Active Galaxies
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Active Galaxies. Chapter 21. Centaurus A. What are Active Galactic Nuclei (AGN)? Galaxies with a source of non-stellar light coming from the nucleus (excessive UV, IR, radio and X-ray light), sometimes showing jets and variability.

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Active Galaxies

Chapter 21

Centaurus A

  • What are Active Galactic Nuclei (AGN)?

    • Galaxies with a source of non-stellar light coming from the nucleus (excessive UV, IR, radio and X-ray light), sometimes showing jets and variability

Central Supermassive Black Hole accreting material from surrounding host galaxy (Lynden-Bell 1969)

Quasars– luminous (MB<-23) objects with broad emission lines, distant, many are strong radio sources

Seyferts– fainter (MB>-23), most identified locally, spiral hosts

Sey 1s– broad emission lines (e.g. Hydrogen)

Sey 2s– narrow emission lines

Radio Galaxies – excess radio emission and jets

Starbursts– extreme starformation in addition to blackhole accretion in the nucleus

ULIRGS – ultra-luminous infrared galaxies

Quasars (or QSOs)

By 1960, several hundred radio sources were cataloged with no obvious optical counterparts (due to poor positional accuracy).

In the early 1960’s, positional improvements allowed for the detection of optical counterparts for two radio sources: 3C48 and 3C273.

Both sources looked like normal stars – “quasi-stellar objects or QSOs”

They appeared bluer than normal stars with strong, broad emission lines.

Maartin Schmidt was the first to recognize that these lines were normal Hydrogen emission lines redshifted by a large amount and indicating high velocities and great distances (according to Hubble’s Law)

D = 660 Mpc (2.2 billion light years) for 3C273

1340 Mpc (4.4 billion light years) for 3C48.

The large distances implied large luminosities:

L = 20 trillion Lsun (or 1000 Milky Way’s) for 3C273

These point-like sources were the most luminous objects known in the Universe!

Within ~2 years, quasars were discovered that were 10 billion light years away and L  100 trillion Lsun. The most distant quasar known today is 12.7 billion light years away!

  • Quasars are embedded in host galaxies

    • difficult to detect due to the brightness of nuclear quasar emission

    • appear to be a mixture of galaxy types - from disturbed galaxies to normal E’s and early type spirals – brightest tend to be in E’s


  • Some quasar spectra not only show broad emission lines but also broad absorption lines

  • (BAL) Quasars: normal quasars viewed at angle along the line-of-sight of intervening, fast-moving material.

Trump et al. 2006

Lyman Alpha Forest

QSO spectra also reveal some absorption lines not associated with the quasar at all.

These narrow absorption lines are caused by intervening galaxies (halos) between us and the QSO.

Redshifts of the material will be equal to or less than the QSO.

Lyman alpha is the lowest excitation level for Hydrogen gas.

Seyfert galaxies - first identified by Carl Seyfert in 1943.

Defined class based on observational characteristics:

Almost all the luminosity comes from a small (unresolved) region at the center of the galaxy – the galactic nucleus.

Nuclei have MB > -23 (arbitrary dividing line between quasars/seyferts) – corresponds to L ~ 108 to 1012 Lsun

Light from nucleus is variable on timescales of months

NGC 4151

  • Seyfert galaxy spectra fall into two classes

    • Broad emission line spectra (Seyfert type 1; similar to quasars)

    • Narrow emission line spectra (Seyfert type 2)

Seyfert 1s:

Broad and narrow lines


Seyfert 2s:

Only narrow lines


  • Seyfert 2 galaxies (Narrow-Line AGN)

    • can be differentiated from normal emission line galaxies through the flux ratios of certain emission lines.

    • shape of the underlying ionizing source determines how many photons are available to produce particular emission lines.

Kewley et al 2006 – red line shows extreme starburst, dashed line is

classification line

Variability in AGNs

  • QSOs and Seyfert nuclei have long been recognized as variable

  • Optical flux changes occur on timescales of months to years

  • Cause of variability? – instabilities in accretion disk, SN or starbursts, microlensing…..

Quasar light curve ~25 years

Seyfert light curve over ~11 months

Hawkins 2002

Variability occurs at most wavelengths - X-rays through radio

This indicates that the fluctuations are originating from a very tiny object.


Why does rapid variability indicate small physical size of the emitting object?

Consider an object like the Sun. Any instantaneous flash would appear “blurred” in time by  t = RSun / c.



