The Milky Way Galaxy • Introduction • Structure • Formation • Neighborhood

Introduction In greek mythology: Milky Way = milk of Hera feeding Heracles 1610: Galileo realizes that the milky aspect is due to the accumulation of millions of stars Original definition: dim glowing band arching across the night sky whose individual stars cannot be distinguished by the naked eye Disk-shaped structure of stars, gas and dust (galactic disk) in which the solar system is located → viewed from inside 360° panorama of the Milky Way as seen from Earth

Modern definition: the Milky Way = our galaxy Introduction - 2 Modern definition: the Milky Way = our galaxy 1785: William Herschel tries to determine its shape by star counts Drawbacks: • distances to stars unknown • solar system assumed close to the center Our galaxy’s shape as determined by Herschel 1930: Robert Trumpler studies the effect of dust (extinction) and gets a much more accurate view of our galaxy

Global properties • Contains ~200 billion stars + gas and dust Introduction - 3 Global properties • Contains ~200 billion stars + gas and dust • `Visible´ mass ~ 1011 M • Type: barred spiral • Diameter ~ 100 000 L.Y. • Center in the direction of the Sagittarius constellation • Differential rotation • The Sun is located ~ 26 000 L.Y. from the center, orbits in ~ 220 million years (already completed ~ 20 orbits) Position of the Sun in the Milky Way

Structure • Galactic bulge (includes the galactic center) • Galactic halo globular clusters dwarf galaxies • Galactic disk thin disk + thick disk open star clusters spiral arms

The galactic bulge The galactic center • Diameter ~ 7000 L.Y. Structure - 2 The galactic bulge • Diameter ~ 7000 L.Y. • Contains 5% of the `visible´ mass, mostly old stars • Hard to observe because of dust in the galactic plane The galactic center • Densely populated in stars • Strong magnetic field (synchrotron radiation – see AGN chapter) Towards galactic bulge in Sagittarius

Structure - 3 The galactic center (2) Highly energetic environment: distorted gas clouds, hot gas, arcs, plasma filaments, supernova remnants Radio image of the galactic center Orbits of stars around the center: → central mass ~ 2.6 106 M with size < a few light-months → supermassive black hole very likely IR image of the galactic center

The galactic disk Contains ~ 90 % of the visible matter in our galaxy Structure - 4 The galactic disk Contains ~ 90 % of the visible matter in our galaxy Flat structure of diameter ~ 70 000 L.Y. Thin disk Young stars often found in open clusters (Pop I, thickness ~ 2000 L.Y.) Giant molecular clouds and dust (thickness ~ 200 L.Y.) Thick disk Progressive dispersion of think disk stars (collisions) Atomic hydrogen extending over 100 000 L.Y. diameter

Open clusters Clusters of ~ 100 to 1000 young stars bound by gravity Structure - 5 Open clusters Clusters of ~ 100 to 1000 young stars bound by gravity Diameter ~ 100 L.Y. Sometimes not very concentrated → hard to identify Stars born from the collapse of a giant molecular cloud → same age same chemical composition progressively dispersed in the disk The Pleiades open cluster

Structure - 6 The galactic halo Spheroidal structure, much more extended and less dense than the bulge Contains ~ 5 % of visible matter in our galaxy Oldest stars in our galaxy Orbits with any inclination with respect to the disk Negligible global rotation → `high velocity stars´ with respect to the Sun + globular clusters and dwarf galaxies

Structure - 7 Globular clusters Gravitationally bound structures, ± spherically symmetric Formed in the very first stages of the galactic evolution Among the oldest objects in the Universe Contain ~105 old stars each (Pop II) ± spherical distribution ~ 100 L.Y. in diameter ~ 200 have been indentified Contain ~1% of halo stars (the other 99% are field stars, not belonging to clusters) Globular cluster M80

Structure - 8 Dwarf galaxies Some small satellite galaxies are tidally disrupted by the gravitational field of the Milky Way Their stars are progressively stripped away and added to the halo stars (`cannibalism´) → form a 2nd (younger) halo population `Stellar streams´ of tidally stripped stars are found along the orbits of currently or recently disrupted galaxies Stellar stream in the Milky Way halo

Spiral arms 2 main spiral arms identified in 1951 by William Morgan Structure - 9 Spiral arms 2 main spiral arms identified in 1951 by William Morgan 2 additional ones identified in 1976 by Yvon and Yvonne Georgelin Evidence for a bar extending over ~ 10 000 L.Y. Structure determined from radio/optical/UV observations of tracers Main difficulty: determine their distance Sketch of the Milky Way spiral structure

Structure - 10 Origin of spiral arms First idea: effect of differential rotation (rejected since arms would be much more tightly wound after ~20 rotations) Best hypothesis: density wave that propagates at a speed different from the stars and gas Origin of the density wave: – gravitational instability caused by differential rotation? – tidal effect from satellite galaxies? – chaotic phenomenon? Spiral galaxy similar to the Milky Way

Why are there tracers in the spiral arms? Structure - 11 Why are there tracers in the spiral arms? Density wave and matter do not propagate at the same velocity When a molecular cloud crosses an overdensity region → compression → collapse of the cloud → star formation Massive stars → HII regions Short-lived stars → already at the end of their life when the density wave moves away → tracers disappear

Structure - 12 Rotation of the disk In the outer regions, the rotation velocity is ~ constant while the predictions based on visible matter would imply a Keplerian decrease → two possibilities: (1) the gravitation theory is wrong in these `extreme´ conditions (a <<) (2) there is more mass than what we can `see´ Most popular hypothesis: dark matter halo composed of some kind of WIMPs (Weakly Interacting Massive Particles) Rotation curve of the Milky Way

Structure - 13 The dark matter halo Most specialists consider that our Galaxy is embedded in a huge dark matter halo, made of massive fundamental particles interacting only via gravitation (and maybe the weak force) Total mass: ~ 5 1011 M Luminous (ordinary, baryonic) matter: ~ 1011 M Dark matter: ~ 4 1011 M Dark halo size: ~ 600 000 L.Y. Sketch of our Galaxy’s structure

Formation of our Galaxy • Giant gas cloud of primordial (Big Bang) matter: H + He +… • Gravitational collapse with simultaneous collapse of smaller parts (→ globular clusters and field halo stars: old stars, low metallicity) • The remaining gas continues collapsing and flattens because of conservation of angular momentum → formation of the disk • Density waves appear in the disk and give birth to a 2nd, 3rd… stellar generations • Successive stellar generations enrich the interstellar gas in metals (stellar winds, supernova explosions…) → newborn disk stars have higher and higher metallicities