Active Galactic NucleiActive Galactic Nuclei

Questions to be addressed:

(1)What are active galactic nuclei (AGN)?

(2) What are the main properties of AGN?

(3) What is the source of power for AGN?

Observationally,What are Observationally,What are AGNs?AGNs?

Objects, Objects, sometimessometimes looking like galaxies looking like galaxies, , other times other times apparently stellarapparently stellar, which show , which show extreme amount of radiationextreme amount of radiation, and , and sometimes powerful jets of material, from sometimes powerful jets of material, from deep in their centers. deep in their centers.

Radiation very different from that of starsRadiation very different from that of stars Brightness can change significantly in Brightness can change significantly in

several months, so the size must be very several months, so the size must be very small, only a few light months acrosssmall, only a few light months across

(Milky Way: 100,000 Ly across)(Milky Way: 100,000 Ly across)

The bottom line about AGN. The bottom line about AGN. II

All the various AGN types are manifestation of All the various AGN types are manifestation of the SAME physical phenomenon: the SAME physical phenomenon: accretion of accretion of matter onto the central super-massive Black Hole matter onto the central super-massive Black Hole (~billion solar mass)(~billion solar mass)

When there is accretion: we have an AGNWhen there is accretion: we have an AGN When there is NOT accretion: AGN is dormant, When there is NOT accretion: AGN is dormant,

and galaxy looks normaland galaxy looks normal AGN are transitory: short duty cycleAGN are transitory: short duty cycle All galaxies are believed to be AGN at some point All galaxies are believed to be AGN at some point

during their evolutionduring their evolution Interaction/Merging can trigger accretion onto Interaction/Merging can trigger accretion onto

the SMBH and “feed the monster”: the result is the SMBH and “feed the monster”: the result is an AGNan AGN

The bottom line about AGN. The bottom line about AGN. IIII

All the various AGN types…All the various AGN types… LinersLiners Seyfert I and Seyfert IISeyfert I and Seyfert II Radio GalaxiesRadio Galaxies Quasars (QSOs)Quasars (QSOs)

… … are all due to combinations of only two very simple are all due to combinations of only two very simple phenomena:phenomena:

1.1. Amount of accretion onto the central SMBH:Amount of accretion onto the central SMBH: LUMINOSITYLUMINOSITY

2.2. Orientation angle of the galaxy/AGN respect to the observer:Orientation angle of the galaxy/AGN respect to the observer: AGN typeAGN type

The number of observed AGN depends on two factorsThe number of observed AGN depends on two factors The number of galaxies (active and dormant)The number of galaxies (active and dormant) The fraction of galaxies that are active (monster is being fed) The fraction of galaxies that are active (monster is being fed)

at the time of the observationsat the time of the observations

All AGN have:

1. SM Black Hole2. Accretion disk3. Obscuring Torus4. Jets5. Narrow-line region6. Broad-line region

Orientation angle iskey variable thatdetermines AGN type

Small size of the BH isthe reason of the Variability

Conversion of large amount of mass intoenergy is the reason ofthe extreme luminosity

Active Galactic NucleiActive Galactic Nuclei• Seyfert GalaxiesSeyfert Galaxies

• spiral galaxies spiral galaxies with an incredibly with an incredibly bright, star-like bright, star-like center (nucleus)center (nucleus)

• they are very they are very bright in the bright in the infraredinfrared

The luminosity can vary by as much as the entire brightness of the Milky Way Galaxy!!

Circinus

Active Galactic NucleiActive Galactic Nuclei• Seyfert GalaxiesSeyfert Galaxies

• spiral galaxies with an spiral galaxies with an incredibly bright, star-incredibly bright, star-like center (nucleus)like center (nucleus)

• they are very bright in they are very bright in the infraredthe infrared

• their spectra show their spectra show strong strong emissionemission lines lines

The luminosity can vary by as much as the entire brightness of the Milky Way Galaxy!!

Circinus

Active Galactic NucleiActive Galactic NucleiRadio GalaxiesRadio Galaxies

• galaxies which emit large amounts of radio galaxies which emit large amounts of radio waveswaves

• the radio emission come from the radio emission come from lobeslobes on either on either side of the galaxy; side of the galaxy; notnot the galaxy itself the galaxy itself

Cygnus A

Radio galaxies emit strongly in radio band, and show jet like structures. Often they are hosted by elliptical galaxies

Radio Galaxy LobesRadio Galaxy Lobes

NGC 1265

These lobes are swept back because the galaxy is moving through an intergalactic medium.

