3.3ACTIVE GALAXIES AND SUPERMASSIVE BLACK

HOLES

Active Galaxies

Galaxies with extremely violent energy release in their nuclei Active Galactic Nuclei (AGN) Up to many thousand

times more luminous than the entire Milky Way.

Energy is released within a region approximately the size of our solar system.

Spectra of Galaxies

Taking a spectrum of the light from a normal galaxy:

The light from the galaxy should be mostly star light, and should thus contain many absorption lines from the individual stellar spectra.

Seyfert Galaxies

Seyfert Galaxies are one of the two largest groups of active galaxies; also an unusual type of spiral galaxy. Make up about 2% of spiral galaxies. Variability ~ 50% in a few months.

Type I Seyfert: very luminous at X-ray and UV wavelengths; typical broad emission lines with sharp, narrow cores.

Type II Seyfert: weaker X-ray emission and emission lines are narrower than Type I Seyfert but still broader than normal galaxy lines.

Seyfert Galaxies

Despite their small size, the cores of Seyfert galaxies produce tremendous amounts of energy. The brightest emit a hundred

times more energy than the entire Milky Way galaxy.

Lots of energy produced in a very small volume with extremely high temperatures and velocities … sound familiar? Astronomers conclude the

centers of these galaxies contain supermassive black holes into which matter is flowing from surrounding hot accretion disks.

NGC 1068 (M77)

Interacting Galaxies

Active galaxies are often associated with interacting galaxies, possibly the result of recent galaxy mergers.

Cosmic Jets and Radio Lobes Many active galaxies show powerful radio jets.

Material in the jets moves at near the speed of light (“relativistic jets”).

Hot spots within the jets (bright red/orange colors) is energy released through interaction with the surrounding material.

Cygnus A, the brightest radio

source in Cygnus, is a pair of giant lobes

with jets leading from the nucleus of a highly disturbed

galaxy.

Radio Galaxies

Infrared image of Centuarus A (NGC 5128) reveals warm gas at its nucleus.

Radio Galaxies

The radio jets from NGC 1265 are being left behind as the galaxy moves

rapidly through the gas of the intergalactic medium. Twists in the

tails are presumably caused by motions of the active nucleus.

The radio source 3C 75 is produced by two galaxies experiencing a close encounter. As the active nuclei whip around each other,

their jets twist and turn.

Radio Galaxies

Radio galaxy 3C 31 is one of a chain of galaxies. It has ejected jets from its

core that twist, presumably because the active nucleus is orbiting another object such as

the nucleus of a recently absorbed galaxy.

Formation of Radio Jets

Jets are powered by accretion of matter onto a supermassive black hole. Conservation of angular momentum forces the matter to form a whirling

accretion disk around the black hole.

Twisted magnetic field confines the jets in a narrow beam and causes synchronous radiation.

Active Galaxies in Galaxy Clusters

The powerful radio lobes of radio galaxies can push away intergalactic gas in galaxy clusters.

Hundreds of millions of years after the galaxy’s activity has calmed down, there are still “ghost” cavities in the X-ray emission from the intergalactic gas.

Quasars

Quasars are the other largest group of active galaxies; primarily in elliptical galaxies with even more powerful central sources than Seyfert galaxies. Also show strong

variability over time scales of a few months.

Show very strong, broad emission lines in their spectra.

Quasars

Spectral lines from Quasar 3C 273 show a large red-shift:z = / Dl l0 = 0.158

The original photographic plate holding a spectrum of 3C 273 contains three hydrogen Balmer lines, Hδ, Hγ, and Hβ . The spectrum is red-shifted by 15.8%.

Quasars

The study of high red-shift quasars allows astronomers to investigate questions of:1. Large scale structure of the universe

2. Early history of the universe

3. Galaxy evolution

4. Dark matter Quasars can be far away, but are not very faint.

Ultra-luminous 10 – 1000 times the luminosity of a large galaxy.

How could they be this bright but also very small? Overwhelming evidence now suggesting quasars are the active

cores of very distant galaxies.▪ Most extreme type of active galactic nuclei (AGN)

NGC 4261: Radio image reveals double-lobed jet structure; close-up view by Hubble

Space Telescope reveals a bright central source embedded in a dust torus.

Supermassive Black Holes

AGN Unification

Astronomers studying active galaxies have developed a unified model of active galaxy core that is well supported by evidence … a monster black hole is the centerpiece. According to the unified model, what you see when you view

the core of an active galaxy depends on how the black hole’s accretion disk is tipped with respect to your line of sight.

Unified model far from complete: Actual structure of accretion disks is poorly understood. Does not explain all differences among active galaxies and

quasars.

Radio Galaxy:

Powerful “radio lobes” at the end points of the jets, where power in the jets is dissipated.

Cyg A (radio emission)

Observing direction

AGN Unification

Emission from the jet pointing towards us is enhanced (“Doppler

boosting”) compared to the jet moving in the other direction

(“counter jet”)

Quasar or BL Lac object (properties very similar to quasars, but no

emission lines)

Observing direction

AGN Unification

Components of a Seyfert Galaxy or Quasar

Broad Line Region

Narrow Line Region

AGN Unification

Orbiting X-ray telescopes observing active galaxies

sometimes detect X-ray flares equaling the energy of a

supernova explosion.

Supermassive Black Holes

Such flares are evidently caused when a star wanders too close to a supermassive black hole at the

center of a galaxy and tidal forces rip the star apart.

Elliptical galaxies; often merging / interacting galaxies

Quasar Host Galaxies

Quasar activity in the universe was most abundant at

redshifts z ~ 2 – 3.

The highest-redshift quasars are seen at z > 6, but those are

very rare.

Quasars Through Time

Evidently astronomers see few quasars at high red-shifts because they are looking back to an age when the universe was so young it

had not yet formed many galaxies.