Milky Way: Galactic Structure and Dynamics

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Milky WayGalactic 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)

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Milky Way Bright Band Across Sky(Resolved by
Galileo)
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Milky Way Galactic Structure
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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)

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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

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The Hydrogen 21-cm radio map of the Sky and the
Galaxy
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H I 21 cm Hyperfine Transition
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Keplers 2nd law velocity varies inversely with
distance from center 3rd Law P2 a3 ? longer
period at larger distance lower velocity
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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

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Measuring Rotational Velocities Doppler Effect
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Measure velocities using Doppler Effect
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Dark Matter

• Why are rotational velocities constant, in
  violation of Keplers laws for gravitationally
  bound orbits ?
• The Suns period P around the center of MW is
  related to the mass inside the Suns orbit (mass
  outside should have no effect on Keplers 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 Suns
  orbit Dark Matter !
• Estimates indicate more than 90 is Dark Matter
  (as much dark matter as visible). NO CLUE !!!

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Galactic StructureSpiral 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

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Galactic Dynamics and Spiral Structure
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Slower traffic creates gaps
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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

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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
  Keplers third law

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Sagittarius A Galactic Center
Radio Source Not visible due to dust extinction
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Stellar Orbits Around the Sgr A
Keplers laws and Law of gravitation yield
Super-Massive Black Hole mass
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Mass of the Black Hole at the Center of Milky Way?
Mass enclosed within a volume at a given
distance Mo/pc3