X-ray/Radio Image of X-ray/Radio Image of Centaurus ACentaurus A

X-ray is blue; radio is red

QuasarsQuasars• In the early 1960s, Maarten Schmidt identified In the early 1960s, Maarten Schmidt identified

the radio source 3C 273 with a faint, blue star.the radio source 3C 273 with a faint, blue star.• the “star’s” spectrum displayed emission lines the “star’s” spectrum displayed emission lines • the wavelengths of these lines matched no know the wavelengths of these lines matched no know

element element • Schmidt realized that the emission lines Schmidt realized that the emission lines

belonged to Hydrogen, but they were highly belonged to Hydrogen, but they were highly redshifted.redshifted.

• This object is very (> 10This object is very (> 101010 light years) far away. light years) far away.• other such objects were subsequently discoveredother such objects were subsequently discovered• they were called they were called quasi-stellar radio sourcesquasi-stellar radio sources or or

quasarsquasars for short for short• The farther away we look out in distance, the The farther away we look out in distance, the

farther back we look in farther back we look in timetime!!

Quasar SpectraQuasar Spectra Star-like objects which:Star-like objects which:

• have spectra that look have spectra that look nothing like a starnothing like a star

• highly redshiftedhighly redshifted• can be strong radio sourcescan be strong radio sources

• turns out that 90% are turns out that 90% are notnot• emit light at emit light at all wavelengthsall wavelengths

Quasars…Quasars…• are extremely luminous. are extremely luminous.

• 10104040 watts watts • 1,000 brighter than the entire Milky Way 1,000 brighter than the entire Milky Way

GalaxyGalaxy• are extremely variable.are extremely variable.

• luminosity changes < 1 hourluminosity changes < 1 hour• implies they have implies they have a very small sizea very small size

• have redshifted emission lines.have redshifted emission lines.• greatest is 6.8 times the rest wavelengthgreatest is 6.8 times the rest wavelength

• have absorption lines at lower redshifts.have absorption lines at lower redshifts.• from gas clouds & galaxies between us and from gas clouds & galaxies between us and

the quasarthe quasar

Hubble ST shows us that Hubble ST shows us that quasars do live in galaxies…quasars do live in galaxies…

they are Active Galactic they are Active Galactic Nuclei!Nuclei!

In bright QSOs, the nuclei are so bright that the host galaxies are difficult, or impossible, to observe

What powers these Active What powers these Active Galactic Nuclei?Galactic Nuclei?

Hubble Space Telescope gave us a clue

NGC 4261

Source of power of AGNSource of power of AGN Jets of matter are shooting out from these galaxies and Jets of matter are shooting out from these galaxies and

emitting radio waves, but the matter is emitting radio waves, but the matter is notnot cold! cold! Synchotron emission --- non-thermal process where Synchotron emission --- non-thermal process where

light is emitted by charged particles moving close to the light is emitted by charged particles moving close to the speed of speed of

light around magnetic fields.light around magnetic fields.

M 87

Gas clouds near the center moving at a speed close to c

Active Galactic NucleiActive Galactic Nuclei• The energy is generated from matter falling The energy is generated from matter falling

onto a onto a supermassive black holesupermassive black hole……• 1.2 x 101.2 x 1099 M M for NGC 4261 for NGC 4261• 3 x 103 x 1099 M M for M87 for M87

• ……which is at the center (nucleus) of the which is at the center (nucleus) of the galaxy.galaxy.

Matter swirls through an Matter swirls through an accretion disk before accretion disk before crossing over the event crossing over the event horizon.horizon.

Gravitational pot. energy lostGravitational pot. energy lost• = mc= mc22 the mass energy the mass energy• 10 – 40% of this is radiated 10 – 40% of this is radiated

awayaway Process is very efficient for Process is very efficient for

generating energy.generating energy.

Implied speed of motion: 800 km/s; there must be a super-massive black hole near the center

AGNs emit in all AGNs emit in all wavebandswavebands

Stars emit mostly in optical, near infrared and ultraviolet.

