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Our Place in the Cosmos

Our Place in the Cosmos. Lecture 15 Quasars and Active Galactic Nuclei. Quasars. Galaxies shine with the luminosity of hundreds of billions of stars However, even galaxies pale beside quasars , the most luminous objects in the Universe

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Our Place in the Cosmos

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  1. Our Place in the Cosmos Lecture 15 Quasars and Active Galactic Nuclei

  2. Quasars • Galaxies shine with the luminosity of hundreds of billions of stars • However, even galaxies pale beside quasars, the most luminous objects in the Universe • Quasar is a contraction of the term quasi-stellar radio source, so-called because they were first identified in the 1950s-60s as unresolved points at radio wavelengths • Quasars radiate with a luminosity of a trillion to a thousand trillion (1012 - 1015) Suns

  3. Quasars • Due to their luminosity, quasars can be seen to huge distances - currently the most distant known object in the Universe is a quasar at redshift 6.4 • The nearest quasar is approximately 1 billion light years away - there are billions of galaxies closer to us than that • Quasars are thus objects of the distant Universe • Finite travel speed of light means that we see more distant objects when they were younger • Quasars were common in the early Universe but are rare today  Universe is evolving

  4. HST Quasar Images

  5. Quasars and AGN • HST images show that quasars are not isolated but are located at the centres of large galaxies • About 3% of all galaxies contain brilliant points of light at their centres that may outshine the light from all of the stars • These are known as active galactic nuclei (AGN) • Quasars are the most luminous type of AGN

  6. Seyfert and Radio Galaxies • The first AGN were discovered by Carl Seyfert in 1943 • These were all spiral galaxies with a bright nucleus of luminosity 10 billion - 100 billion Solar luminosities, comparable to the rest of the galaxy • AGN in elliptical galaxies are most prominent at radio wavelengths, giving the host galaxies the name radio galaxies • Many radio galaxies emit jets of radiation extending millions of light years from the galaxy

  7. Blue = visible starlight Red = radio emission

  8. What Powers AGN? • The first clue is that AGN emit synchrotron radiation • This radiation is named after an early particle accelerator called a synchrotron • Synchrotron radiation is produced by the acceleration of charged particles to almost the speed of light by strong magnetic fields • AGN must be extremely energetic to accelerate particles to these high speeds

  9. Synchrotron radiation is instantaneously beamed in the direction of motion of a charged particle Since the particles spiral around the magnetic field lines, the overall radiation is emitted in directions perpendicular to the field lines

  10. What Powers AGN? • The second clue is that AGN are about the size of the Solar system • How do we know this? - AGN are unresolved point sources even with HST • The answer is that AGN intensity varies on a time-scale of order a day or so • This tells us that the AGN power source can be no more than a light-day across, since no signal can travel faster than light

  11. A note played simultaneously by musicians in a widely-spaced marching band will reach the listener at different times. A tight apparent ensemble means that the players must be close together.

  12. Supermassive Black Holes • AGN emit the light of 10,000 galaxies from a region smaller than Pluto’s orbit! • How can we make so much energy production into such a small volume? • Only one explanation makes sense - AGN are powered by accretion disks surrounding supermassive black Holes • We have already encountered accretion disks in star formation and around white dwarfs and neutron stars in binary systems • In the case of AGN, the central black hole has a mass of order one billion solar masses, compared with around 3-10 solar masses for a supernova remnant

  13. Unified Model of AGN • We now believe that all types of AGN - Seyfert galaxies, radio galaxies and quasars are described by a unified model • In this model, a supermassive black hole is surrounded by an accretion disk • Much further out lies a large torus of gas and dust - material that is feeding the central engine • Our classification of an AGN depends on how we view such a system

  14. Captions

  15. Unified Model of AGN • As material is accelerated by gravity towards the black hole it slams into the accretion disk, heating it to around 100,000 degrees • Radiation is emitted in visible, UV and X-rays • Around 50% of the mass of infalling matter is converted into luminous energy, the rest is pulled into the black hole itself, causing it to grow in mass • Interaction of accretion disk with black hole gives rise to powerful radio jets • Magnetic fields accelerate charged particles  synchrotron radiation

  16. Unified Model of AGNEdge-On View • The outer dust torus is ionized by UV radiation, giving rise to emission lines in AGN spectra • Most importantly, the dust torus obscures our view of the central engine in different ways depending on our viewing angle • Seen edge-on, we see emission lines from the torus and other surrounding gas • We may also see the torus in absorption NGC 7052 (HST)

  17. Unified Model of AGNPartially Face-On • Viewing more face-on, one can see over the edge of the torus and obtain a more direct view of the accretion disk and black hole • We see more synchrotron emission and Doppler-broadened lines produced in the accretion disk

  18. Radio jets hundreds of thousands of light-years in size originate in a central engine no larger than our Solar System

  19. Doppler Broadening Motion of emitting gas broadens observed lines in the spectrum

  20. Unified Model of AGNFace-On View • Our view of an AGN seen face-on is dominated by radiation from the jet coming straight towards us - we call this object a quasar or radio galaxy • The jet is so bright that frequently we cannot see the surrounding galaxy unless a coronagraph is used to block the light from the brilliant central quasar 3C 273 - HST

  21. Success of Unified Model • Note that the unified model of AGN is an empirical one which has been developed since the 1980s to explain the observations • To be a good theory this model must also make testable predictions • The essential features of the model are an accretion disk surrounding a supermassive black hole being “fed” by infalling material • Without a source of matter falling onto the black hole the AGN would no longer be active, but the supermassive black hole would remain

  22. Success of Unified Model • Only about 3% of present-day galaxies contain AGN • In more distant, and hence older, galaxies, that fraction is much higher • There were many more AGN in the young Universe than there are today • If the unified model is correct, then the supermassive black holesthat powered these AGN should still be around today • We expect most normal galaxies to contain a supermassive black hole

  23. Supermassive Black Holes • We can search for supermassive black holes via the gravitational pull on stars in the centres of galaxies • Spectroscopic observations have revealed evidence for 10,000 - 5 billion M black holes at the centre of every normal galaxy studied • Furthermore, there is a correlation between the black hole mass and the mass of the elliptical galaxy or spiral galaxy bulge in which it resides

  24. Supermassive Black Holes • All large galaxies appear to contain a supermassive black hole • The only difference between normal galaxies and AGN is whether the black hole is being “fed” by infalling matter at the time we see the galaxy • Adding large amounts of gas and dust to the centre of a galaxy would re-ignite AGN activity • This can happen when galaxies interact - tidal forces redistribute the material within the galaxies

  25. The Antennae - a merging pair of galaxies (HST)

  26. Galaxy Formation and AGN • Deep HST images show that galaxy interactions were more frequent in the past • AGN activity was most frequent then • How do supermassive black holes form? • Does the black hole form first, then the galaxy accrete around it, or does the galaxy form first? • There is clearly a link between galaxy formation SMBH formation, but what is that link? • These are unanswered questions!

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