Active Galactic Nuclei - PDF Free Download

Active Galactic Nuclei Prof. Jeff Kenney Class 18 June 20, 2018

the first quasar discovered 3C273 (1963) very bright point source (the quasar ) jet

the first quasar discovered 3C273 (1963) very bright point source (the quasar ) when light from bright central source is blocked, can see surrounding host galaxy jet artifact from removing bright central source

Quasars the most luminous of the different types AGN ENORMOUS luminosities up to L~10 46 erg/sec ~ 10 13 L sun this is more than the luminosity from all the stars of the largest galaxies!! Not only that, but we know all this energy must come from a very small region, much smaller than the size of a galaxy. We know this because the luminosity from AGN are highly time-variable.

Blazar 3C454.3 AGN time γ-ray variability optical radio AGN luminosities vary on timescales of weeks to years à AGN must be small! Sizes of light-weeks to light-years

Connection: AGN time variability and AGN size d = c Δt some emitting blob Back Δt Front photon from Front has head start, reaches observer Δt = d/c before photon from Back Brightness Front Back Time It takes time Δt=d/c for light to travel from back of source to front. This smears out any instantaneous pulse to a pulse which lasts Δt=d/c. If d = 1 light-year, then Δt = 1 year. Since pulse might not be instantaneous, source could be smaller: d<cδt

For the most luminous AGN, the quasars, the size is < 1 light-year. This is much smaller than a galaxy, whose typical size is 100,000 light years!

For the most luminous AGN, the quasars, the size is < 1 light-year. This is much smaller than a galaxy, whose typical size is 100,000 light years! What can possibly produce enormous luminosities from such small volumes? Supermassive BHs in the centers of large galaxies.

How can a black hole emit lots of energy? A. By the process of Hawking radiation B. Energy can go into 1 black hole and out another, via a wormhole. C. It can t, but some stuff falling TOWARD the black hole gets shot outwards D. It can t, but relativistic boosting effects make the small amounts of energy released appear large to us E. It can t, but stuff falling in can emit lots of energy just before it goes in

Main components of AGN

Optical light from AGN accretion disk & jet Optical light image of M87 elliptical galaxy (light from stars) Light from accretion disk Light from stars Light from jet Optical light image of center of M87 elliptical galaxy (light from accretion disk & jet & stars)

How do BHs provide the energy for AGN? Enormous energy rate from small volume L~10 46 erg s -1 from r < 100 AU Q: What is energy source?

How do BHs provide the energy for AGN? Enormous energy rate from small volume L~10 46 erg s -1 from r < 100 AU Q: What is energy source? Could it be nuclear fusion?

How do BHs provide the energy for AGN? Enormous energy rate from small volume L~10 46 erg s -1 from r < 100 AU Q: What is energy source? Could it be nuclear fusion? NO!!

How do BHs provide the energy for AGN? Enormous energy rate from small volume L~10 46 erg s -1 from r < 100 AU Q: What is energy source? Could it be nuclear fusion? NO!! basic problem with fusion is low efficiency Fusion efficiency = [photon energy output / rest-mass energy input ] ~ 0.007 i.e. <1% efficiency

Gravitational accretion energy can be much more efficient! Suppose a mass m falls from ~infinity (large distance) to R toward mass M. If it starts with zero velocity (v=0), when it reaches R it will have velocity v = v esc = sqrt[2gm/r] KE gained can be high if M big and R small! e.g. M = 1 M sun, R = R Sch = 2GM/c 2 = 3 km KE acc = ½ m v esc 2 = ½ mc 2 (since escape speed for R Sch is c) (ignoring relativity effects) Actually, incorporating relativity effects KE acc =~ 0.30 mc 2 (falling to R Sch of BH) A mass can produce ~30% of its rest mass energy just by falling!

Gravitational accretion energy can be very efficient! A mass can produce ~30% of its rest mass energy just by falling! (if it falls from large distance to event horizon of BH) Not all of this KE gets converted to photons. We think that for BHs, a maximum of ~1/3 of the KE can get converted to photons (by stuff colliding with other stuff and getting heated up). Net result: up to ~10% of rest mass energy of stuff falling into BHs can get converted to photons à gravitational accretion energy for BHs can be ~10x more efficient than nuclear fusion!

Note that this is the same energy source we use on the earth as hydroelectric power letting stuff fall, converting gravitational potential energy into kinetic energy.

Note that this is the same energy source we use on the earth as hydroelectric power letting stuff fall, converting gravitational potential energy into kinetic energy. The difference is that: on the surface of the earth, M is small and R is large. with BHs, M is large and R is small. So the energy efficiency is much greater with BHs!

Gravitational energy powering AGN E ~ 0.1 mc 2 i.e., material falling into a black hole near the nucleus of a galaxy may release up to about 10% of its rest energy The release of gravitational energy by a massive black hole (about 100 million solar masses) "eating" one star per year would power a typical quasar.

