Spiral Structure of the Milky Way Mapping of the spiral structure of the Milky Way ¾ Spiral arms look strung out along the line of sight ¾ Different parts overlapped ¾ Need distances to bright spiral structure tracers Local Arm ¾ Our understanding entirely depends on observational data. Cygnus Arm Perseus Arm Carina – Sagittarius Arm Norma Arm Outer (distant) spiral arms
Young Stellar Clusters and Galactic Structure Moffat and Vogt (1974)
Figure 72.01 The spiral structure of the Milky Way
Carina-Sagittarius Norma-Centaurus or Norma-Scutum
1 Figure 72.02b Figure 72.06 Various Types of Interstellar Matter
Reddish nebulae: glow with light emitted by hydrogen atoms HII regions in the MW
Darkest areas – clouds of dust
Bluish glow – reflected light
Radio waves from Location of the interstellar dust cold interstellar gas
•30-300 µm : 10-90 K : distribution of warm dust •Disk (40 kpc (50 kpc ?) : 0.6 kcp); bulge (diameter 2 kpc); bar Cold Hydrogen clouds: Spin-flip transition
2 Location of the HI regions A Map of Neutral Hydrogen in the Milky Way
Leiden-Dwingeloo Survey of Galactic Neutral Hydrogen using the Dwingeloo 25-m radio telescope
Figure 72.10 Giant molecular clouds as tracers of Radio waves from molecular clouds Galactic structure
Molecular clouds: cold (10K) dense regions containing molecules: H2, CO and others.
3 The Milky Way The revolution of the Sun around Overview: the Galactic center
•Number of Stars •Mass 2π d •Shape P = •Size v •Age •Sun’s location
•First ideas about MW structure How do we know d and v?
The revolution of the Sun M83 around the Galactic center Hydra Mapping the MW disk in visible light
4π 23d P2 = Gm()+ M 4π 23d P2 = GM 2π d P = Mass of v material inside Sun’s vd2 M = orbit: G 100 billion Solar masses
4 • The predicted and observed rotation curve of a typical spiral galaxy
Structure of galactic disk and halo Density of visible matter Density of non-luminous matter
5 Inner 0.3 Light Years (24,000AU) Infrared Pictures Inner 6 Light Years Can see stars move over time Infrared Picture (must be moving fast!) Packed With Stars! 100 000 stars per cubic parsec Mass required to hold stars in orbit: 2.6 Million M Sun
The Sgr A* Radio Source:
Less than 10 AU across
Emissions from charged Sgr A* particles in B.H. accretion disk
Ring of Gas Around Sgr A* T ~ 300K Ring of Gas Around Sgr A* T ~ 300K From Motion of Gas: 2-8 pc across 2.6 Million MSun inside gas orbit From Motion of Gas: 2.6 Million M inside gas orbit Sun
6 X-Ray Mosaic Of Galactic Center:
The nature of spiral arms Winding Dilemma M 83 • Rotation of material about galactic center – differential rotation • Are the spiral arms “ material arms” ?
• Spiral arm tracers
• Overall stellar density within and between spiral arms
• Spiral arms outlined by massive O&B stars
• Star-formation takes place only in spiral arms
7 Fig. 22.37 The spiral structure is a wave Density Waves phenomenon
¾A wave is a disturbance of a medium which transports energy through the medium without permanently transporting matter
¾Spiral arms are made up of different material at different Waves in a Guitar String times Density waves
Fig. 22.38 Galactic density waves are a consequence of gravity
• Density of matter is higher in the spiral arms • Within the spiral arms additional gravitational influence is exerted on stars and ISM • In spiral arms matter moves slow and piles up • Increase in density, change of direction of motion • Not “material” arms • Entire spiral pattern rotates with the same orbital period (500 mil yr)
8 Star-formation within the spiral arms How do spiral arms get started?
• Small irregularities can grow into a full spiral pattern • Spiral structure is produced by tidal influence of a companion galaxy • Gravitational phenomena that cause formation of stars
Self-Propagating Star-Formation Model (triggered star formation)
Sa Sb Sc
Star-formation triggered by supernova explosion and strong stellar wind of massive OB stars
SBa SBb SBc http://www.mpifr-bonn.mpg.de/staff/tpreibis/
9 Star- Scorpius – Centaurus Association forming activity near Sun
Scorpius – Centaurus Association
http://www.mpifr-bonn.mpg.de/staff/tpreibis/
Sequentially triggered star- Sequentially triggered star- formation in OB associations formation in OB associations
5 Myr ago Massive stars formed Shock wave reached Upper Sco Wind from massive stars halted SF SF began
15 Myr upper Cen-Lup has been formed
12 Myr ago – SN explosion
Large shock wave
http://www.mpifr-bonn.mpg.de/staff/tpreibis/ http://www.mpifr-bonn.mpg.de/staff/tpreibis/
10 Sequentially triggered star- Sequentially triggered star- formation in OB associations formation in OB associations
1 Myr ago Shock wave reached ρ Oph molecular cloud Ongoing SF 1.5 Myr ago - SN explosion Molecular cloud fully dispersed
http://www.mpifr-bonn.mpg.de/staff/tpreibis/ http://www.mpifr-bonn.mpg.de/staff/tpreibis/
The Upper Scorpius association and ρ Ophiuchus Figure 73.08 molecular cloud Self-Propagating Star-Formation
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