Quasars – The worst kind of light pollution

There are some monster objects in the universe that are truly frightening in their power. A while ago at Milky-Way.Kiwi we had a look at the Super Massive Black Hole at the centre of our own galaxy but it’s these huge monsters (the Quasars) that are responsible for the brightest objects in the night sky. But don’t go rushing outside expecting to see any of these fantastic objects, you might be a little disappointed because they are a long distance away – it’s better to talk about them in terms of time. There is in fact one that can be easily seen with a telescope and the cool thing about it is that it’s the most distant object that an amateur astronomer can see. The other amazing thing about quasars is that they are super bright, so bright that they outshine their host galaxies. So what are these monsters of the deep (space)? And why do they put out so much energy that we can see them billions of light years away?

Our understanding of quasars started in 1963 when a radio source was examined in detail by Maarten Schmidt and was found to have a number of anomalies. For one, he determined that it had a significant red-shift, meaning it was a long way away, in fact a considerable long way outside of our own galaxy. Schmidt figured that this strange object had two possible explanations, it was either a star with a large gravitational red-shift or an active centre of a galaxy. The redshift he measured was calculated to be 0.158 which works out to be a velocity of 47,400km per second. That is the distance at which the quasar receded from us, how fast was rushing into the cosmic void. Cosmologists figured out that the relationship between speed and distance could be related by Hubble’s constant and Schmidt calculated the possible distance of this mystery object to be around 1.6 billion light years (Schmidt 1963, p126). You can see this quasar with a telescope as it has a magnitude of 12.85 and is in the constellation Virgo. Modern measurements put it at a distance of 2 billion light years. It is the most distant object that amateur astronomers can see, to go any further you’ll need a massive telescope and a huge backyard.

Best image of bright quasar 3C 273
Quasar 3C 273 imaged by Hubble (Credit: Hubble Space Telescope via Wikipedia)

Astronomers believe that quasars are emissions given off from the material around super massive black holes. These black holes started out in the early universe small and grew rapidly attaining the size of about billion times the mass of the Sun. This happened because they formed in areas that had a lot of dense material, which formed large accretion disks around the black hole. Quasars are exceptionally bright (the one in the picture above looks just to the untrained eye just like a star) with observed luminosities of one large population being between 10^41 and 10^49 ergs* (Hopkins et al 2007, p750). Compare this to the Sun’s luminosity which is about 3.8 x 10^33 ergs so it’s easy to see how some of these things would be brighter than an entire galaxy. This material fuelled the massive mass of the blackholes and probably also sparked the growth of galaxies in their vicinity (Bouwens, R 2017, p418). These blackholes soon grow into super massive blackholes and these are thought to reside at the centre of almost every galaxy (White, S 2015, p5).

The material in an accretion disk around the super massive black hole (SMBH) heats up to very high temperatures as it gets closer and closer to the SMBH and the energy released by this process is what gives them their luminosity across many wavelengths, including UV. The size of these accretion disks are quite small measuring only about 4.5 light days (Jimenez-Vicente et al 2014, p6) which is about 20 times the distance from the Sun to Pluto. Studies of the million of quasars that have been observed since they were first reported in 1963 have shown that the population rose rapidly until about 10.8 billion years ago and then started to taper off (Mortlock 2014, p44). It seems that conditions in the early universe favoured the formation of these monsters and then they started to diminish. The result is that there are no quasars very close to the Milky Way as very few have been formed recently. Quasars last around 1,000,000 to 100,000,000 years (Haiman & Hui 2001, p546). This estimation combined with observations of the population of quasars and their spread throughout the observable age of the universe has enabled astronomers to understand a lot about these powerful objects.

The closest quasar to us is about 600 million light years away in the galaxy Mrk 231. In 2015 the Hubble Space Telescope imaged the centre of this galaxy and scientists determined that there may be two black holes orbiting each other in the middle of the donut-shaped accretion disk. They worked this out by looking at the ultraviolet radiation coming off the quasar and found it was not uniform towards the centre to where a single super massive black hole would be and they inferred that there must be two of them orbiting each other. This added another flavour to the whole discussion around quasars as the likelihood of two super massive black holes being in close proximity to each other is suggestive of a galaxy merger (Hille 2017).

8x10.ai

Models of how quasars form are based on galaxy mergers and the theory being that in the early universe there were more galaxy mergers and the as the universe continued to expand there were less (Mortlock 2014, p44). This might explain how the SMBHs got so big. For Mrk 231 the two black-holes are predicted to collide in a few hundred thousand years (Hille 2017).

Given there are a lot of quasars and they tend to be in larger numbers in the earlier universe and our galaxy is very old and has merged with a lot of other galaxies then it seems to be possible that the Milky Way once had a quasar blazing out from it’s centre at some point in its early life. What a spectacle that would have been and how incredible it would have been to view it by the galaxy’s inhabitants (if any were able to live). I can imagine it would be the worst kind of light pollution you could get.

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* ergs – unit of energy and work equal to 10^-7 jules

References:

Bouwens, R 2017 ‘Quasars Signpost Massive Galaxies’, Nature, vol 545, 25 May 2017.

Jimenez-Vicente, J, Mediavilla, E, Kochanek, CS, Munoz, JA, Motta, V, Falco, E & Mosquera, AM 2014, ‘The Average Size and Temperature Profile of Accretion Disks’, The Astrophysical Journal, 783:47, 1 March 2018, <http://iopscience.iop.org/article/10.1088/0004-637X/783/1/47/pdf&gt;

Haiman, Z & Hui, L 2001, ‘Constraining the Lifetime of Quasars From Their Spatial Clustering’, The Astrophysical Journal, 547, 20 January 2001, viewed 3 October 2018, <http://iopscience.iop.org/article/10.1086/318330/pdf&gt;

Hille, K 2017, Hubble Finds The Nearest Quasar is Powered by a Double Black Hole, NASA, Goddard Space Flight Center, viewed 3 October 2018, <https://www.nasa.gov/feature/goddard/hubble-finds-that-the-nearest-quasar-is-powered-by-a-double-black-hole&gt;

Hopkins, PF, Richards, GT & Hernquist, L 2007, ‘An Observational Determination of the Bolometric Quasar Lminsoity Function’, The Astrophysical Journal, 654:731, 10 January 2007, viewed 3 October 2018, <http://iopscience.iop.org/article/10.1086/509629/pdf&gt;

Mortlock, D 2014, ‘The Age of Quasars’, Nature, Vol 514, 2 October 2014.

Schmidt, M 1963, ‘3C 273 a star like object with large red-shift’, A Century of Nature, Twenty One Discoveries That Changed Science and the World, University of Chicago Press, Chicago, viewed 3 October 2018, <https://books.google.co.nz/books?hl=en&lr=&id=p_P1Wm5MnVAC&oi=fnd&pg=PA125&dq=3c+273&ots=XnFVHD9y7x&sig=e18_sC1mpy2srfDmWTRJODW0LWc#v=onepage&q=3c%20273&f=false&gt;

White, SV 2015 Accretion and Star Formation in Quasars, University of Oxford, Oxford, United Kingdon, <https://ora.ox.ac.uk/objects/uuid:94fa7a0c-83be-4283-9bf5-558b9354044d/download_file?safe_filename=thesis.pdf&file_format=application%2Fpdf&type_of_work=Thesis&gt;

The featured image shows a lensed quasar taken by the Hubble Space Telescope.