3C273 Credit: SDSS
The central object looks just like any other star in the image above, but it’s as far from a star as you can get. It is in fact a Quasar (the less catchy name is Quasi-Stellar Radio Source), which are some of the most distant and most luminous objects seen in the observable universe. This one however isn’t as distant as many of its type, at just a redshift (Z) of 0.15 it’s just 1.88 billion (yes, just) light years away [1].
Some of the most distant Quasars have a Z of 6, placing them around 12 billion light years away, or 24 billion light years away in commoving distance. One such example is SDSS J1148+5251 which has a Z of 6.41. It was until recently the most distant quasar found, but this record has been replaced by CFHQS J232908-030158 at a Z of 6.43, placing it 12.8 billion light years away [2].
Most of these objects appear as point-like objects, which is why they look exactly like stars in images. Most galaxies at high redshift are hard to see, so why are Quasars so luminous that they can be seen at such great distances; and what are they anyway?
Quasars are a type of Active Galactic Nuclei (AGN). If you could see in detail the nucleus of any galaxy that is classified as ‘active’ you’ll see a doughnut shaped torus of matter surrounding a more compact, thinner accretion disk which in turn surrounds the galaxy’s super massive black hole. And all this could fit snugly inside a solar system!
The black hole at the centre of this powers the AGN. The huge gravitational pull from the black hole drags matter into its vicinity, surrounding itself with an accretion disk in the process. As the matter travels round the disk, it releases energy due to the friction caused by the material in the disk interacting with each other as it races round at thousands of kilometres per hour. The radiation emitted ranges from gamma rays to radio waves, and the sheer amount of radiation emitted can cause an AGN to completely outshine its host galaxy by hundreds of times!
The powerful magnetic fields caused by the AGN can also cause material to collimate into jets of plasma. This plasma races out of the galactic nucleus along the black hole’s spin axis at relativistic speeds, stretching out for thousands upon thousands of light years. The creation of these jets is still under a lot of debate, but currently it is thought that the material in the accretion disk escapes via the hole in the disk – which compared to the rest of the disk, is relatively dust free. The material escapes in this direction simply because there’s less resistance.
There are different types of AGN, from radio galaxies to Blazars. The AGN are basically the same thing, just viewed at different angles:
AGN at various angles; Credit: Aurore Simonnet, SSU NASA E/PO.
A Seyfert 1 for instance is where the AGN is viewed at a 30 degree angle and a Seyfert 2/ Radio galaxy is viewed at a 90 degree angle and so on. A Quasar is viewed at a 30 degree angle, so is it not just a Seyfert galaxy? What differentiates a Quasar from a Seyfert is the luminosity; an AGN with a luminosity of over 1011 L?is classed as a Quasar [3].
Back to the Quasars and 3C 273…
3C273 was the first Quasar to be indentified in 1963, when Maarten Schmidt published a paper in Nature, after the star-like object was associated with a radio source already documented in the Third Cambridge Catalogue of Radio Source [4]. Schmidt’s paper showed that 3C273 has a high redshift, placing it billions of light years outside our own galaxy.
This particular Quasar has a jet that stretches out for 60 kilo parsecs in length, making it almost twice the diameter of our galaxy [5]! You can see the jet in figure one; it’s the faint streak just below the quasar on the bottom right. This jet makes 3C273 one of just 10% of Quasars that have large scale jets as big as 3C273’s [6]. The other 90% have less powerful jets that are just parsecs in length.
Quasars aren’t seen (to the best of our knowledge) after a redshift of 0.06; they become more common the higher the redshift. Why are there more Quasars in the early universe? The universe was full with young galaxies absolutely brimming with new stars and therefore plenty of gas for a super massive black hole to consume. Eventually the fuel runs out, either by the AGN exceeding the Eddington luminosity, where the AGN becomes so bright the photons it emits buffets the gas out of the way and prevents it from falling in, or simply by the AGN having consumed all the available material.
List of references:
[1] SDSS DR7 ObjID: 587726014535237707
[2] Willot C. J et al (2007) “Four quasars above redshift 6 discovered by the Canada-France High-z Quasar Survey” Astro-ph:arXiv:0706.0914v2.
[3] Sparke, L.S. and Gallagher, J.S. (2000) ‘’Galaxies in the Universe: An Introduction’’ 2nd ed. New York: Cambridge University Press.
[4]Schmidt. M. (1963) “3C273: A Star-Like Object with Large Red-Shift”. Nature 197: 1040–1040. doi:10.1038/1971040a0.
[5] Uchiyama. Y. et al (2006) “Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope” Astrophys. J. 648 910 doi: 10.1086/505964
[6] “UT Austin scientists find evidence that all radio-loud quasars may be blazars’’. The University of Texas (14/2001)http://www.utexas.edu/news/2001/06/14/nr_blazers/
