Download Ch 17n18 AGN Cosmology

Active Galactic Nuclei (AGN)
and Cosmology
Chapters 17-18
AGN Again?
Galaxies that emit powerful radiation in
many parts of the EM spectrum are called
active galaxies. However, the power
source is in the nucleus—hence, AGN.
One type of active galaxy is called the
seyfert galaxy, in honor of Carl Seyfert.
The spectra of normal galaxies contain
the combined light of billions of stars
(mostly absorption lines), but seyfert
galaxies also contain broad emission
lines.
Seyfert Characteristics
Seyfert galaxies contain large amounts of
hot, low-density gas. They are broadened
because this gas is moving extremely fast
(30 times greater than normal).
Their luminosities fluctuate quickly*,
suggesting a compact power source.
A large majority of these galaxies show
signs of interaction with another galaxy.
The evidence points to the presence of a
supermassive black hole in their
centers.
Supermassive Evidence?
*1. An astronomical body cannot change
its brightness in a time shorter than the
time it takes light to cross its diameter! A
black hole is the only power source that is
small enough to explain this.
2. These galaxies have matter around
their centers that are swirling so fast, that
nothing short of a black hole is massive
enough hold it in orbit. Ex. M87 Dust
orbiting only 60 ly from its center is
traveling at 750 km/s!
Mass = 2.4 billion suns!!
The Core of M87
Note the Doppler shift in the
above picture and the jet in
the right one.
Disky Evidence: NGC 7052
The gas in this
disk (only 57 pc
from the center)
spins at 155
km/s, which
suggests a black
hole of 300
million solar
masses.
Disks and Jets: NGC 4261
Modeling AGNs
There are 2 types of Seyfert galaxies that
each behave differently—their properties
may depend on the angle at which we see
the accretion disk (or its jet) in the center
(NOT necessarily related to the galaxy as a
whole).
Seyfert Type 1: bet. 0o & 90o from the disk
Seyfert Type 2: 0o (looking at the side of
the disk)
What would we see if we looked straight
down the jet (at 90o)? A blazar!
90o
0o
Blazars and Quasars
Blazar: what we see when we look straight
down the jet: 10,000 x more luminous than
the MW; fluctuates in only hours!
A quasar (or quasi-stellar object, or QSO)
looked like stars, but had strange spectra,
were thousands of times brighter than
average galaxies, and gave off powerful
radio emissions. It turns out that they have
highly redshifted lines. The first quasar
had a redshift of 0.158 (15.8%). Largest =
6!!
A picture (left) and spectrum
(below) of the first quasar,
3C273. Note how it looks very
“starlike.”
Invisible Lenses
Gravity can bend light in such a way that a
galaxy can act like a “gravitational lens”
and create distorted or multiple images.
The galaxies that appear stretched and bent result from
gravitational lensing.
Calculating z for Galaxies
Classical (slow) redshift (z): z  

 For “normal” galaxies
0
2
v =zc
v
(
z

1
)

1
Relativistic redshift: r 
2
c
(
z

1
)

1
 For quasars
Ex. If z = 6, then we have to use the
relativistic redshift equation. We get:
vr 49  1 48


or 96% the speed of light!
c 49  1 50
Such a large redshift also implies a huge lookback time (several billion years!). There were
more quasars long ago, when galaxies were
closer and collisions more common.
Cosmological Assumptions
The cosmological principle assumes
homogeneity and isotropy in the universe.
1. Homogeneity is the assumption that
matter is spread evenly throughout the
universe on the large scale.
2. Isotropy is the assumption that the
universe looks the same in all directions
on the large scale.
3. Universality is the idea that the laws we
observe on Earth operate the same way in
any part of the universe.
Cosmological Observations
1. The fact that the night sky is dark seems
obvious, but it has a profound answer: the
universe is finite in size and age!
2. The universe is expanding! The redshifts
that we measure do NOT result from
galaxies flying through space (so NOT
Doppler shifts!), but space-time itself
expanding and carrying the galaxies with
it—as if on an expanding balloon. There is
no edge or center of the universe!
Which Universe Do We Live In?
1. Space-time could have a positive
curvature [curvature is due to gravity
according to General Relativity]: like the
surface of a sphere, finite and closed.
2. Space-time could have zero curvature:
like a flat surface, infinite or it would
have an edge. An open universe.
3. Space-time could have negative
curvature: like a saddle, infinite or it
would have an edge. An open universe.
Curvature Possibilities
Most
evidence
supports a
flat
universe
(middle)
and
inflation.
More later.
The Ultimate Assumption!
If our universe is expanding, then it must
have started from a small, high-density,
high-temperature state (run the “video”
backwards). There must be a beginning!
This beginning was derisively called “the
big bang” by Fred Hoyle, a strong critic.
Many astronomers were repulsed at the
idea of a finite universe—favored
alternatives that made the universe infinite.
Myths: not an explosion in space and time,
but of space and time; did not occur at one
point (everywhere).
The Ultimate Evidence?
Radiation from the big bang should still be
detectable from hot gas formed just after the
bang. About 3000 K (near IR).
However, this light has a redshift of 1000!
Therefore, its observed wavelength and
temperature would be about 1000 times larger.
Expected about 3 K (microwave).
Arno Penzias and Robert Wilson detected this
in the mid-sixties: about 2.7 K! Now called the
cosmic microwave background radiation.
Seen in all directions! 1978 Nobel Prize
Other Theories
The steady state model posited that the
universe is eternal and unchanging. New
matter continuously appeared in order to
maintain a constant density. Abandoned.
The oscillating universe model stated
that the universe undergoes stages of
alternating expansion and collapse so that
the universe can be “infinite.” Abandoned.
Current ideas: infinite universes (“branes”);
time can flow both forward and backward;
time does not really exist.
Refinements
The work of Penzias and Wilson was rough, but later
measurements have refined their work.
The COBE (Cosmic Background Explorer)
satellite mapped the entire sky, refined
the black body temperature (2.735  .06),
and discovered tiny variations in the
background temperature (from density
differences, “seeds” of larger structure).
BOOMERANG, WMAP, and other satellite
data support a big bang/inflationary/flat
universe model.
CMBR Blackbody Curve
COBE and WMAP Data
The COBE team,
including
George Smoot,
won the 2006
Nobel Prize.
A (Very) Brief History of Time*
*Based on the book by Stephen W. Hawking
1. Early: What occurred within the first fraction
of a second is uncertain. The laws of Physics
can’t operate as we understand them today.
Inflation (sudden expansion) and separation
of 4 forces.
2. Matter Era: Cool enough for basic particles
(protons, neutrons, electrons) and photons to
appear. Matter wins; atoms form; finally
~75% H; 25% He.
3. Structure Era: Small structures become
bigger structures; quasars and early stars
form, then conventional galaxies, later stars,
and planets.
Future Fate of the Universe
The fate of the universe depends on its
density. The critical density (  0), which
determines the universe’s geometry and its
fate equals 4 x 10-30 g/cm3.
If the universe is less than or equal to  0 ,
then the universe will expand forever. If the
universe is greater than  0 , then it will
eventually collapse (the “big crunch”).
The % of dark matter and dark energy (?)
determines the density. Probably flat.