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Quasar Wind

Quasar wind is one of the hot topics discussed in cosmology today. It is basically a 'wind' generated by the accretion disc around a quasar. Before investigating the wind, first a look at what quasars and accretion disks are all about.

What is a Quasar?

The word 'QUASAR' is a contraction of QUASi-stellAR radio source. The intense radio waves are generated near the active nucleus of a distant galaxy. The active nucleus is though to comprise of a very massive black hole that is 'swallowing' surrounding matter, mostly gas and dust, but including the occasional star. The in-falling matter swirls around the black hole in what is called an 'accretion disk'. The accretion disk is doing the radio 'broadcasts'.

Quasar images

Pictured here is an artists impression of the accretion disk of a quasar with the plasma jets swirling out of the two poles. The image at the top left is an actual Chandra x-ray image of the same quasar in green. (Source: Wikipedia)

Why does the Accretion Disk Broadcast?

As the particles of matter swirl around the black hole, it forms a flat disk-like structure that is in orbit around the black hole, almost like the water swirling around the outlet of a bathtub. The gas and dust are compressed as they move closer to the black hole and the friction between the molecules and dust particles heat them up. They radiate away some of that energy, casing them to lose orbital energy and hence swirl in closer, where the process of heating up and radiating away intensifies. Eventually they get so hot that they radiate radio waves.

What is a Quasar Wind?

Quasars were long known to blast out some material in the form of polar jets of plasma (extremely hot ionized gas). They are though to form due to the twisting of magnetic fields in the accretion disk and were not called 'winds', but one can think of them as such. Now it is becoming apparent that there is also a quasar wind "of the second kind". This one is formed in the accretion disk by a still to be determined mechanism. The big difference to the polar jets 'wind' is that the disk wind is dusty. It rips some of the in-falling gas and dust 'vertically' out of the disk and flings them back into the galaxy.

Quasar wind source

Picture: Martin Elvis, Harvard-Smithsonian Center for Astrophysics.

Note the difference between the disk-wind and the polar jets of the previous picture. The disk wind originates in the disk en starts off at right angles to the disk, before centrifugal forces drags it out into a funnel shape.

The role of the these winds is huge in the formation process of young galaxies and even for the larger scale intergalactic medium. The quasar winds from the accretion disk can be as fast as 10,000 km/s, exceeding the escape velocity of the black hole and even sometimes the escape velocity of the whole galaxy. This helps with 'seeding' of the galaxy's star forming process and new galaxy forming in the intergalactic space.

Are we all just Quasar Dust?

It was long considered that we (and all the stuff we see around us) are literally 'star dust'. This dust was thought to originate only from supernovae, the explosions of super-heavy stars running out of fuel. Now there is competition for the supernovae and the competiton is bigger and much more massive - QUASARS.

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