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Apr 12, 2006 Astro 100 Lecture 32 1

Lecture 32

QuasarsBlazars

Black Hole/ Jet Model

Active Galaxies and Quasars

Apr 12, 2006 Astro 100 Lecture 32 2

Quasars

� When radio telescopes were first invented astronomers discovered many distant radio sources, and tried to identify them with optical counterparts.

� In the optical these radio sources looked just like stars, hence they were given the name quasi-stellar objects, now shortened to quasars.

� Quasars have very unusual spectra that display:� Very blue continuous emission� Very high redshift emission lines

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Apr 12, 2006 Astro 100 Lecture 32 3

Quasar RedshiftsExample: Quasar 3C273� From the spectrum we can measure the redshift:

z = ∆λ/λ = 104 / 656 = 0.1583 for z < 0.5 or so, v/c = z, v = 47,000 km/sline width of ~20 nm means internal velocities of ~ 8000 km/s

� If the mean velocity is due to the expansion of the Universe, Using Hubble's Law (v = Hd) we can calculate the distance to the quasar:� d = 47,000 km/s / 65 km/sec/Mpc = 720 Mpc

= 2.4 billion light years !� This sort of calculation OK for z < 0.5, d < 2.5 Gpc, 8 b lyExample: redshift 5 quasar

z = 608 / 122 = 5.0for z > 0.5, use formula from relativity, v/c = 0.95, v = 285,000 km/s

� Naive application of Hubble's Law (really need cosmology model):d = 4.4 Gpc = 14.2 billion light-years

look-back time ~ age of Universe

Apr 12, 2006 Astro 100 Lecture 32 4

Quasar Facts� Quasars are being seen when Universe was younger, since

they are typically billions of light years away. (First example of �look-back time� being important: a good fraction of the age of the Universe)� Most quasars are high-redshift objects. Most are found in the

range z = 2 - 4. They are not seen at the current time (i.e. at low redshift), so they are evidence for evolution of the Universe.

� Quasars are extremely luminous. From distance and brightness get luminosity

L = 4π d2 B � A typical quasar has a luminosity 100 times as large as the Milky

Way's luminosity.� Because of luminosity and distance, observing quasars can tell us

about the intergalactic medium (IGM) between us and the source.

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Apr 12, 2006 Astro 100 Lecture 32 5

Blazars and Radio Galaxies� BL Lac

� a variable point-like object, changing its brightness markedly over months. (Initially it was mistaken for a variable star)

� a smooth spectrum, with no absorption or emission lines. It is now the prototype for a class of object known as blazars.

� Blazars- distance and size� many blazars are now seen to have faint, distant elliptical galaxies

around them- they are in the centers of galaxies and far outshine them

� Some blazars have variability time of days. Using the size-variability argument, size < 1 light�day ~ size of solar system

� Radio galaxies are elliptical galaxies with strong radio emission. They are often accompanied by jets and lobes, with motions approaching the velocity of light

Apr 12, 2006 Astro 100 Lecture 32 6

A Unified Model of Quasars and Active Galaxies

� A model trying to explain these phenomena must account for the following properties:� High luminosity released in a very small space� High variability� High velocity motions� Production of jets and lobes

� The current model: energy is released by accretion of matter onto a supermassive black hole at the center of a galaxy. These are all Active Galactic Nuclei ("AGN's")

� Accretion onto compact object is a great way of producing energy, since the efficiency is so high.� hydrogen fusion in the Sun: E(out) = 0.007 Mc2

� In accretion disks: E(out) = 0.25 Mc2

� 25% of the mass is converted to light energy! To generate 100 L(MW): A typical QSO needs to eat 1 solar mass of material per year. So in 1 million years it will swallow 1 million stars.

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Apr 12, 2006 Astro 100 Lecture 32 7

Verifying the AGN Model� Jets and high velocities formed by magnetic field squirting gas

out the poles before it is swallowed (like star formation jets)� Can account for different types of AGN's by

� how much matter is being ingested� accretion disk is surrounded by obscuring disk, type depends on the

viewing angle (eg: Blazars: down the pole)

� How can we check to see if supermassive black holes are really there? High velocity motions in the center of Milky WayAndromeda Galaxy (M31) Sombrero Galaxy (M104). � Kepler's 3rd Law. For M31 there is 10 billion solar masses of material

in a volume 3 light years across.

Apr 12, 2006 Astro 100 Lecture 32 8

Formation

� How would a supermassive black hole form?� Maybe low angular momentum (i.e. slowly spinning)

gas settled down into the center of a galaxy.� Compact star clusters might have collapsed together

and merged.� Once formed, the black hole swallows nearby gas and

stars. � But why do we not see quasars in nearby galaxies?

� Current thought: central black holes formed and fed only when galaxy is first formed

� Now (low redshift) most galaxies have already formed, black holes are not being fed.

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Apr 12, 2006 Astro 100 Lecture 32 9

Quasar 3C273

Lab: λ=656 nmObs: 760 nm

∆λ=104 nm

width 20 nm

Apr 12, 2006 Astro 100 Lecture 32 10

z = 5.0 QuasarLab: λ=122 nm Obs: 730 nm∆λ=608 nm

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Apr 12, 2006 Astro 100 Lecture 32 11

Blazar Variabilitybr

ight

ness

Apr 12, 2006 Astro 100 Lecture 32 12

Radio Galaxies and Jets

M87 Centaurus AFigure 10.20, p319, Arny

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Apr 12, 2006 Astro 100 Lecture 32 13

AGN Black Hole Model

Figure 10.25, p323, Arny