Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
Lecture 30: Active Galaxies & Quasars
Readings: Ch 27, sections 27-1 thru 27-5
- Active Galactic Nuclei
- Powerful energy sources in the nuclei of some galaxies.
- Types of Active Galaxies:
- Seyfert Galaxies
- Radio Galaxies
- Power source:
- Accretion of matter by Supermassive Black Holes
- Exact center of a galaxy and its immediate surroundings
- If a spiral galaxy, it is also the center of rotation.
- Dense central star cluster
- Show a composite stellar absorption-line spectrum
- May also show weak nebular emission lines.
Active Galactic Nuclei
About 1% of all galaxies have bright active nuclei.
Bright, compact nucleus:
- Sometimes brighter than the entire galaxy.
- Strong, broad emission lines from hot, dense, highly excited
In general, about 30-50% of spiral galaxies show some level of activity
in their nuclei, but only about 1% are truly dominant.
- Means they are small: only a few light days across.
Discovery of Active Galaxies
1943: Seyfert 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
- Photographs revealed slightly fuzzy or "quasi-stellar"
objects at these locations.
- The spectra were bizarre and full of unrecognized broad emission
Named them Quasars, short for Quasi-Stellar Radio Sources.
The Riddle of Quasars: Solved
1963: Maarten Schmidt (Caltech)
This led to a problem:
- 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.
Quasars are the most luminous objects in the Universe:
- The brightest Quasars can have luminosities of
up to ~1014 Lsun
- 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
Most Active Galaxies are related to each other
Radio Loud: powerful radio sources
- Low Power: Radio Galaxies
- High Power: Quasars
Radio Quiet: very weak radio sources
All types of AGNs share many characteristics. A problem of modern research
is to sift through the similarities and differences to figure out how
they might be related to each other.
- Low Power: Seyfert Galaxies
- High Power: Quasi-Stellar Objects (QSOs).
What powers AGNs?
Properties that need to be explained:
- Luminosities of Billions to Trillions of Lsun
- Emit wavelengths from from Radio to Gamma-rays
- 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
The energy source of active galaxies is the steady accretion of matter onto
a supermassive Black Hole.
- Supermassive = 106 - 109 Msun
- 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
The Central Engine
Black Hole accretion is very efficient:
- up to ~10% efficiency
- ~1 Msun/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:
How do supermassive black holes form?
- 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?
All of these questions make the study of Active Galaxies a very
exciting area of current research. As an aside, it is the primary
area that I work in.
- 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 Msun black hole at its center).
- There were many more AGNs in the distant past, but few today - where
are all the dead quasars?
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Updated: 2006 February 16
Copyright © Richard W. Pogge, All Rights Reserved.