Download 21. The Milky Way Galaxy

4/12/12 21. The Milky Way Galaxy
Take a giant step outside the Milky Way
The Milky Way Galaxy
Artist's Conception
c
Example
(not to
scale)
b
d
a
1 4/12/12 Supermassive (3x106 M) Black Hole in the
Galactic Center
Perseus arm
Orion arm
Sun
from above
("face-on")
see disk and
bulge
Cygnus arm
Carina arm
from the side
("edge-on")
Another galaxy: NGC 4414. The Milky Way roughly resembles it.
M51
2 4/12/12 The Tree Main Structural Components
of the Milky Way
1.  Disk
•  30,000 pc diameter (30 kpc)
3.  Bulge
•  About 4 kpc across
• 
Old stars, some gas, dust
• 
Contains young and old stars, gas, dust. Has spiral structure.
• 
Central black hole of 3x106 M"
• 
Vertical thickness roughly 100 pc -2 kpc (depending on component).
Most gas and dust in thinner layer, most stars in thicker layer.
• 
Spherical
2.  Halo
•  At least 30 kpc across
• 
Contains globular clusters, old stars, little gas and dust, much ‘dark
matter’
• 
Roughly spherical
Shapley (1917) found that Sun was not at center of Milky Way
Precise Distance to Galactic Center
Distance = 7.94 +/- 0.42 kpc
SgrA*
Shapley used distances to variable “RR Lyrae” stars (a kind of Horizontal Branch
star) in Globular Clusters to determine that Sun was 16 kpc from center of Milky
Way. Modern value 8 kpc.
Eisenhauer et al. 2003
Orbital motion 6.37 mas/yr
3 4/12/12 Stellar Orbits
Clicker Question
Where is out solar system located?
A. Near the center of the milky Way Galaxy in the bulge.
B. 4 kpc from the center of the Milky Way in the halo.
C. 8 kpc from the center of the Milky Way in the disk.
Halo: stars and globular clusters swarm around center of Milky Way. Very elliptical
orbits with random orientations. They also cross the disk.
D. 20 kpc from the center of the Milky Way in the
disk.
Bulge: similar to halo.
Disk: rotates.
Clicker Question
What lurks at the center of our galaxy?
A. A 3 million solar mass black hole.
B. A giant star cluster.
Rotation of the Disk
Sun moves at 220 km/sec around center. An orbit takes 240 million years.
Stars closer to center take less time to orbit. Stars further from center take
longer.
=> rotation not rigid like a phonograph record or a merry-go-round. Rather,
"differential rotation".
Over most of disk, rotation velocity is roughly constant.
C. A 30 solar mass black hole.
D. Darth Vader
The "rotation
curve" of the
Milky Way
4 4/12/12 Spiral Structure of Disk
Spiral arms best traced by:
Young stars and clusters
Emission Nebulae
HI
Molecular Clouds
(old stars to a lesser extent)
Disk not empty between arms, just
less material there.
Problem: How do spiral arms survive?
The spiral should end up like this:
Given differential rotation, arms should be stretched and smeared out after a few
revolutions (Sun has made 20 already):
The Winding Dilemma
Real structure of Milky Way (and
other spiral galaxies) is more
loosely wrapped.
5 4/12/12 Proposed solution:
Now replace cars by stars and gas
clouds. The traffic jams are
actually due to the stars' collective
gravity. The higher gravity of the
jams keeps stars in them for
longer. Calculations and
computer simulations show this
situation can be maintained for a
long time.
Traffic jam on a loop caused by merging
Arms are not material moving together, but mark peak of a compressional
wave circling the disk:
A Spiral Density Wave
Traffic-jam analogy:
90% of Matter in Milky Way is Dark Matter
Gives off no detectable radiation. Evidence is from rotation curve:
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Rotation
Velocity
(AU/yr)
Solar System Rotation Curve: when almost all
mass at center, velocity decreases with radius
("Keplerian")
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1
1
10
R (AU)
20
30
observed curve
Molecular gas clouds pushed together in arms too => high density of clouds => high
concentration of dust => dust lanes.
Also, squeezing of clouds initiates collapse within them => star formation. Bright young
massive stars live and die in spiral arms. Emission nebulae mostly in spiral arms.
