1. Structure of the Milky Way
Evolution and Stability
C&M Sections 23.3 – 23.5

2. Videos http://beltoforion.de/galaxy/galaxy_en.html
check website above for all videos in this lecture.
Lecture by Erica Franzmann & Ben Guest

3. Structure Components Spiral Galaxies are made up of 3 main components
Halo
Bulge
Disk
Each has different
motion
composition
structure

4. Halo Halo contains old stars and globular clusters
Orbit randomly in 3D
Contains almost no gas or dust
No star formation (stars ~10 B years old)
Roughly spherical
No obvious substructure
Stars are Population II (old)
Reddish

5. Bulge Spherical to football shaped region
High density of stars and gas
Individual motions are still random
General net rotation about galactic centre
Mix of Population I & II stars

6. Disk A flat disk shaped region Thickness dependent on objects studied
Young stars, ISM tightly confined to plane of disk
Sol-type stars less confined
K-M stars least confined
Reason for differences: stars form from ISM and drift
Orbital motion is ordered
All stars orbit in the same direction in the plane of the disk
Mostly Population I Stars (young)
Drift due to interactions with ISM and other stars

7. Milky Way Stats: Barred Spiral Halo Bulge Disk ~30 kpc ~6 kpc across
~4 kpc perpendicular to disk
Disk
~300 pc thick (solar neighbourhood)
~30 kpc wide
Sun ~8 kpc from galactic centre

8. Milky Way Measurement Difficulties
Dust lanes obscure view of
Disk stars
Bulge
Different wavelengths show things more effectively

10. Overall Structure

11. Milky Way Formation Merger of several smaller systems
Combined cloud has some overall rotation
Gas cools, falls into disk
Halo and halo stars remnant of stages a) and b)

12. No strongly preferred direction when stars in the Halo formed
No ongoing star formation in the Halo
Infall of halo gas is continuing today
Metalicity of disk stars would be much higher if the gas wasn’t being diluted by relatively unevolved gas from the halo
Requires an infall rate of 5-10 solar masses per year

13. Orbital Motion Disk stars and gas move in circular orbits about bulge
Sun: 220 km/s -> 225 M y!
Differential rotation
Orbit governed by gravity
objects closer to centre move faster than objects at the edge
GESTICULATE WILDLY TO EXPLAIN WHAT UP

14. Orbital Motion
Bulge and Halo stars move with comperable velocity to corresponding disk radius
But they do it in all directions!
Halo stars & clusters occasionally pass through disk on orbit
Arcturus!

15. Milky Way Radio Maps
Differential rotation => different doppler shifts
Velocity measurements from Doppler shift of 21cm emission
Observations in many directions allow the gas distribution of our galaxy to be mapped

16. Spiral Galaxies

17. Why do they look like that?
Gravitational interactions from nearby galaxies?
Isolated galaxies also show spiral structure
NGC1300 – Hubble
Credit1: Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)
Credit2: Hubble data: NASA, ESA, and A. Zezas (Harvard-Smithsonian Center for Astrophysics); GALEX data: NASA, JPL-Caltech, GALEX Team, J. Huchra et al. (Harvard-Smithsonian Center for Astrophysics); Spitzer data:NASA/JPL/Caltech/Harvard-Smithsonian Center for Astrophysics
M81-Hubble/GALEX/Spitzer
M101 – Hubble Credit3: NASA, ESA,

18. The space between the arms is not empty!
Its full of matter but is not a site of plentiful star formation and appears dark in optical observations

19. Stability of Spiral Arms
Galaxy Rotates as a solid disk
Preserves the spiral structure

20. Solid disk rotation video

21. Not Observed
Requires a linear rotation Curve
Non-physical

22. Observed disk galaxies show differential rotation

23. Not observed
Would cause spiral structure to be short-lived

24. Winding problem video

25. Density Waves Wave patterns move across material
think water waves
Density wave -> wave of compression and expansion sweeping through galaxy
Mundane example:
blockage causes high traffic density
cars enter high density region, slow down, pass through and speed up again
Cars flow through, wave pattern remains

26. Density Wave model Arms are not solid
Instead they are regions of higher density
“spiral density waves”
wave of gas compression that moves through disk
Density waves rotate slower than stars and gas
Stars and gas flow into arm
slowed and compressed
compression triggers star formation
Stars form in these regions then flow out
Young stars appear blue and bright
Arms appear brighter and blue in colour

27. Density Wave Model In a reference frame rotating with the spiral arms:
arms are fixed in position
stars and gas are not.

28. Density wave video 1

29. Density wave video 2

30. How do we test this model?
Look for observational tracers of different stages of star formation
How does this change with radius?
Separation distance should change with radius in agreement with the rotation curve and a constant pattern rotation speed
Studies that have done this have been inconclusive

31. What started the density wave?
Gravitational interactions?
Gravitational instability with the gas near the bulge?
Bar-like asymmetry within the bulge?
Unknown

32. Self-Propagating Star Formation
Another possibility: star formation driving waves
Stars form, die, and go SN -> send out shockwaves
Shockwaves compress gas
New stars!
Repeat ad nauseam
Problem: produces only pieces of spirals, not galaxy wide
More going on

33. END