Our Our Place Place in in the the Cosmos Cosmos the most - PDF document

Quasars • Galaxies shine with the luminosity of hundreds of billions of stars • However, even galaxies pale beside quasars, Our Our Place Place in in the the Cosmos Cosmos the most luminous objects in the Universe • Quasar is a contraction of the term quasi- stellar radio source, so-called because they Lecture 15 were first identified in the 1950s-60s as Quasars and unresolved points at radio wavelengths • Quasars radiate with a luminosity of a trillion Active Galactic Nuclei to a thousand trillion (10 12 - 10 15 ) Suns Quasars HST Quasar Images • 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 Quasars and AGN Seyfert and Radio Galaxies • HST images show that quasars are not • The first AGN were discovered by Carl Seyfert in isolated but are located at the centres of 1943 large galaxies • These were all spiral galaxies with a bright nucleus of luminosity 10 billion - 100 billion Solar luminosities, • About 3% of all galaxies contain brilliant comparable to the rest of the galaxy points of light at their centres that may outshine the light from all of the stars • AGN in elliptical galaxies are most prominent at radio wavelengths, giving the host galaxies the name radio • These are known as active galactic nuclei galaxies (AGN) • Many radio galaxies emit jets of radiation extending • Quasars are the most luminous type of AGN millions of light years from the galaxy

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 Blue = visible starlight Red = radio emission Synchrotron radiation What Powers AGN? is instantaneously beamed in the direction of motion of • The second clue is that AGN are about the a charged particle size of the Solar system • How do we know this? - AGN are unresolved Since the particles spiral around the point sources even with HST magnetic field lines, • The answer is that AGN intensity varies on a the overall radiation is emitted in time-scale of order a day or so directions • This tells us that the AGN power source can perpendicular to the field lines be no more than a light-day across, since no signal can travel faster than light 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 A note played simultaneously by musicians in a widely- • In the case of AGN, the central black hole has a spaced marching band will reach the listener at different mass of order one billion solar masses, compared with times. A tight apparent ensemble means that the players around 3-10 solar masses for a supernova remnant must be close together.

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 Captions Unified Model of AGN Unified Model of AGN Edge-On View • As material is accelerated by gravity towards • The outer dust torus is ionized by UV the black hole it slams into the accretion disk, radiation, giving rise to emission lines in AGN heating it to around 100,000 degrees spectra • Radiation is emitted in visible, UV and X-rays • Most importantly, the dust torus obscures our view of the central engine in different ways • Around 50% of the mass of infalling matter is depending on our viewing angle converted into luminous energy, the rest is pulled into the black hole itself, causing it to • Seen edge-on, we see emission lines from the grow in mass torus and other surrounding gas • Interaction of accretion disk with black hole • We may also see the torus gives rise to powerful radio jets in absorption • Magnetic fields accelerate charged particles � synchrotron radiation NGC 7052 (HST) Unified Model of AGN Partially 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 Radio jets hundreds of thousands of accretion disk light-years in size originate in a central engine no larger than our Solar System

Unified Model of AGN Doppler Broadening Face-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 Motion of emitting gas broadens observed lines in the spectrum Success of Unified Model Success of Unified Model • Note that the unified model of AGN is an • Only about 3% of present-day galaxies empirical one which has been developed since contain AGN the 1980s to explain the observations • In more distant, and hence older, galaxies, • To be a good theory this model must also that fraction is much higher make testable predictions • There were many more AGN in the young • The essential features of the model are an Universe than there are today accretion disk surrounding a supermassive • If the unified model is correct, then the black hole being “fed” by infalling material supermassive black holes that powered these • Without a source of matter falling onto the AGN should still be around today black hole the AGN would no longer be active, • We expect most normal galaxies to contain a but the supermassive black hole would remain supermassive black hole Supermassive Black Hole s Supermassive Black Hole s • We can search for supermassive black holes • All large galaxies appear to contain a via the gravitational pull on stars in the supermassive black hole centres of galaxies • The only difference between normal galaxies • Spectroscopic observations have revealed and AGN is whether the black hole is being evidence for 10,000 - 5 billion M � black holes “fed” by infalling matter at the time we see at the centre of every normal galaxy studied the galaxy • Furthermore, there is a correlation between • Adding large amounts of gas and dust to the the black hole mass and the mass of the centre of a galaxy would re-ignite AGN elliptical galaxy or spiral galaxy bulge in activity which it resides • This can happen when galaxies interact - tidal forces redistribute the material within the galaxies

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 The Antennae - a form first? merging pair of • There is clearly a link between galaxy galaxies (HST) formation SMBH formation, but what is that link? • These are unanswered questions!