So AGNs are not starsBecause they are bright, can be observed at very large distances

Active Galactic NucleiActive Galactic Nuclei Formation of the JetsFormation of the Jets

• magnetic fields in accretion disks are twistedmagnetic fields in accretion disks are twisted• they pull charged particles out of the disk and they pull charged particles out of the disk and

accelerate them like a slingshotaccelerate them like a slingshot• particles bound to magnetic field; focused in particles bound to magnetic field; focused in

a beam a beam

Orientation of beam determines Orientation of beam determines what we see:what we see:• if beams points at us, we see a quasarif beams points at us, we see a quasar• if not, the molecular clouds/dust of the if not, the molecular clouds/dust of the

galaxy block our view of the nucleusgalaxy block our view of the nucleus• so we see a radio galaxyso we see a radio galaxy• lobes are where jets impact lobes are where jets impact

intergalactic mediumintergalactic medium

Current idea about the structure of an AGN

Central engine is powered by super-massive black hole: with a mass 100 million Msun

AGN AnimationAGN Animation

Quasars are observed in the distant past Quasars are observed in the distant past (high redshift).(high redshift).• this implies that many galaxies had bright this implies that many galaxies had bright

nuclei early in their histories, but those nuclei nuclei early in their histories, but those nuclei have since gone dormanthave since gone dormant

So many galaxies which look “normal” So many galaxies which look “normal” today have supermassive black holes at today have supermassive black holes at their centers.their centers.• such as such as AndromedaAndromeda and Milky Way? and Milky Way?

Movie. Click to launch.

Survey QuestionsSurvey Questions

(1)What are active galactic nuclei (AGN)?

(2)What are the main properties of AGN?

(3) What is the source of power for AGN?

What have we learned?What have we learned?• What two starting assumptions do What two starting assumptions do

we make in most models of galaxy we make in most models of galaxy formation?formation?• (1) Hydrogen and helium gas filled (1) Hydrogen and helium gas filled

all of space when the universe was all of space when the universe was young. (2) The distribution of young. (2) The distribution of matter in the universe was nearly matter in the universe was nearly but not quite uniform, so that but not quite uniform, so that some regions of the universe were some regions of the universe were slightly denser than others.slightly denser than others.

What have we learned?What have we learned?• Describe in general terms how galaxies are Describe in general terms how galaxies are

thought to have formed.thought to have formed. • Gravity slowed the expansion of matter in Gravity slowed the expansion of matter in

regions of the universe where the density was regions of the universe where the density was slightly greater than average. Within about a slightly greater than average. Within about a billion years after the birth of the universe, billion years after the birth of the universe, gravity had stopped the expansion of these gravity had stopped the expansion of these regions and had begun to pull matter together regions and had begun to pull matter together into protogalactic clouds. Halo stars began to into protogalactic clouds. Halo stars began to form as the protogalactic cloud collapsed into form as the protogalactic cloud collapsed into a young galaxy. In galaxies that had enough a young galaxy. In galaxies that had enough remaining gas after this initial star formation, remaining gas after this initial star formation, conservation of angular momentum ensured conservation of angular momentum ensured that the gas flattened into a spinning disk.that the gas flattened into a spinning disk.

What have we learned?What have we learned?• What does careful study of our Milky Way Galaxy What does careful study of our Milky Way Galaxy

tell us about galaxy formation?tell us about galaxy formation?• The Milky Way’s halo stars are very old and their The Milky Way’s halo stars are very old and their

orbits have random orientations, suggesting orbits have random orientations, suggesting that they did indeed form before the that they did indeed form before the protogalactic cloud collapsed into a disk. The protogalactic cloud collapsed into a disk. The low abundances of heavy elements in halo stars low abundances of heavy elements in halo stars tell us they were born before the star-gas-star tell us they were born before the star-gas-star cycle significantly enriched the interstellar cycle significantly enriched the interstellar medium with heavy elements. However, the medium with heavy elements. However, the relationship between heavy element abundance relationship between heavy element abundance and distance from the galactic center suggests and distance from the galactic center suggests that our Milky Way formed not from a single that our Milky Way formed not from a single protogalactic cloud but rather from the merger protogalactic cloud but rather from the merger of several smaller protogalactic clouds.of several smaller protogalactic clouds.

What have we learned?What have we learned?• How might a galaxy’s birth properties have How might a galaxy’s birth properties have

determined whether it ended up spiral or elliptical?determined whether it ended up spiral or elliptical? • There are two basic ways in which birth conditions could There are two basic ways in which birth conditions could

have determined whether a galaxy ended up as a spiral have determined whether a galaxy ended up as a spiral galaxy with a gaseous disk or as an elliptical galaxy galaxy with a gaseous disk or as an elliptical galaxy without a disk. (1) Angular momentum tends to shape a without a disk. (1) Angular momentum tends to shape a collapsing gas cloud into a spinning disk. Thus, ellipticals collapsing gas cloud into a spinning disk. Thus, ellipticals may have formed from protogalactic clouds with may have formed from protogalactic clouds with relatively small amounts of angular momentum, while relatively small amounts of angular momentum, while the clouds that formed spirals had greater angular the clouds that formed spirals had greater angular momentum. (2) Dense clouds tend to cool and form momentum. (2) Dense clouds tend to cool and form stars more rapidly. Thus, ellipticals may have formed stars more rapidly. Thus, ellipticals may have formed from protogalactic clouds that started out with greater from protogalactic clouds that started out with greater density, leading to a high rate of halo star formation that density, leading to a high rate of halo star formation that left little or no gas to collapse into a disk. Spirals may left little or no gas to collapse into a disk. Spirals may have started form lower-density protogalactic clouds in have started form lower-density protogalactic clouds in which a lower rate of halo star formation left enough gas which a lower rate of halo star formation left enough gas to form a disk.to form a disk.

