1	Astro 1050 Fri. Mar 26, 2004 Today: Astronomy Articles End Ch. 12: The Milky Way
2	The Galactic Rotation Curve Keplerian fall-off near center indicates compact mass at center Flat curve throughout disk indicates much distributed mass Lack of fall-off beyond visible “edge” indicates “dark matter”
3	Stellar Population and Galaxy Evolution “Metal” abundance during time “Metals” are elements heavier than He A given star’s atmospheric abundance is approx. fixed at birth Interstellar metal abundance grows with each new generation of stars Red giants and supernova eject new heavy elements into interstellar gas Orbits during time A given star’s orbit is approx. fixed at birth – just plows through gas Orbits of gas clouds evolve with time since they can collide Orbits get more circular and disk-like with time
4	Traditional Model of Galaxy Evolution Stars are stuck with their original orbits They plow through gas like bullets Orbits of gas can evolve Gas clouds collide and only average motion (rotation) survives Metal abundance grows with time System starts out with little organized motion,few metals Halo stars form at this time It contracts, velocities average out leaving only rotation Gas collapses to form the disk Disk stars form after this has happened Some problems with traditional model Globular clusters not all same age Gap in ages between halo and disk objects Presence of some metals in even in oldest stars System may have formed by merger of smaller galaxies Galactic Cannibalism
5	M51: The Whirlpool Galaxy
6	Possible Origin of Spiral Arms Differential rotation smears features out into spiral patterns But can’t be whole story: Number of times Sun has orbited the galaxy: 10 billion yr/200 million yr = 50 times Spiral arms would have been wound up very tightly Something must continuously rebuild them
7	Degree of Organization of the Spiral Arms Different degrees of organization Grand Design Spirals: M51 Flocculent (“wooly”) Spirals
8	Degree of Organization of the Spiral Arms Different degrees of organization Grand Design Spirals: M51 Flocculent (“wooly”) Spirals
9	Tracing the Spiral Arms Arms NOT obvious if you look at: Old objects like the sun Arms ARE obvious if you look at: Maps of gas clouds 21 cm Hydrogen Radio maps of CO Far infrared observations of dust Young stars O, B stars “HII” ionized hydrogen regions surrounding O,B stars Clouds somehow form in arms , then dissipate between them Short lived objects only get a short distance from their places of birth O stars, Lifetime = few million years, at 250 km/s Þ500 pc
10	M51: The Whirlpool Galaxy
11	Density Wave Theory SPIRAL WAVE rotates with galaxy, but slower than individual stars Like moving traffic jam after an accident has been cleared Gas (and stars) catch up with wave, move through it, eventually reach front Just like cars catching up with moving traffic jam, eventually get through it Gas is more crowded in wave – clouds collapse to form new stars More collisions in the traffic jam There are slightly more old stars in the arm too, because they speed up slightly coming into it and slow down slightly moving out of it. But the best tracers are the things that mark recent cloud collapses: O,B stars, etc.
12	Self Sustaining Star Formation Cloud collapse Þ New stars New stars Þ Supernova after few million years Supernova Þ Shock Waves Shock Waves Þ Nearby clouds collapse Differential Rotation twists pattern into spiral
13	Two limiting cases of spirals Grand Design: Density Wave Flocculent: Self Sust. Star Form. + Diff. Rot. In most Galaxies you have some combination of the two
14	The Nucleus of the Galaxy Likely Black hole High velocities Large energy generation At a=275 AU P=2.8 yr Þ 2.7 million solar masses Radio image of Sgr A about 3 pc across, with model of surrounding disk
15	A movie of stars at the core www.mpe.mpg.de/www_ir/GC Very cool, and worth a look! This is the best evidence to date for a massive black hole at the Galactic core. Now essentially “proven.”