21. The Milky Way Galaxy

Transcription

1 21. The Milky Way Galaxy The Milky Way Galaxy c Take a giant step outside the Milky Way Artist's Conception Example (not to scale) b a d 1

2 Supermassive (3x10 6 M ) Black Hole in the Galactic Center Perseus arm Orion arm Sun from above ("face-on") see disk and bulge Carina arm Cygnus arm from the side ("edge-on") Another galaxy: NGC The Milky Way roughly resembles it. M51 2

3 The Tree Main Structural Components of the Milky Way 1. Disk 30,000 pc diameter (30 kpc) Contains young and old stars, gas, dust. Has spiral structure. Vertical thickness roughly 100 pc -2 kpc (depending on component). Most gas and dust in thinner layer, most stars in thicker layer. 3. Bulge About 4 kpc across Old stars, some gas, dust Central black hole of 3x10 6 M " Spherical 2. Halo At least 30 kpc across Contains globular clusters, old stars, little gas and dust, much dark matter Roughly spherical Shapley (1917) found that Sun was not at center of Milky Way Precise Distance to Galactic Center Distance = / kpc SgrA* Shapley used distances to variable RR Lyrae stars (a kind of Horizontal Branch star) in Globular Clusters to determine that Sun was 16 kpc from center of Milky Way. Modern value 8 kpc. Eisenhauer et al Orbital motion 6.37 mas/yr 3

4 Stellar Orbits Clicker Question Where is out solar system located? A. Near the center of the milky Way Galaxy in the bulge. B. 4 kpc from the center of the Milky Way in the halo. C. 8 kpc from the center of the Milky Way in the disk. Halo: stars and globular clusters swarm around center of Milky Way. Very elliptical orbits with random orientations. They also cross the disk. Bulge: similar to halo. Disk: rotates. D. 20 kpc from the center of the Milky Way in the disk. Clicker Question What lurks at the center of our galaxy? A. A 3 million solar mass black hole. B. A giant star cluster. C. A 30 solar mass black hole. D. Darth Vader Rotation of the Disk Sun moves at 220 km/sec around center. An orbit takes 240 million years. Stars closer to center take less time to orbit. Stars further from center take longer. => rotation not rigid like a phonograph record or a merry-go-round. Rather, "differential rotation". Over most of disk, rotation velocity is roughly constant. The "rotation curve" of the Milky Way 4

5 Spiral Structure of Disk Spiral arms best traced by: Young stars and clusters Emission Nebulae HI Molecular Clouds (old stars to a lesser extent) Disk not empty between arms, just less material there. Problem: How do spiral arms survive? The spiral should end up like this: Given differential rotation, arms should be stretched and smeared out after a few revolutions (Sun has made 20 already): The Winding Dilemma Real structure of Milky Way (and other spiral galaxies) is more loosely wrapped. 5

6 Proposed solution: Arms are not material moving together, but mark peak of a compressional wave circling the disk: A Spiral Density Wave Traffic-jam analogy: Traffic jam on a loop caused by merging Now replace cars by stars and gas clouds. The traffic jams are actually due to the stars' collective gravity. The higher gravity of the jams keeps stars in them for longer. Calculations and computer simulations show this situation can be maintained for a long time. 90% of Matter in Milky Way is Dark Matter Gives off no detectable radiation. Evidence is from rotation curve: Molecular gas clouds pushed together in arms too => high density of clouds => high concentration of dust => dust lanes. Also, squeezing of clouds initiates collapse within them => star formation. Bright young massive stars live and die in spiral arms. Emission nebulae mostly in spiral arms. So arms always contain same types of objects, but individual objects come and go. Rotation Velocity (AU/yr) Milky Way Rotation Curve Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian") 1 10 R (AU) observed curve Curve if Milky Way ended where visible matter pretty much runs out. 6

7 Not enough radiating matter at large R to explain rotation curve => "dark" matter! Dark matter must be about 90% of the mass! Composition unknown. Probably mostly exotic particles that don't interact with ordinary matter at all (except gravity). Some may be brown dwarfs, dead white dwarfs Most likely it's a dark halo surrounding the Milky Way. Mass of Milky Way 6 x solar masses within 40 kpc of center. More Evidence for Dark Matter - Abell 1689 Dark Matter halos At large radii there is little starlight. There is 5-10 times as much dark matter associated with galaxies, as ordinary matter. 7

8 Dark Matter candidates MACHOs (Massive compact halo objects) Brown dwarfs (low mass stars) White dwarfs (burn-out stars) Neutron stars (dead stars) Stellar black holes (dead stars) mini (primordial) black holes massive (primordial) black holes WIMPS (Weakly interacting massive particles: neutrons, axions, etc). Clicker Question How long does it take our solar system to orbit once around the Milky Way? A. 1 year. B. 2 million years. C. 240 million years. D. 250 billion years (longer than the age of the Universe). Clicker Question What makes up most of the mass (90%) of the Milky Way Galaxy? A. Hydrogen gas. B. Stars. C. Dead stars (white dwarfs, neutron stars and black holes). D. We don t know. 8

9 4/12/12 The Galactic Center Up to 30 mag extinction in the optical (only 1 in 1012 photons reaches us!)" Solar motion up through the disk will give us a clear view in 15 Myrs" Meanwhile, forced to work at radio, IR, X-ray and gamma ray wavelengths." Seeing into the center of the Milky Way GC is about 8 kpc away, at a declination of -29, well in the southern hemisphere. Most information we have about the Galactic Center region is thus from IR and radio: 1kpc: expanding gas at 100km/s (explosive event 10Myr ago?) 100 pc: Sgr A (strongest radio source on the sky). 10 pc: pinwheel gas structure <1 pc: Sgr A*, pointlike radio/x-ray/ir source It is very busy, as shown by this 1x1 kpc VLA map. Nucleus: Supermassive black hole ~3x106 M Stars are very tightly packed in the central region, reaching densities of up to 107 M pc-3 near the Sun the density is only about 0.05 M pc-3 in globular clusters the central density reaches 104 M pc-3 => Stellar collisions probably frequent (1 every 106 yrs). 9

10 Moving closer to the center, the picture gets more complex. Seeing into the center of the Milky Way The center-most source is called Sgr A, divides up into Sgr A West Sgr A East Sgr A * SgrA * is extremely compact, and is almost certainly due to the central black hole itself. Sgr A West appears like a mini spiral in an image at 6cm - complicated structure in the ionized gas. Scale of central region about 0.3 pc, IR (1.6, 2.2 and 3.8 microns). Sgr A * located near the center of image (not bright in IR). Most of these stars are very young and massive and heavily reddened so they don't appear blue. Spectroscopic studies indicate that the stars are luminous super giants and only a few 10s of millions years old. Consequently, IR stars can be observed very close to thecenter of the Galaxy. They move so fast (500 to 1000 km/s) that their proper motions are measurable even after a few years. In recent years even their orbits around the central massive source has been detected

11 Is the black hole the dynamical center? Supporting evidence comes from the proper motion of Sgr A *. 4/17: Reading Assignment Chapter : Galaxies At 8 kpc from us its proper motion is very small, about 10 mas/yr. It moves at a straight line across the sky. The amount turns out to exactly reflect the motion of the Sun around the Galaxy => Sgr A * is clearly the Milky Way's dynamical heart. 11