What have we learned?What have we learned?• How might interactions between galaxies How might interactions between galaxies

cause spiral galaxies to become elliptical?cause spiral galaxies to become elliptical? • Computer models show that two Computer models show that two

colliding spiral galaxies can merge to colliding spiral galaxies can merge to form a single elliptical galaxy. The form a single elliptical galaxy. The collision randomizes the orbits of the collision randomizes the orbits of the stars, while their combined gas sinks to stars, while their combined gas sinks to the center and is quickly used up in a the center and is quickly used up in a burst of rapid star formation. Spirals burst of rapid star formation. Spirals may also turn into ellipticals when their may also turn into ellipticals when their gas disks are stripped out by gas disks are stripped out by interactions with other galaxies.interactions with other galaxies.

What have we learned?What have we learned?• What do observations of galaxy clusters tell us What do observations of galaxy clusters tell us

about the role of galaxy interactions?about the role of galaxy interactions?• Observations of clusters of galaxies support Observations of clusters of galaxies support

the idea that at least some galaxies are shaped the idea that at least some galaxies are shaped by collisions. Elliptical galaxies are more by collisions. Elliptical galaxies are more common in the centers of clusters — where common in the centers of clusters — where collisions also are more common — suggesting collisions also are more common — suggesting that they may have formed from collisions of that they may have formed from collisions of spiral galaxies. The central dominant galaxies spiral galaxies. The central dominant galaxies found in cluster centers also appear to be the found in cluster centers also appear to be the result of collisions, both because of their large result of collisions, both because of their large size and the fact that they sometimes contain size and the fact that they sometimes contain multiple clumps of stars that probably were multiple clumps of stars that probably were once the centers of individual galaxies.once the centers of individual galaxies.

What have we learned?What have we learned?• What is a starburst galaxy?What is a starburst galaxy?

• A starburst galaxy is a galaxy that is A starburst galaxy is a galaxy that is forming new stars at a very high rate — forming new stars at a very high rate — sometimes more than 100 times the star sometimes more than 100 times the star formation rate of the Milky Way. This high formation rate of the Milky Way. This high rate of star formation leads to supernova-rate of star formation leads to supernova-driven galactic winds.driven galactic winds.

• How do we know that a starburst must be How do we know that a starburst must be only a temporary phase in a galaxy’s life?only a temporary phase in a galaxy’s life? • The rate of star formation is so high that The rate of star formation is so high that

the galaxy would use up all its interstellar the galaxy would use up all its interstellar gas in just a few hundred million years – far gas in just a few hundred million years – far shorter than the age of the universe.shorter than the age of the universe.

What have we learned?What have we learned?• What can cause starbursts?What can cause starbursts?

• Many starbursts apparently result from collisions between Many starbursts apparently result from collisions between galaxies. The collision compresses the gas and leads to the galaxies. The collision compresses the gas and leads to the high rate of star formation. Some starbursts may occur as a high rate of star formation. Some starbursts may occur as a result of close encounters with other galaxies rather than result of close encounters with other galaxies rather than direct collisions. The starburst underway in the nearby direct collisions. The starburst underway in the nearby Large Magellanic Cloud may have resulted from the tidal Large Magellanic Cloud may have resulted from the tidal influence of the Milky Way.influence of the Milky Way.

• What are active galactic nuclei and quasars?What are active galactic nuclei and quasars? • Active galactic nuclei are the unusually bright centers Active galactic nuclei are the unusually bright centers

found in some galaxies. The brightest active galactic nuclei found in some galaxies. The brightest active galactic nuclei are called quasars. Active galactic nuclei (including are called quasars. Active galactic nuclei (including quasars) generally radiate energy across much of the quasars) generally radiate energy across much of the electromagnetic spectrum. In some cases, we see electromagnetic spectrum. In some cases, we see spectacular jets of material shooting out of these objects, spectacular jets of material shooting out of these objects, sometimes forming huge lobes (revealed by radio sometimes forming huge lobes (revealed by radio observations) at great distances from the center of the observations) at great distances from the center of the galaxy.galaxy.

