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) ~ 10 6 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)

Period-Luminosity Relation: Pulsating Cepheid, RR Lyrae Stars

Apparent Magnitude (m) vs. T(d)

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 H2 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

a. Constant Velocity with radius b. Velocity increases with radius c. Kepler ’ s third law P 2 = a 3, says that outer orbits must have longer periods and slower velocities Velocity and Radius

Observed and Keplerian Rotation Curves

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) 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: M o /pc 3