So arms always contain same types of objects, but individual objects come and go.
Milky Way
Rotation
Curve
Curve if Milky
Way ended
where visible
matter pretty
much runs out.
6 4/12/12 Not enough radiating matter at large R to explain rotation curve => "dark"
matter!
Dark matter must be about 90% of the mass!
Composition unknown. Probably mostly exotic particles that don't interact
with ordinary matter at all (except gravity). Some may be brown dwarfs, dead
white dwarfs …
Most likely it's a dark halo surrounding the Milky Way.
Mass of Milky Way
6 x 1011 solar masses within 40 kpc of center.
More Evidence for Dark Matter - Abell 1689
•  Dark Matter halos
At large radii there is little starlight. There is 5-10 times as much dark
matter associated with galaxies, as ordinary matter.
7 4/12/12 Dark Matter candidates
• MACHOs (Massive compact halo objects)
• Brown dwarfs (low mass stars)
• White dwarfs (burn-out stars)
Clicker Question
How long does it take our solar system to orbit once around
the Milky Way?
• Neutron stars (dead stars)
A. 1 year.
• Stellar black holes (dead stars)
B. 2 million years.
• mini (primordial) black holes
• massive (primordial) black holes
• WIMPS (Weakly interacting massive particles: neutrons, axions, etc).
C. 240 million years.
D. 250 billion years (longer than the age of the
Universe).
Clicker Question
What makes up most of the mass (90%) of the Milky Way
Galaxy?
A. Hydrogen gas.
B. Stars.
C. Dead stars (white dwarfs, neutron stars and black
holes).
D. We don’t know.
8 4/12/12 The Galactic Center
•  Up to 30 mag extinction in the optical (only 1 in 1012 photons reaches us!)"
•  Solar motion up through the disk will give us a clear view in 15 Myrs"
•  Meanwhile, forced to work at radio, IR, X-ray and gamma ray wavelengths."
Seeing into the center of the Milky Way
• 
GC is about 8 kpc
away, at a declination
of -29°, well in the
southern hemisphere.
Most information we have about the Galactic Center region is thus from IR
and radio:
•  1kpc: expanding gas at 100km/s (explosive event 10Myr ago?)
•  100 pc: Sgr A (strongest radio source on the sky).
•  10 pc: pinwheel gas structure
•  <1 pc: Sgr A*, pointlike radio/X-ray/IR source
• 
It is very busy, as
shown by this 1x1
kpc VLA map.
•  Nucleus: Supermassive black hole ~3x106 M
•  Stars are very tightly packed in the central region, reaching densities of up
to 107 M pc-3
–  near the Sun the density is only about 0.05 M pc-3
–  in globular clusters the central density reaches 104 M pc-3
=> Stellar collisions probably frequent (1 every 106 yrs).
9 4/12/12 Seeing into the center of the Milky Way
•  Moving closer to the center, the picture gets more complex.
•  The center-most source is called Sgr A, divides up into
–  Sgr A West
–  Sgr A East
–  Sgr A*
•  SgrA* is extremely compact, and
is almost certainly due to the
central black hole itself.
•  Sgr A West appears like a mini
spiral in an image at 6cm complicated structure in the
ionized gas.
•  Scale of central region about 0.3
pc, IR (1.6, 2.2 and 3.8 microns).
•  Sgr A* located near the center of
image (not bright in IR).
•  Most of these stars are very young
and massive and heavily reddened
so they don't appear blue.
•  Spectroscopic studies indicate that
the stars are luminous super giants
and only a few 10s of millions
years old.
•  Consequently, IR stars can be observed very close to thecenter of the
Galaxy.
•  They move so fast (500 to 1000 km/s) that their proper motions are
measurable even after a few years.
•  In recent years even their orbits around the central massive source has
been detected.
1992-2008
10 4/12/12 4/17: Reading Assignment
•  Is the black hole the dynamical center?
Supporting evidence comes from the
proper motion of Sgr A*.
•  Chapter 15.1-15.3: Galaxies
–  At 8 kpc from us its proper motion is
very small, about 10 mas/yr. It moves
at a straight line across the sky.
–  The amount turns out to exactly reflect
the motion of the Sun around the
Galaxy => Sgr A* is clearly the Milky
Way's dynamical heart.
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