Astronomy 162: Introduction to Stars, Galaxies, & the Universe Prof. Richard Pogge, MTWThF 9:30

Lecture 30: Active Galaxies & Quasars

Key Ideas

Active Galactic Nuclei

Powerful energy sources in the nuclei of some galaxies.

Types of Active Galaxies:

Quasars

Seyfert Galaxies

Radio Galaxies

Power source:

Accretion of matter by Supermassive Black Holes

Galactic Nuclei

• Exact center of a galaxy and its immediate surroundings
• If a spiral galaxy, it is also the center of rotation.

Normal Galaxies:

• Dense central star cluster
• Show a composite stellar absorption-line spectrum
• May also show weak nebular emission lines.

Active Galactic Nuclei

Bright, compact nucleus:

• Sometimes brighter than the entire galaxy.
• Strong, broad emission lines from hot, dense, highly excited gas.

Rapidly Variable:

• Means they are small: only a few light days across.

Discovery of Active Galaxies

• Carl Seyfert identified 6 galaxies with strong, broad emission lines coming from a compact, bright galaxy nucleus

1950s: Radio Galaxies

• First radio telescopes found faint galaxies at the location of intense radio emission.
• Also show broad emission-lines in their spectra (sometimes).

The Riddle of the Quasars

• Radio astronomers found intense, point-like sources of radio emission.
• Photographs revealed slightly fuzzy or "quasi-stellar" objects at these locations.
• The spectra were bizarre and full of unrecognized broad emission lines.

Named them Quasars, short for Quasi-Stellar Radio Sources.

The Riddle of Quasars: Solved

• Recognized that the lines in Quasars were normal Hydrogen lines with extreme redshifts.
• Made them extremely luminous objects located very far away.
• The "fuzz" is the host galaxy lost in the glare of an intense active nucleus.

• Great distances imply extreme luminosities for Quasars.

Cosmic Beacons

• The brightest Quasars can have luminosities of up to ~10¹⁴ L_sun

Brightest Quasars:

• Among the most distant objects in the Universe.
• Most distant is almost 4 Gpc away
• Probes of the Universe on very large scales.

The Active Galaxy Zoo

Radio Loud: powerful radio sources

• Low Power: Radio Galaxies
• High Power: Quasars

Radio Quiet: very weak radio sources

• Low Power: Seyfert Galaxies
• High Power: Quasi-Stellar Objects (QSOs).

What powers AGNs?

Powerful:

• Luminosities of Billions to Trillions of L_sun
• Emit wavelengths from from Radio to Gamma-rays

Compact:

• Visible light can vary on timescales of a few days
• X-rays can vary on timescales of a few hours or less!

The Black Hole Paradigm

• Supermassive = 10⁶ - 10⁹ M_sun
• Schwarzschild Radii: ~0.01 - 10 AU

Infalling matter releases gravitational binding energy

• Gas settles into an accretion disk.
• The hot inner parts of the disk shine brightly, especially at X-rays.

The Central Engine

• up to ~10% efficiency
• ~1 M_sun/year of matter needed to power bright active galaxies
• Get their "fuel&fuel; from surrounding gas and stars

Rapidly Spinning Black Hole:

• Acts like a particle accelerator
• Leads to the jets seen in radio-loud AGNs.

Some Nagging Questions:

• We don't really know for sure, but recent work is strongly suggests that the formation and growth of supermassive blackholes is tightly coupled to the formation of the bulge of a galaxy.

How are they fueled?

• Galaxy interactions might dump gas into the nuclear regions to feed the Black Hole.

• Stellar bars might be able to funnel gas into the nucleus from the disk of the galaxy.

• Cannibalism of a gas-rich dwarf?

Do most galaxies have supermassive black holes?

• Nearly all spirals show some level of "activity" if perhaps only very faintly.

• There is a growing body of dynamical evidence for the presence of massive black holes in many nearby, but otherwise "inactive" galaxy nuclei (including our own Galaxy, which has a relatively inactive 3 Million M_sun black hole at its center).

• There were many more AGNs in the distant past, but few today - where are all the dead quasars?