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Our Our Place Place in in the the Cosmos Cosmos

Lecture 15 Quasars and Active Galactic Nuclei

Quasars

• Galaxies shine with the luminosity of hundreds
• f 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

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

HST Quasar Images 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

• utshine the light from all of the stars
• These are known as active galactic nuclei

(AGN)

• Quasars are the most luminous type of AGN

Seyfert and Radio Galaxies

• The first AGN were discovered by Carl Seyfert in

1943

• These were all spiral galaxies with a bright nucleus
• f 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

Blue = visible starlight Red = radio emission

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

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

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

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.

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

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

• 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

Unified Model of AGN Edge-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)

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 accretion disk Radio jets hundreds of thousands of light-years in size originate in a central engine no larger than our Solar System

Doppler Broadening

Motion of emitting gas broadens observed lines in the spectrum

Unified Model of AGN 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

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

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 holes that powered these AGN should still be around today

• We expect most normal galaxies to contain a

supermassive black hole

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

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

The Antennae - a merging pair of galaxies (HST)

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!