Time Delay =  t = RSun / c

700,000 km / 300,000 km/s = 2.3 sec

Seyfert continuum luminosity varies significantly in less than a year (some variation occurs on timescales of days or weeks).

This implies an emitting source less than a few light-weeks across!

BL Lac objects – extremely variable AGN

  • Very bright radio sources

  • Variability faster and higher amplitude than normal quasars and Seyferts

  • Look like QSOs with extremely bright nucleus and faint fuzz “host” galaxy surrounding

  • Rapid and strong variability will “flood” emission lines so that they can’t be detected

  • Variability light curve indicates emitting region less than one light-day across (or ~200 AU)

Light Curve


Radio Galaxies

  • Emit most light at radio wavelengths (~107 to 1011 Lsun)

  • Radio emission is highly polarized synchrotron radiation

  • Morphology can be extended or compact

  • Host galaxies are generally Ellipticals

  • Radio morphology first classified by Fanaroff & Riley (1974)

    • FR I: less luminous, 2-sided jets brightest closest to core and dominate over radio lobes  compact

    • FR II: more luminous, edge-brightened radio lobes dominate over 1-sided jet (due to Doppler effect “brightening” of approaching jet over receding jet)  extended

FR I - 3C 47

FR II - 3C 449

Radio “Light”

Centaurus A

0.8 Mpc

Visible Light

FR II radio galaxies: most emission comes from lobes

The radio lobes span about 10 degrees on the sky!

Lobes consist of material ejected from the nucleus.

~1 Mpc

Radio image of the FR II radio galaxy Cygnus A.

The lobes occur where the jets plow into intracluster gas.

FR I radio galaxy: most of the energy comes from a small nucleus with a halo of weaker emission around the nucleus and jets close to nucleus.

Visible image of the FR I radio galaxy M87.

This giant elliptical (E1) galaxy is ~100 Kpc across.

Radio jet is visible in radio, optical and X-ray light

~2 kpc

galaxy nucleus, i.e. the radio core

Close-up view of the jet in M87 at radio wavelengths.

The jet is apparently a series of distinct “blobs”, ejected by the galaxy nucleus, and moving at up to half the speed of light.

Superluminal Expansion

Some radio “blobs” in jets appear to move faster than the speed of light – superluminal

For object moving distance r below:

x = r cos , y = r sin  and t = r/v

Light from P takes x/c less time to reach us than light emitted from O.

Time the observer sees for object to travel from O to P is

tapp = t - x/c

tapp = (r/v) - (r/c) cos 

tapp = (r/v) (1 -  cos )

Apparent velocity across sky

vapp = y/tapp

vapp = (v sin )/(1 -  cos )

object moving at v

radio jet in 3C273

Superluminal Expansion

vapp = (v sin )/(1 -  cos )

For v << c,  is close to 0 and vapp = v sin

For v close to c, vapp can be much more than v and even greater than c

To see this, find the angle that gives the maximum vapp for a given v by dividing this equation by v, differentiate wrt theta and set equal to zero

Substitute this into the above equation gives

(vapp/v)max = 1/(1- 2)1/2

or tmax = /(1- 2)1/2(21.35)

where t is the tangential or apparent velocity

What is the maximum apparent velocity that could be observed for a “blob” moving at 0.95c? What is the angle required to get this velocity?

  • ULIRGs (ultra luminous IR galaxy) - Starburst or AGN?

  • A starburst galaxy:

  • May result from a galaxy collision/merger

  • Gas streams converge from different directions causing shocks which compress material and trigger star formation

  • Gas which loses enough angular momentum falls into the galaxy center  bar formation  funnels more gas inward  violent star formation near center of disk and further out

  • ULIRGS are:

  • Galaxies that emit most of their light in the IR - LIR > 1012 Lsun

  • more common beyond z > 1

  • nearly all are undergoing mergers

  • IR light is a combination of dust reprocessed AGN emission and star formation

  • Some AGN may appear as ULIRGs during different stages of galaxy evolution.

Nuclear close-up (HST) of NGC 1808 starburst galaxy. Galaxy has barred-spiral morphology.