What have we learned?What have we learned?• The nature of quasars was once hotly debated. The nature of quasars was once hotly debated.

What evidence supports the idea that they are What evidence supports the idea that they are the active galactic nuclei of distant galaxies?the active galactic nuclei of distant galaxies? • The debate centered on the question of whether The debate centered on the question of whether

quasar redshifts really indicated the great distances quasar redshifts really indicated the great distances that we calculate for them with Hubble’s law. The that we calculate for them with Hubble’s law. The key evidence showing that these distances are key evidence showing that these distances are correct comes from the fact that we see quasars correct comes from the fact that we see quasars located in the centers of galaxies in distant clusters located in the centers of galaxies in distant clusters — and the redshifts of the quasars, the surrounding — and the redshifts of the quasars, the surrounding galactic material, and the neighboring galaxies in galactic material, and the neighboring galaxies in the clusters all match. In addition, the fact that the clusters all match. In addition, the fact that quasars are quite similar to other active galactic quasars are quite similar to other active galactic nuclei supports the idea that they are simply nuclei supports the idea that they are simply unusually bright members of this class of object.unusually bright members of this class of object.

What have we learned?What have we learned?• What do we think is the source of power for What do we think is the source of power for

active galactic nuclei?active galactic nuclei? • We suspect that active galactic nuclei are powered We suspect that active galactic nuclei are powered

by supermassive black holes that can exceed one by supermassive black holes that can exceed one billion solar masses. Observations of the rapid billion solar masses. Observations of the rapid variability of active galactic nuclei tells us that their variability of active galactic nuclei tells us that their energy output comes from quite a small region, energy output comes from quite a small region, while Doppler shifts of orbiting gas clouds tell us that while Doppler shifts of orbiting gas clouds tell us that the central region contains an enormous amount of the central region contains an enormous amount of mass. The only known way that so much mass could mass. The only known way that so much mass could be concentrated in such a small region is if it be concentrated in such a small region is if it contains a black hole. As matter falls into one of contains a black hole. As matter falls into one of these supermassive black holes, it releases these supermassive black holes, it releases tremendous amounts of energy. This is the only tremendous amounts of energy. This is the only mechanism we know of that can account for the mechanism we know of that can account for the prodigious energy output of active galactic nuclei.prodigious energy output of active galactic nuclei.

What have we learned?What have we learned?• Do quasars still exist?Do quasars still exist?

• Most quasars are found at very large distances, meaning Most quasars are found at very large distances, meaning that we are seeing them at a time when the universe was that we are seeing them at a time when the universe was much younger than it is today. Very few quasars are much younger than it is today. Very few quasars are found nearby, although we do find some nearby active found nearby, although we do find some nearby active galactic nuclei that are less bright than quasars. These galactic nuclei that are less bright than quasars. These observations suggest that quasars are essentially a observations suggest that quasars are essentially a phenomenon of the past, and that active galactic nuclei phenomenon of the past, and that active galactic nuclei may in most cases occur as part of the galaxy formation may in most cases occur as part of the galaxy formation process.process.

• Where do active galactic nuclei fit into the story of Where do active galactic nuclei fit into the story of galaxy evolution?galaxy evolution? • Because quasars were much more common in the past, it Because quasars were much more common in the past, it

is likely that many galaxies once had very bright nuclei is likely that many galaxies once had very bright nuclei that have now gone dormant. If so, then many galaxies that have now gone dormant. If so, then many galaxies that now look quite normal have supermassive black that now look quite normal have supermassive black holes at their centers.holes at their centers.

What have we learned?What have we learned?• How do quasars let us study gas between the How do quasars let us study gas between the

galaxies?galaxies? • Quasars are bright enough to be easily detected Quasars are bright enough to be easily detected

at distances most of the way to the at distances most of the way to the cosmological horizon. Each cloud of gas cosmological horizon. Each cloud of gas through which the quasar’s light passes on its through which the quasar’s light passes on its long journey to Earth leaves a fingerprint in the long journey to Earth leaves a fingerprint in the quasar’s spectrum. We can distinguish the quasar’s spectrum. We can distinguish the different clouds of gas because each one different clouds of gas because each one produces hydrogen absorption lines with a produces hydrogen absorption lines with a different redshift in the quasar spectrum. Study different redshift in the quasar spectrum. Study of these absorption lines in quasar spectra of these absorption lines in quasar spectra allows us to study gas — including protogalactic allows us to study gas — including protogalactic clouds — that we cannot otherwise detect.clouds — that we cannot otherwise detect.