Determining the Information

1
Determining the Information

• Stars in the Milky Way
• all types are present
• mass function
• of stars of each mass
• determines evolution of galaxy
• most common star
• dim, low mass
• selection effect

2
Spiral Arms

• neutral hydrogen map
• spiral structure
• wedge shape
• no Doppler shift

3
M83

• In constellation Hydra
• 5 million pc from Earth

4
Spiral Arms

• Four major arms
• Perseus
• Sagittarius
• Centaurus
• Cygnus
• Minor arms
• We live in Orion

5
Rotation of the Galaxy

• Stability requires rotation
• Gravitational collapse
• Rotational velocity
• Determined by Doppler shift of 21 cm line
• Relative to Sun
• Need reference frame to determine speed of Sun
• Distant galaxies and globular clusters
• Speed of Sun
• Period T distance/speed
• T 2p (8000 pc)(3.09 x 1013 km/pc)/220 km/s
• 7.1 x 1015 s 220 million years
• Suns orbit roughly circular
• Mass inside orbit affects motion

6
Rotation of Galaxy

• Stars and gas all orbit in the same direction
• Orbital speed roughly constant
• Rotation curve of galaxy
• Orbital speed as a function of distance

7
Rotation of the Galaxy
Actual galactic rotation
If galaxy rotates as a disk
Keplerian rotation
8
Disk like rotation
9
Keplerian Rotation
10
Galactic Rotation Curve
11
Galactic Rotation Curve

• Dark Matter
• What is it?
• Brown dwarfs, white dwarfs, black holes
• MAssive Compact Halo Objects (MACHOs)
• Detecting
• Gravitational microlensing

12
Microlensing
13
Microlensing
14
Microlensing
15
Spiral Arms

• Disk-like Structure
• Conservation of angular momentum
• Spiral Arms
• ????
• Pattern in place from the start
• Only if galaxy rotates as a disk
• Our galaxy
• objects have approximately the same speed
• Winding dilemma

16
Winding Dilemma
17
Winding Dilemma
18
Winding Dilemma
19
Winding Dilemma

• Spiral Structure disappears quickly
• Few hundred million years
• Spiral arms cannot be like spokes
• Bertil Lindblad
• Swedish Astronomer
• Density Waves
• Waves
• Crests and troughs
• Spiral Arms are the crests

20
Spiral Arms

• Density Waves move slower than stars or
  interstellar material

21
Spiral Arms

• Density Waves
• areas of higher densities propagating around
  galaxy
• Interstellar dust and gas moves into region
• Higher density - star forming regions
• brighter than surroundings
• OB stars seen close to high density regions
• Do not have time to move out
• What maintains the waves?
• Energy source?
• Bar type galaxy
• Asymmetric pull could generate waves
• Gravitational interaction between galaxies

22
Spiral Arms

• Density waves may not be complete
• Grand-design spiral
• Thin, graceful, well-defined spiral arms
• Flocculent
• Broad, fuzzy, chaotic, poorly defined arms

23
Spiral Arms

• Self-propagating Star Formation (SSF)
• disturbance produced by supernovae
• causes new stars to form
• star forming regions move outward
• inner regions move faster
• results in spiral structure
• Explains flocculent spirals

24
The Galactic Center

• in constellation Sagittarius
• radio source
• Karl Jansky
• SgrA
• Invisible at optical wavelengths
• too much dust
• use radio, infrared, and gamma ray

25
The Galactic Center

• Large number of stars
• packed densely
• about 1/1000 separation near Sun
• intense, small radio source
• synchrotron radiation
• nonthermal emission
• high-energy electrons in magnetic field
• 5 kpc
• a necklace of clouds of molecular hydrogen,
  ionized hydrogen
• 3 kpc
• arc of cold hydrogen outward at 100 km/s

26
The Galactic Center

• Core
• giant swarm of stars
• 1000 light years across
• millions of stars/cubic light year
• at center - IRS16
• extremely luminous stars at 20 light years
• 30,000 solar masses
• luminosity of 20 million Suns

27
Core

• Powerful Energy Source
• SgrA
• 10 AU in diameter
• million times the mass of the Sun
• no light
• Black hole?
• How Formed?
• Starts small- few solar masses
• Grows by collecting gas
• Eventually gets big enough to "eat" stars

28
History and Formation of the Milky Way

• Explain
• shape
• motion
• stellar populations

29
(No Transcript)
30
History and Formation of the Milky Way

• Intergalactic gas cloud
• pure H and He
• 100 billion solar masses
• slow spin
• Due to interactions with other proto-galaxies
• Collapses under its gravity
• some stars form in collapsing gas
• collapse is slow
• takes several 100 million years
• retained motion once they formed

31
History and Formation of the Milky Way
32
History and Formation of the Milky Way

• massive stars
• evolve
• generate heavy elements in their core
• explode as supernova
• add heavy elements to gas
• additional stars form in collapsing gas
• now contain some heavy elements
• these become Pop II stars
• some gas left over
• "Polluted" gas collects in disk
• angular momentum (like solar system)

33
History and Formation of the Milky Way

• Spiral arms form as "density wave
• Clouds in arms collapse
• form Pop I stars in disk
• Pop III?
• early conditions unsuitable for low mass stars
• surface layers may have been contaminated