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Take a giant step outside the Milky Way The Milky Way Galaxy c Artist's Conception Example (not to scale) b
d a
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6 Supermassive (3x10 M) Black Hole in the Perseus arm Galactic Center from above ("face-on") Orion arm see disk and Sun bulge
Cygnus arm Carina arm
from the side ("edge-on")
Another galaxy: NGC 4414. The Milky Way roughly resembles it. M51
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The Tree Main Structural Components
of the Milky Way 3. Bulge • About 4 kpc across 1. Disk • 30,000 pc diameter (30 kpc) • Old stars, some gas, dust
• Contains young and old stars, gas, dust. Has spiral structure. 6 • Central black hole of 3x10 M
• Vertical thickness roughly 100 pc -2 kpc (depending on component). • Spherical Most gas and dust in thinner layer, most stars in thicker layer.
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
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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.
D. 20 kpc from the center of the Milky Way in the
Halo: stars and globular clusters swarm around center of Milky Way. Very elliptical disk. orbits with random orientations. They also cross the disk. Bulge: similar to halo. Disk: rotates.
Rotation of the Disk Clicker Question 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 What lurks at the center of our galaxy? longer.
A. A 3 million solar mass black hole. => rotation not rigid like a phonograph record or a merry-go-round. Rather, "differential rotation". B. A giant star cluster. 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
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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.
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Now replace cars by stars and gas Proposed solution: Traffic jam on a loop caused by merging clouds. The traffic jams are
actually due to the stars' collective Arms are not material moving together, but mark peak of a compressional gravity. The higher gravity of the wave circling the disk: jams keeps stars in them for A Spiral Density Wave longer. Calculations and computer simulations show this Traffic-jam analogy: situation can be maintained for a long time.
90% of Matter in Milky Way is Dark Matter
Gives off no detectable radiation. Evidence is from rotation curve:
10 Solar System Rotation Curve: when almost all Rotation mass at center, velocity decreases with radius ("Keplerian") Velocity 5 (AU/yr)
1 1 30 10 R (AU) 20 observed curve Molecular gas clouds pushed together in arms too => high density of clouds => high Milky Way concentration of dust => dust lanes. Rotation Curve if Milky Also, squeezing of clouds initiates collapse within them => star formation. Bright young Curve Way ended massive stars live and die in spiral arms. Emission nebulae mostly in spiral arms. where visible matter pretty So arms always contain same types of objects, but individual objects come and go. much runs out.
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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.
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Dark Matter candidates Clicker Question • MACHOs (Massive compact halo objects)
• Brown dwarfs (low mass stars) How long does it take our solar system to orbit once around • White dwarfs (burn-out stars) the Milky Way?
• Neutron stars (dead stars) A. 1 year.
• Stellar black holes (dead stars) B. 2 million years. • mini (primordial) black holes C. 240 million years. • massive (primordial) black holes D. 250 billion years (longer than the age of the • WIMPS (Weakly interacting massive particles: neutrons, axions, etc). 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.
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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 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?) • GC is about 8 kpc away, at a declination • 100 pc: Sgr A (strongest radio source on the sky). of -29°, well in the southern hemisphere. • 10 pc: pinwheel gas structure
• <1 pc: Sgr A*, pointlike radio/X-ray/IR source
• It is very busy, as 6 • Nucleus: Supermassive black hole ~3x10 M shown by this 1x1 kpc VLA map. • Stars are very tightly packed in the central region, reaching densities of up 7 -3 to 10 M pc -3 – near the Sun the density is only about 0.05 M pc 4 -3 – in globular clusters the central density reaches 10 M pc
=> Stellar collisions probably frequent (1 every 106 yrs).
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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.
• Consequently, IR stars can be observed very close to thecenter of the • Scale of central region about 0.3 Galaxy. pc, IR (1.6, 2.2 and 3.8 microns). • They move so fast (500 to 1000 km/s) that their proper motions are • Sgr A* located near the center of measurable even after a few years. image (not bright in IR). • In recent years even their orbits around the central massive source has • Most of these stars are very young been detected. and massive and heavily reddened so they don't appear blue.
• Spectroscopic studies indicate that 1992-2008 the stars are luminous super giants and only a few 10s of millions years old.
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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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