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

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

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

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

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

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

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

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

Overall Structure

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)

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

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

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!

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

Spiral Galaxies

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,

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

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

Solid disk rotation video

Not Observed Requires a linear rotation Curve Non-physical

Observed disk galaxies show differential rotation

Not observed Would cause spiral structure to be short-lived

Winding problem video

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

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

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

Density wave video 1

Density wave video 2

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

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

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

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