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 11.