Milky Way Galactic Structure and Dynamics Milky Way

Milky Way: Galactic Structure and Dynamics • • Milky Way has spiral structure Galactic Bulge surrounds the Center Powerful radio source Sagittarius A at Center Contains Super-Massive Black Hole M(SMBH) ~ 106 M(Sun)

Milky Way: Bright Band Across Sky (Resolved by Galileo)

Milky Way: Galactic Structure

Our Galaxy – Milky Way • Spiral galaxy, e. g. Milky way, Andromeda • Sun is located in the disk, but far away from the center, about 8000 parsecs (1 pc = 3. 28 LY) • MW appears as a bright band across the sky • Determine the size of the galaxy from RR Lyrae stars in globular clusters that lie above the plane of the galaxy (a globular cluster has about a million stars) • Period-Luminosity relation for Cepheid variables and RR Lyrae gives the magnitude M of absolute luminosity L • Distance modulus relation relates the magnitude m of the apparent luminosity to distance d at 10 pc m – M = 5 log (d/10)

Shape and Constituents of the Galaxy • Most of the matter in the Galaxy is not visible (at visible wavelengths) • But observations in other wavelength ranges, such as the IR and Radio reveal most of the matter • Matter too cold to emit shorter wavelength radiation • Spiral arms have hot O and B stars, with ionized H II regions, but interstellar matter is molecular H 2 or atomic H I • H I is traced by a “hyper-fine structure” transition due to magnetic interaction between spins of the electron and the proton, at radio wavelength = 21 cm

The Hydrogen 21 -cm radio map of the Sky and the Galaxy

H I 21 cm Hyperfine Transition

Kepler’s 2 nd law: velocity varies inversely with distance from center 3 rd Law: P 2 = a 3 longer period at larger distance; lower velocity

Mass, Velocity, Radius of a Galaxy • Mass at radius R has rotational velocity given by this relation • Rotational velocity should decrease with R, since M should decrease with R

Measuring Rotational Velocities – Doppler Effect

Measure velocities using Doppler Effect

Dark Matter • Why are rotational velocities constant, in violation of Kepler’s laws for gravitationally bound orbits ? • The Sun’s period P around the center of MW is related to the mass inside the Sun’s orbit (mass outside should have no effect on Kepler’s Laws) • If the rotational velocity of matter (H I) is the same in the outer regions of the galaxy, then there must be unseen matter outside the Sun’s orbit – Dark Matter ! • Estimates indicate more than 90% is Dark Matter (as much dark matter as visible). NO CLUE !!!

Galactic Structure: Spiral Arms – Density Waves • Spiral arms made of stars, dust, and gas should dissipate after some time, arms should unwind • But they maintain a stationary pattern like the spokes on a wheel • Gravitational density waves through galactic matter, propagating like water waves; matter itself is not dislocated after the wave disturbance passes (periodically) • Concentration of matter in spiral arms leads to formation of massive O, B stars that are short-lived

Galactic Dynamics and Spiral Structure

Slower traffic creates gaps

Stellar Populations and Ages • Population I (Pop I) – metal-rich, young, massive, bright stars found in the spiral arms, e. g. Cepheid variables (“metals” – all elements besides H, He); the Sun is an intermediate Pop I star • Pop II – old, metal-poor stars found in the halo and globular clusters above the plane of the galaxy (e. g. RR Lyrae stars) • The Bulge of the galaxy has a mixture of Pop I and Pop II stars • Pop III are metal-poor stars in the early Universe, predominantly primordial H and He

Center of our Galaxy • A massive Black Hole ? How do we know ? • Extremely luminous, indicating huge concentration of interacting matter • Observations of stellar orbits around the Center – must be moving extremely fast ! Can calculate mass at the center of the orbit using modified Kepler’s third law

Sagittarius A – Galactic Center Radio Source Not visible due to dust extinction

Stellar Orbits Around the Sgr A Kepler’s laws and Law of gravitation yield Super-Massive Black Hole mass

Mass of the Black Hole at the Center of Milky Way? Mass enclosed within a volume at a given distance: Mo/pc 3