Astronomy 114. Lecture 29: Internal Properties of Galaxies. Martin D. Weinberg. UMass/Astronomy Department

Transcription

1 Astronomy 114 Lecture 29: Internal Properties of Galaxies Martin D. Weinberg UMass/Astronomy Department A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 1/16

2 Announcements Exam #2 redux: due Friday (27 Apr) PS#7: due next Monday (30 Apr) A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 2/16

3 Announcements Exam #2 redux: due Friday (27 Apr) PS#7: due next Monday (30 Apr) Questions on Exam #2? Dynamics of Galaxies... Galaxies, Chap. 26 A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 2/16

4 Population I Location: Age: Composition: Environment: Disk and Open Clusters Mix of young and old stars Metal rich (roughly solar composition) Often gas rich, especially for the young stars 70% Hydrogen 28% Helium 2% "metals" A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 3/16

5 Population II Location: Spheroid and Globular Clusters Ages: Oldest stars, >10 Gyr Composition: Metal Poor (0.1-1% solar) Environment: gas poor, no star formation 75% Hydrogen 24.99% Helium 0.01% metals A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 4/16

6 Stellar Kinematics Disk Stars: Ordered, roughly circular orbits in a plane. All orbit in the same general direction. Orbit speeds similar at a given radius. Spheroid Stars: Disordered, elliptical orbits at all inclinations. Mix of prograde and retrograde orbits Wide ranges of orbital speeds. A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 5/16

7 Stellar Kinematics A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 6/16

8 Major clue to formation of galaxy Spheroid: Old, formed first No gas left to settle onto disk Disk: Formed later Gas settled on disk Continues to form stars A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 7/16

9 Major clue to formation of galaxy Spheroid: Old, formed first No gas left to settle onto disk Disk: Formed later Gas settled on disk Continues to form stars Both populations of stars can be used to measure mass of Milky Way... A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 7/16

10 Dark matter! Problems: Flat rotation curve M(r) r Disk ends at 16 kpc Mass of galaxy appears to increase with radius even beyond the visible disk! Brightness from stars, gas accounts for only 10% of mass implied by rotation A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 8/16

11 Dark matter! Problems: Flat rotation curve M(r) r A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 8/16

12 Dark matter! Problems: Flat rotation curve M(r) r Disk ends at 16 kpc Mass of galaxy appears to increase with radius even beyond the visible disk! Brightness from stars, gas accounts for only 10% of mass implied by rotation 90% of the mass in the Galaxy is unobserved! A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 8/16

13 Missing mass: unsolved mystery What, where is this stuff? Dark, compact objects: MACHOS (MAssive Compact Halo ObjectS) Microlensing A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 9/16

14 Missing mass: unsolved mystery What, where is this stuff? Dark, compact objects: MACHOS (MAssive Compact Halo ObjectS) Microlensing A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 9/16

15 Missing mass: unsolved mystery What, where is this stuff? Dark, compact objects: MACHOS (MAssive Compact Halo ObjectS) Microlensing Some undiscovered subatomic particle (neutralino) Change laws of physics??? Modify gravity A114: Lecture Apr 2007 Read: Ch. 26 Astronomy 114 9/16

16 Spiral arms Wave pattern occur in the disk of the Milky Way and other galaxies Earliest thought: material waves A114: Lecture Apr 2007 Read: Ch. 26 Astronomy /16

17 Spiral arms Wave pattern occur in the disk of the Milky Way and other galaxies Earliest thought: material waves A114: Lecture Apr 2007 Read: Ch. 26 Astronomy /16

18 Density Waves Not material waves:densitywaves Example: waves in traffic flow Cars bunch together and spread out as a "density wave" passes through traffic Slow vehicle, line painting A114: Lecture Apr 2007 Read: Ch. 26 Astronomy /16

19 Solution to thewinding problem The orbits of stars are not quite circles but ellipses Where orbits bunch, gravity is enhanced Extra gravitycorrelates the orbits and prevents the wave from wrapping quickly A114: Lecture Apr 2007 Read: Ch. 26 Astronomy /16

20 Young stars in the arms The spiral density waves cause a density enhancement which triggers star formation In a galaxy like the Milky Way there will be a progression of star formation across the spiral arm. Gasfalls into arm, compresses Forms molecular clouds Forms stars Asssociations and clusters trail arm A114: Lecture Apr 2007 Read: Ch. 26 Astronomy /16

21 Example: M51 Note location of O-B stars A114: Lecture Apr 2007 Read: Ch. 26 Astronomy /16

22 Halo stars Pop II stars compose two areas of galaxy: Halo - spherical distribution of stars and Globular clusters. Halo radius 3-5 kpc. Bulge - dense swarm of stars centered on the Galactic center. Bulge radius 1.5 kpc, height 0.7 kpc Very little gas and dust. Randomly tipped elliptical orbits. A114: Lecture Apr 2007 Read: Ch. 26 Astronomy /16

23 Interesting and Puzzling Aspects Rotation curve: The Milky Way has a differential rotation, but the rotation curve showing speeds at different distances from the center cannot be explained by the matter we see. Anomalous motions: Some packs of stars move on paths unrelated to that of the rest of the galaxy s stars; They seem to be remnants of small galaxies absorbed by the Milky Way. Immediate surroundings: There is a giant ring of several hundred million stars outside the galactic disk, high-velocity clouds in anomalous motions, and small satellite galaxies. A114: Lecture Apr 2007 Read: Ch. 26 Astronomy /16