Astronomy 101 Lecture 25, Apr. 28 2003 Active Galaxies and Quasars – (Chapter 25 in text) Active galaxies are much more luminous than normal galaxies; they emit much of their radiation in non-visible light. That is, they give Non thermal radiation (not blackbody radiation from hot gas in the stars). Large energy output: Active galaxies emit between 10 and 1000 times the Milky Way luminosity ( LMW = 1037 watts) Normal galaxies emit as black bodies; active galaxies produce a great amount of longer wavelength radio and infrared (IR) radiation. Several types of active galaxies exist – Seyfort & radio galaxies and also the quasars. We believe that there is no fundamental distinction between them. Long wavelength radio and infrared Thermal blackbody radiation
Seyfort galaxies Seyfort galaxies have very bright central cores. Overall, the Seyforts are about 10 times more luminous than the Milky Way. The core (or nucleus) may be 10,000 times more intense (per unit area) than the Milky Way and is a strong emitter in the radio and IR ranges. There is a ‘normal’ blackbody component to their spectra coming from the disk; and the larger component of radio and IR from the disk. The spectral lines seen in the core are very broad, indicating that the nucleus is rotating very rapidly (1000 km/s)
Short time variation means a small object: Seyforts and other active galaxies show a striking short-term variability of their luminosity. This Seyfort shows striking variations on the time scale of a month or so. This means that the emitting region is small. Why? Dt2 = time to cross object = diameter/speed of light Short term variation in light (or any other form of radiation) indicates the object is small. If a variable object were large enough that ‘light’ takes longer to travel across it than the duration of the spike in radiation, the spike would be smeared out. If Dt1 = Dt2, the two spikes are separated in Speed of light time. Time structure is washed out. Dt1= duration of spike
Radio Galaxies: Radio galaxies emit an even larger fraction of total radiation in radio region than Seyforts. They typically show huge lobes of large radio emission – often 10 times the size of the visible galaxy, with comparable luminosity to the visible galaxy. There are often ‘jets’ of material that seem to be ejected from the central core; the jets emit radio and IR. Centaurus A seen in visible & radio superimposed, and X-rays Seyforts and radio galaxies emit comparable energy (~ 10 x Milky Way) and both have very intense central cores with often rapid variations.
Jets: Jets are common features of active galaxies; example in M87 in nearby Virgo cluster M87 – giant elliptical long exposure short exposure shows a jet emerging from galaxy Magnified jet viewed in radio Magnified jet viewed in IR
Quasars: • Around 1960, odd, very strong radio sources were found. They were later associated with visible objects whose spectral lines were found to be shifted far to the red, so are moving away from us rapidly. Using the Hubble relation v = H0 d, we deduce that they lie very far away, and must therefore be intrinsically very bright. • Typical red shifts z =Dl/l = 2-3 ! (largest observed z=6) • Typical distances (at time light was emitted) = 3000 Mpc (up to 8000 Mpc) • light emitted typically 10 billion years ago • Luminosity 1040 – 1042 watts (1000 – 100,000 times Milky Way) • Non thermal emitters • Rapid time variations z = 0.4 = Dl/l; distance =700 Mpc (a nearby quasar!) Named quasi-stellar radio sources = QUASARS Quasi-stellar because at large distances, they are nearly point-like. (we now know them to be galaxies)
Quasars look pretty unimpressive compared to nearer spiral galaxies since they are so far away. But their energy output is prodigious. Quasars often show jets which emit radio, IR and visible light Quasars seem to be more powerful versions of Seyfort or radio galaxies. They seem to have existed only for one 2-3 billion year period in the history of the universe, around 10 billion years ago.