Structure of AGNs and Unified Theory

Central Black Hole

Rs = 3x1011 m or 2 AU for 108 Msun

Accretion disk

UV/visible light from region 1012 - 1013m (X-rays probably come from a corona that surrounds the disk)

Jets – ionized gas from accretion disk spirals along magnetic field lines away from the disk

  • Broad Line Region

  • from reverberation mapping

  • size is few light-days to months across

  • gas densities > 109 /cm3

  • stratified (higher-ionization lines from clouds closer to nucleus)

  • outer edge defined by dust sublimation radius (1500K)

  • Obscuring Torus

  • Dusty structure that blocks view of inner region

  • Narrow Line Region

  • 1017-1019 m (10s to 100s of pc)

  • lower gas density ~103/cm3

  • part of the host galaxy ISM near galaxy center


Seyfert 1

Structure of AGNs and Unified Theory

Seyfert 2

Observer is looking at blackhole “edge-on” through the surrounding dusty torus - does not see broad emission lines produced by gas near BH

Quasar/Seyfert 1

Observer is looking into the center of the accretion disk, viewing motions of gas near blackhole - sees broad emission lines

/BL Lac

Observer sees mostly synchrotron emission from jet - highly variable with hardly any emission lines/absorption lines

What Powers Active Galactic Nuclei??

  • A compact central source provides a very intense gravitational field. For active galaxies, the black hole has MBH = 106 - 109 Msun

  • Infalling gas forms an accretion disk around the black hole.

  • (3) As the gas spirals inward, friction heats it to extremely high temperatures; emission from the accretion disk at different radii (T>104 K) accounts for optical thru soft X-ray continuum.

  • (4) Some of the gas is driven out into “jets,” focused by magnetic fields.

How efficient is the energy production?

Before entering the black hole, some fraction of the mass is converted into energy. Matter is heated to high temps by dissipation in the accretion disk and radiates away its gravitational potential energy.

BH radius is Rs=2GMbh/c2 = 0.25 M8 light hours. What energy is available (via gravitational potential energy) for a mass m falling from far away to the Rs?

Emax = GMbhm/Rs

Substituting equation for Rs

Emax = (1/2)mc2

Half the rest energy of the infalling mass is converted to kinetic energy. If the mass is decelerated (via the accretion disk), KE can be converted to thermal energy and then photon energy.

Ephot = ηmc2 where η is efficiency and should be ≤ 0.5

The efficiency of this conversion is typically ~10% (η=0.1) for AGN.

AGN luminosity is L = η(dm/dt)c2. If we know AGN’s L, we can deduce infall rate (e.g. MW “AGN” luminosity is just 1000 Lsun implying dm/dt of 10-9 Msun per year)

Calculate L for dm/dt of one solar mass per year with 10% efficiency

So...can you just shovel in mass at higher and higher rates to create a superluminous AGN?

The Eddington Limit – the maximum L for a given black hole mass (any higher and radiation pressure will blow away surrounding gas!)

Force caused by outward flow of photons balances gravitational force of infalling matter (assuming pure ionized H gas)

σeL / (4πr2c) = GMbhmp / r2

LE = 4πcGMbhmp/σe = 3.3x1012L(Mbh/108M)

If L is greater than LE, ionized gas will be accelerated outward and accretion will cease. This leads to a maximum accretion rate for black holes:

ME = LE / ηc2 = 2M/yr (Mbh/108M) (0.1/η)

The Eddington ratio is then m = M / ME = Lbol/LE where m = dm/dt






  • Black Hole Mass - Bulge Mass Relation

  • Evidence for SMBHs has been found in most galaxies with a significant spheroidal component. It is generally believed that all galaxies contain SMBHs.

  • A clear relationship between the mass of the spheroidal component of a host galaxy and the black hole mass has been found.

  • indicates connection between galaxy formation (star formation) and growth/evolution of central black hole

(Gebhardt et al. 2000; Ferrarese & Merritt 2000)

Active Galaxies as part of Galaxy Evolution

Quasars are relatively short-lived phenomena – only need 20 Myr to grow largest BHs known

Number density of quasars peaked when universe was about 3 Gyr old

Where do those BH’s go? Become dormant – go into “hibernation”

  • As small galaxies merge to form larger ones,black holesmay form at the nucleus (ULIRG stage?)

  • With plenty of fuel available early on, the galaxy light is dominated by emission of the black hole (Quasar). Feedback from AGN radiation may squelch star formation in the host galaxy – setting spheriodal component size

  • Additional mergers and depletion of fuel may result in radio galaxies and Seyfert galaxies.

  • Further fuel depletion results in a normal galaxy with a dormant black hole at the nucleus.

Active Galaxies as part of Galaxy Evolution

AGN phase may be the result of galaxy mergers

Hopkins et al. 2008

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