Different types of AGN Quasars Radio galaxies Blazars Seyfert galaxies What are fundamental differences?

Main components of AGN

Unified model of AGN contains: massive black hole + accretion disk (+ possibly a jet) What varies among galaxy nuclei: A. intrinsic properties 1. black hole mass (M sun ) 2. feeding rate (M sun /yr) 3. black hole spin? B. Viewing Angle effects 1. Obscuring torus? 2. Jet direction

Unified model of AGN contains: massive black hole + accretion disk (+ possibly a jet) What varies among galaxy nuclei: A. intrinsic properties 1. black hole mass (M sun )

Bigger black holes in bigger galaxies (or bigger bulges of galaxies) We think every galaxy has BH at its center & Black hole mass ~ 10-3 galaxy bulge mass

Why aren t the nuclei of most nearby large galaxies active? A. They don t have nuclear black holes B. Their nuclear black holes have small masses C. They are not accreting much matter at present D. Their nuclear black holes aren t spinning E. Our view of these nuclei is obscured, but they really are active

AGN or not AGN: how much matter is in the accretion disk? If there is a lot of matter in the accretion disk, it means: we can detect light from the accretion disk (and so it is an AGN) the black hole is being fed & is growing! something happened recently to put matter in the accretion disk

We think all galaxies have BHs at their centers, but most are NOT being fed now If there is little or no gas in accretion disk, it means: There is nothing around the black hole to produce light to detect (not an AGN) Black hole is not feeding or growing

Black hole disrupting star, fueling accretion disk

Galaxy interactions cause central black holes to be fed, making AGN The best way to feed nuclear BH is to have a galaxy interaction, which drives material toward the center This happened more often in early universe, which is why quasars are rare today (the SMBHs in most galaxies today are fairly dormant).

Unified model of AGN contains: massive black hole + accretion disk (+ possibly a jet) What varies among galaxy nuclei: A. intrinsic properties 1. black hole mass (M sun ) 2. feeding rate (M sun /yr) how much matter is flowing through accretion disk 3. black hole spin? (or something else related to presence of jet not understood!)

Thick disk of gas & dust (beyond accretion disk) can block view of accretion disk

animation 24-2

Thick disk of gas & dust can block view of accretion disk If you view accretion disk nearly face-on, you can see it. But if you view from other angles, view of accretion disk may be blocked (although still might see jets).

Unified model of AGN contains: massive black hole + accretion disk (+ possibly a jet) What varies among galaxy nuclei: B. Viewing Angle effects 1. Obscuring torus? -- whether direct view of accretion disk is blocked by gas & dust 2. Jet direction -- if jet is aimed nearly at us, two relativistic effects occur: a. apparent superluminal motion b. flux (light) of jet is boosted

Radio Galaxy RG is associated with the big Elliptical Galaxy Blue optical emission from starlight (blackbody radiation) Red radio emission from relativistic particles in magnetic fields (synchrotron radiation)

Radio jets and radio lobes Hercules A: optical image (HST) plus radio image (VLA)

What is the Radio emission from radio galaxies and AGN jets? It is EM radiation, but it is not blackbody (or thermal) radiation, like that produced by stars very different spectra Blackbody: characteristic shape with peak at frequency which depends on Temperature Synchrotron: no peak intensity declines steadily with frequency (radio jets) (stars) radio optical Similarities: in both cases it is EM radiation produced by electric charges (electrons) which are accelerated

Thermal (blackbody) radiation Electron accelerated by electric field (of proton) emits photon

Synchrotron radiation Relativistic (very fast) electron accelerated by magnetic field emits photons

M87 jet

Apparent superluminal motion in jet of M87 From series of optical HST images taken from 1994-1998

1992 1994 1996 1998 (apparent) Superluminal motion Blobs in jets appear to move faster than the speed of light! Outer blob appears to move 24 light-years in only 6 years so apparent speed is about 4 times the speed of light!! 0 20 40 60 80 (Light years)

apparent superluminal motion an optical illusion that happens when stuff moving at nearly the speed of light is beamed nearly in your direction

Jet direction affects observed properties Jet flux boosted in jet direction by relativistic effects blue indicates flux from jet light from face-on jet can outshine accretion disk & stars in galaxy!

Jet direction affects observed properties Jet is much brighter if it is beamed right toward us (within ~10 degrees) Blazar is a quasar whose jet is directed toward us

Death Star Galaxy Actual image (optical & radio) AGN Jet from one galaxy hits neighboring galaxy Artist s version

During the merger of our Galaxy with M31, which way are we most likely to die? A. Colliding with star or planet B. Falling into supermassive black hole at nucleus C. Blasted by AGN jets from black hole D. Sun turns into red giant E. Boredom F. Bugs G. Self-destruction through something like nuclear annihilation or global warming or Trump

Good books on black holes Kip Thorne Black Holes & Time Warps Mitch Begelman & Martin Rees Gravity s Fatal Attraction

Great NOVEL!! Alan Lightman Einstein s Dreams