun and twelve billion times more massive. With somewhere between 200 and 400 billion stars in the Milky Way, this makes the quasar many times brighter than our entire galaxy combined. It is also one of the earliest quasars - dating back to the end of the ‘Cosmic Dark Age’ when the universe was less than one billion years old[1][3]. The quasar is a distance of 12.8 billion light years from Earth, and despite the great distance travelled, we can detect the electromagnetic radiation emitted from the quasar at the Earth’s surface.

Diagram explaining the doppler shift.
Quasar J0100+2802 has a redshift of 6.30, which is very high - Redshift is a measurement that correlates to how far away an object is in the universe. Due to the constant expansion of space, cosmic bodies that are older are moving away from us. The light we observe from them as a result is stretched and hence we see it as redder than the original emitted light, as red light has a longer wavelength[4] - so a high redshift demonstrates the quasar is very far away, and hence very old. Image © Things We Don't Know (CC BY)

These interstellar entities are among the oldest and brightest known structures to be found, and hence offer invaluable information. Aside from their interesting structure, Quasars have been regarded with growing interest as their great distance provides an insight to the conditions of the early universe.

What makes quasar J0100+2802 so impossible?

A quasar can supposedly limit the growth of the internal supermassive black hole. The matter in the accretion disk that eventually ‘disappears’ beyond the event horizon essentially ‘feeds’ the black hole, allowing for growth. However as matter reaches the inner realm of the disk, the force applied accelerates it towards the speed of light, so collisions become far more frequent. The temperature of the inner disk rises as a result, and a torrent of electromagnetic radiation emanates in the form of two jets on opposing sides due to the magnetic conditions. These cosmic jets push the matter in the inner disk further out and hence limit the black hole’s source of energy; every time enough matter is provided to form the surges of radiation, the available matter in the disk decreases, restricting the growth of the system.

It has been suggested that quasars surrounding supermassive black holes in the early universe may have developed at a greater rate than the galaxy in which they resided. This could be attributed to the universe being far smaller than it is today, and hence the matter within it being denser. This could hypothetically prevent some of the matter within the inner disk being pushed back by the jets, and as a result imply that more matter could fall into black holes than is currently believed possible.

For now, however, we are left with an open ended question, and a tantalising insight to the dawn of the universe.

This article was written by Holly Godwin, a physics student from the University of Surrey, conducting a summer internship as a science writer at TWDK.

why don't all references have links?

[1] Beletsky, Y. et al. An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30. Nature 518, 512-515, 2015 doi:10.1038/nature14241
[2] Wanjek, C. Ring Around the Black Hole. NASA, Science and Technology, Solar System Exploration, 2011 (web)
[3] Zrioka, P. Writing the history of the ‘Cosmic Dark Ages’. 2013
[4] Hubble, E. P. The Observational Approach to Cosmology. Clarendon Press, 1937.

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