Our Place in the Cosmos

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!