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21. The Milky Way Galaxy

The Milky Way Galaxy

a

b

c

d

Take a giant step outside the Milky Way Artist's Conception

Example (not to scale)

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Supermassive (3x106 M) Black Hole in the Galactic Center from above

("face-on") see disk and bulge

from the side ("edge-on")

Orion arm

Perseus arm

Cygnus arm

Carina arm

Sun

Another galaxy: NGC 4414. The Milky Way roughly resembles it. M51

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The Tree Main Structural Components of the Milky Way

1.  Disk

•  30,000 pc diameter (30 kpc)

•  Contains young and old stars, gas, dust. Has spiral structure.

•  Vertical thickness roughly 100 pc -2 kpc (depending on component). 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

3.  Bulge

•  About 4 kpc across

•  Old stars, some gas, dust

•  Central black hole of 3x106 M"

•  Spherical

Shapley (1917) found that Sun was not at center of Milky Way

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.

Precise Distance to Galactic Center

Distance = 7.94 +/- 0.42 kpc

Eisenhauer et al. 2003 Orbital motion 6.37 mas/yr

SgrA*

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Stellar Orbits

Halo: stars and globular clusters swarm around center of Milky Way. Very elliptical orbits with random orientations. They also cross the disk.

Bulge: similar to halo.

Disk: rotates.

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 disk.

Clicker Question

What lurks at the center of our galaxy?

A. A 3 million solar mass black hole.

B. A giant star cluster.

C. A 30 solar mass black hole.

D. Darth Vader

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.

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? Given differential rotation, arms should be stretched and smeared out after a few revolutions (Sun has made 20 already): The Winding Dilemma

The spiral should end up like this:

Real structure of Milky Way (and other spiral galaxies) is more loosely wrapped.

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Proposed solution: Arms are not material moving together, but mark peak of a compressional wave circling the disk:

A Spiral Density Wave

Traffic-jam analogy:

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

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.

90% of Matter in Milky Way is Dark Matter

Gives off no detectable radiation. Evidence is from rotation curve:

Rotation Velocity (AU/yr)

Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian")

R (AU)

10

5

1

1 10 20 30

Curve if Milky Way ended where visible matter pretty much runs out.

observed curve

Milky Way Rotation Curve

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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!

Mass of Milky Way

6 x 1011 solar masses within 40 kpc of center.

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.

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

• MACHOs (Massive compact halo objects)

• Brown dwarfs (low mass stars)

• White dwarfs (burn-out stars)

• Neutron stars (dead stars)

• Stellar black holes (dead stars)

• mini (primordial) black holes

• massive (primordial) black holes

• WIMPS (Weakly interacting massive particles: neutrons, axions, etc).

Clicker Question

How long does it take our solar system to orbit once around the Milky Way?

A. 1 year.

B. 2 million years.

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.

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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

•  GC is about 8 kpc away, at a declination of -29°, well in the southern hemisphere.

•  It is very busy, as shown by this 1x1 kpc VLA map.

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

•  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).

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•  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.

Seeing into the center of the Milky Way

•  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

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•  Is the black hole the dynamical center? Supporting evidence comes from the proper motion of Sgr A*.

–  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.

4/17: Reading Assignment

•  Chapter 15.1-15.3: Galaxies