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ASTR 200 : Lecture 28

Supermassive black holes and AGNs

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Strange Phenemona

• As distant galaxies were being studied in detail, starting in roughly the 1940s, several strange phenomena were observed. We now understand that these are all linked to things going on in `active galactic nuclei' (AGNs)

–Seyfert Galaxies

–Radio Active Galaxies

–Quasars

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Seyfert Galaxies

• Around 1940, Carl Seyfert noticed that some spiral galaxies have very broad emission lines in their spectra, compared to normal spiral galaxies

• We now call such objects Seyfert galaxies

• Why are the lines so broad?

Normal galaxy                Seyfert Galaxy

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Doppler(or Thermal)Broadening

• If gas particles are moving in many directions at great speeds, some will be blue shifted and some will be red shifted

• This makes the emission line very broad

• The derived velocities are MUCH larger than the ~300 km/s one finds in galaxy rotation curves

• Seyfert doppler widths can be ~10,000 km/s (!)

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A clue : Bright nucleii

• In optical images, Seyfert galaxies look much like normal spiral galaxies with one exception

• They often have a very bright unresolved nucleus, far brighter than the nucleus of a normal galaxy

The galaxy Markarian 500

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A second phonomenon: Radio active galaxies

• Astronomical objects can sometimes be detected in radio waves.

• Perhaps surprisingly, some of the sky's strongest radio sources are related to distant galaxies

• Often the radio emission does NOT come from the galaxy, even though there is an obvious connection with the central galaxy (linear jets, in this case)

• The jets clearly transport a large amount of energy out from the galaxy and it is deposited into interactions with material beyond the galaxy (producing radio emissions)

Two emission regions on either side of the radio galaxy Cygnus A

~ 50 kpc(~1 arcmin)

map of radiointensity

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Sometimes jets can be also be seen at optical wavelengths

●  <<< at left; an amateur astronomer's image of the cD giant elliptical M87 (in the Virgo cluster)●  A linear jet emerging from the galaxy 

~1 kpc

●  At right  >>>>>>>>> Much higher resolution views from HST (optical) and a radio telescope

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A THIRD phonomenon: Quasi-stellar objects

• By the late 1950s, radio telescopes were detecting many extragalactic sources, which were consistent with unresolved points in the radio (beam is large on the sky).

• Many had no obvious optical source association with them! (in fact, couldn't be sure which of many point sources like stars might be related)

• In 1962, the moon passed in front one of these and helped localize it.

• An optical image yielded a very faint source (with even fainter tiny jet), but the main source looked like a point (and thus nearly stellar), hence the term `quasi-stellar object' (QSO), which became quasar.

• Spectra revealed very broad emission lines, but at strange wavelengths so it was unclear which elements they were

Optical image of 3C273

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Quasars at great distance?• Astronomers eventually realized that

they were looking at lines of hydrogen, but with redshifts of about 0.15

• This implied the source was moving away at 15% of the speed of light!

• Seemed impossible that stars could be moving 45,000 km/second... and ALL the quasars were moving AWAY, none were blue shifted

• The resolution is that the objects are NOT stars, and are at cosmological distances and the motion is due to the expansion of the universe

• 3C273 is about 700 Mpc away, but is is as bright as a typical star viewed in a moderate telescope (13th magnitude)

• It's luminosity is thus 4 trillion solar luminosities (a few hundred times the total luminosity of the Milky Way).

HST Optical image of 3C273

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Quasars at great distance!

• There was a great deal of debate for many years about the nature of quasars

• Some astronomers argued there was some strange process producing the high red shifts.

• The question is now settled, as very deep imaging with the central source blocked out (like is done to image exoplanets around stars) reveals the very faint galaxies (due to distance) that surround the incredibly bright central quasar, which is at the nucleus of the galaxy

HST Optical image of 3C273

Deep image with central source blocked out by coronograph

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So what the heck are these various objects?

• They appear to be related to the very center (nucleus) of the galaxies

• Can we get any clues by looking at the center of OUR galaxy?

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So let's try and study the nucleus of our galaxy• It turns out the center of our galaxy is off in the direction of the

constellation Sagittarius, but as we learned earlier, it is incredibly obscured by dust. In optical light no photons from the center reach us.

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Hard to study the center of our galaxy• Below, an optical all-sky image in galactic coordinates. (central plane is

that of the galaxy and center of the image is at the galactic center).

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But in the infrared, dust absorbs much less light• Below, an IR image in galactic coordinates towards galactic center

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Zooming in on the galactic

center

• Using IR light to be able to peer past the dust

• At the very center of the Milky way there is a tight star cluster

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Stars near the galactic centre

• There are a lot of stars here....

• There about 107 stars per cubic pc at the galactic centre (compare to ~0.1 in the solar neighbourhood)

• Is there a way to actually define where the `center' is?

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Where is the Center of our Galaxy?

• In fact there IS a good way to anchor this question, because there is a very bright radio source that has been known for decades

• It is called Sagittarius A* = Sgr A*

• It is bright and very compact– Using radio interferometry the scale of

the source is ~0.3 au = 44 million km

• Via proper motion studies, this source is right at the distance of the galactic center and also centered in the dense star cluster

• ~1000 solar luminosities, not so huge...

• Many astronomers believed the radio emission was coming from hot gas in orbit around a supermassive black hole

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Motions

• Amazingly, now it has become possible to watch (in the infrared) stars ORBIT in the very center of this cluster very close to Sgr A*

• The periods vary, but some stars go around in 15-50 years and can be watched...! (check youtube with the search : stars orbit galactic center)

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Elliptical orbits!• These stars follow beautifully

elliptical orbits around a point which is centered precisely on Sgr A*

• Semimajor axes are a few thousand au

• The closed orbits means that the central mass must dominate the dynamics

• Applying our friend

to each of these stars always yields the same answer of

M S+M SgrA *=aau3

/P yr2

M SgrA *≈4×106 M ⊙

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S.M. Black holes inhabit the nuclei of other galaxies

• The evidence for SM black holes in the nuclei of other galaxies is provided by high spatial resolution spectroscopy which reveal the rotational dynamics

• At right, the very central core of M31

• The velocity dispersion shows that there are many objects within the central 0.5” moving at ~300 km/s

• A rotation curve also shows ordered fast rotation, with

MM 31bh≈4×107 M ⊙

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`Super massive' Black holes• This is MUCH larger than the 5-100 solar mass black holes we think

may form after supernovae explosions of massive stars

• How could such a black hole form?

• Very likely started as a stellar-mass black hole and then grew via accretion of other material (stars, gas, and other black holes!)

• Some objects just pass directly into the event horizon and disappear

• Others (especially clouds of gas) will be broken up by tidal gravitational forces and because the gas is then dissipational, will settle into an accretion disk around the supermassive black hole

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`Super massive' Black holes• In this model, nuclei of galactic bulges host super-massive BHs

– Chicken or egg argument...unclear which came first

• The material in the accretion disk is very hot, moving very fast, and ionized, so it generates a very powerful magnetic field

• As material spirals into the event horizon, it approaches v~c and if charged some material is ejected along the jet of the magnetic field

• The material in the jet is moving relativistically

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Unified AGN model• In this model, all these energetic Active Galactic Nuclei phenomena

(Seyferts, quasars, etc) are energy released from the region of a SM BH, and our viewing angle determines what is seen

• The very brightest objects (called blazars) are when looking right down the jet

• Less energetic, but still high-energy phonemena occur when viewing from the side

• IMPORTANT: The galaxy will only be an AGN if it is recently accreting a large amount of mass...