Large red shifts for very distant objects: z = Dl/l (fractional shift in wavelength) Earlier we said z = v/c where c = speed of light. So how come we see quasars with z > 1? Are they moving faster than light? NO! The formula z = Dl/l = v/c breaks down when v approaches c and has to be replaced with a more complicated (relativistic) formula. We see a distant quasar at the distance from us that it had at the time it emitted the light we see. But since it is moving away rapidly, it is now considerably more distant. at time of emission now us light galaxy v Lookback time is simply the time that the light has been travelling to reach us. The present distance depends on the way the universe is expanding. z v/clookback timepresent distance 0.5 0.3855,300 M yrs6,500 Mlyr 2000 Mpc 1 0.600 8,10011,300 3500 3 0.88211,80021,300 6500 10 0.98413,40031,400 9600 1.00013,900 46,700 14,300
What generates the energy of Active Galaxies? • Have to explain: • High luminosity (10 – 100,000 x Milky Way) • Non-thermal emission of energy • Rapid time variations • Appearance of jets • Rapid internal motion • There is some analogy here with gamma ray bursters, which were explained in terms of matter falling on a neutron star or black hole from an accretion disk. Also recall that we that we find a supermassive black hole in the center of our galaxy. A combination of these ideas is thought to explain the quasars and active galaxies: • Accretion of matter onto a supermassive black hole (~billion solar masses with Swartschild radius of ~20AU) in the center of a galaxy. • Infalling material (stars, gas) is heated, ionized, accelerated. One can convert about 10 – 20% of the mass energy (E=mc2) of the material to radiation. The amount of stuff needed to generate what we see is ‘only’ about one solar mass per day (bright quasar) to one solar mass per decade (Seyfort galaxy).
Model for an Active galaxy: Supermassive black hole, rotating rapidly (remember spin-up of a neutron star by accretion disk). Accretion disk – stars, gas clouds Swirling ionized material generates high magnetic fields aligned perpendicular to the disk. Jets are due to electrons and ions moving along the magnetic field direction.
Observational evidence for the model: 1. Evidence of a very small, very bright core at the center of an active galaxy. 2. M87 in Virgo cluster is near enough that we can resolve the central core, and see that opposite sides are spinning rapidly from Doppler shifts. 3. Very long baseline radio telescopes have been able to see rapidly rotating gas around an object smaller than 0.2 pc.
Jets and Radio emission: The accretion disks somehow squirt energetic electrons out along the axes of the magnetic fields, giving jets. These electrons spiral around the magnetic field lines, rapidly altering the direction of their velocity. The resulting acceleration generates synchrotron radiation. Synchrotron radiation (discovered in our earthly particle accelerators) is highly non-thermal (non-blackbody) and favors radio and IR frequencies. The electron motion around the magnetic field lines thus explains both jets and the large radio emission.
Quasar lifetimes: The most luminous quasar (1042 watts) consumes 1000 solar mass stars per year. At this rate it eats up all the material in its environment in a relatively short time (compared to the life of the universe) – perhaps in 100 million years. The quasar will eat out a cavity in its galaxy and then go dormant, flaring up periodically as new material wanders by. The finite lifetime of this accretion process explains why the quasars are only observed for a relatively brief interval in the universe’s life about 10 billion years ago. No nearby quasars are observed, indicating that the supermassive black holes long ago ate up the available food supply. Our own galaxy, and most others we can study, have massive black holes in their center. Though the Milky Way central black hole is small on the scale of quasar black holes, it may resemble a dormant quasar galaxy.
Active galaxy formation hypothesis: Early in the history of the universe, small irregular galaxies collide and merge, and form massive central black holes. After successive mergers and buildup of the central black hole, these early galaxies become quasars (if merge large galaxies) or Seyfort/radio galaxies (if more minor mergers). Ultimately the material available to fall into the black hole is exhausted and the activity stops. The galaxy then continues its life as a normal galaxy with its shape dependent on the nature of the mergers that created it.
Quasars as observational tools: • Quasars are so luminous that we can see them from very great distances, so they help us map out the extent and structure of the universe at the largest scale and earliest times. • The light from a quasar is strongly absorbed by the Lyman alpha transition (electrons hopping from lowest to next lowest orbits). In our labs, Lya line is at 122 nm. Lya absorption by quasar itself is red shifted here to 564 nm. Intervening gas clouds are receding less fast, so show Lya lines less red shifted. Thus the extinctions of light reveal the presence and velocities of the clouds. We learn about the distribution of gas in the distant universe. (there are 100’s of intervening clouds seen in this picture.)
Quasars as observational tools: Gravitational lensing * Light from a distant quasar passes close to a nearer galaxy and is bent (general relativity and warped space) so we see multiple images. An unseen object in the path of a distant galaxy creates a pair of mirror images. Ring of images due to gravitational lensing of a distant blue galaxy. Gravitational lensing gives us a way to map dark matter in the distant universe.
Quasars and active galaxies: • Early stages of galaxy evolution. They give us insight into the way that galaxies form. • Their very large luminosity means they can be seen at huge distances. They thus help us map the distant universe. The light they emit interacts with closer galaxies, clouds of gas and dark matter, revealing their distributions in the universe as well.