Download Our Galaxy -- The Milky Way PowerPoint

23. The Milky Way Galaxy
• The Sun’s location in the Milky Way galaxy
• Nonvisible Milky Way galaxy observations
• The Milky Way has spiral arms
• Dark matter in the Milky Way galaxy
• Density waves produce spiral arms
• Infrared & radio galactic nucleus observations
Our View of the Milky Way
The Sun’s Location in Our Galaxy
• William Herschel’s observations
– The Solar System is disk-shaped
• Faint stars cluster in a band extending in all directions
– The Solar System is near the galactic center
• The number of stars is about the same in all directions
• William Herschel’s nemesis
– Interstellar extinction
• Interstellar dust obscures more distant stars
• Dark regions in the Milky Way are obscured, not empty
• A telltale phenomenon
– Globular clusters surround the galactic center
• Spherical distributions of ~ 106 stars
– Globular clusters orbit the galactic center
• Period-luminosity relationship of Cepheid variables
William Herschel’s Milky Way Map
Cepheid Period-Luminosity Relationship
RR Lyrae Light Curves
The Milky Way’s Numbers
• Disk
– Flat disk ~ 5.0 . 104 pc
in diameter
– Earth is ~ 8.0 . 103 pc from the galactic center
– Greatest abundance of stars
• Primarily bright young stars, especially OB associations
• Abundant new star formation
• Central bulge
– Approximately spherical distribution of stars
• Primarily dim old stars
• Little new star formation
• Halo
– Globular clusters
• Approximately spherical distribution of star clusters
Nonvisible Observations of Our Galaxy
• Basic physical processes
– Selective scattering of EMR
• Short visible wavelengths are scattered most
• Long IR & radio wavelengths are scattered least
– Development of non-optical telescopes
• Radio
telescopes
• Thermal IR telescopes
– Near infrared
– Far infrared
Wavelengths relatively close to the visible
Wavelengths relatively far from the visible
• Some benefits
– Ability to see all of the Milky Way galaxy
• Central bulge
• Distribution of interstellar gas clouds
The Infrared Milky Way
Far-Infrared View
(25 µm, 60 µm, 100 µm)
Near-Infrared View
(1.2 µm, 2.2 µm, 3.4 µm)
The Milky Way Galaxy Edge-On
The NGC 4565 Galaxy Edge-On
Our Galaxy Has Spiral Arms
• Observations of other galaxies
– Many disk-shaped galaxies have spiral arms
• Number
of spiral arms varies
• Distinctness of spiral arms varies
– Tentative conclusion
• The disk-shaped Milky Way may have spiral arms
• Observations of the Milky Way galaxy
– Neutral hydrogen proton-electron spin-flip transitions
• Small energy difference between two possible states
• Produces an emission line at the 21 cm radio wavelength
– Neutral hydrogen strongly concentrated in the disk
• Doppler shift of various nebulae reveals arm structure
– Four major spiral arms
– The Solar System is in the small Orion arm
Our Galaxy Has Spiral Arms
Milky Way rotation
Proton-Electron Spin-Flip in Hydrogen
The Entire Sky at 21 Centimeters
Neutral Hydrogen in the Milky Way
M83 Galaxy at Three Wavelengths
Visible
Near-Infrared
21 Centimeter
The Milky Way Galaxy Face-On
The Milky Way’s Dark Matter
• Basic observations
– Stars & nebulae orbit the galactic center
• Identical to the pattern in the Solar System
– Orbital mechanics
• Keplerian
orbits
Speed decreases with distance
– Farthest planets in the Solar System have slowest orbital speeds
• Non-Keplerian orbits
Speed is almost constant
– Milky Way’s rotation curve is nearly constant
– Sun’s speed around galactic center is ~ 7.9 . 105 km . hr–1
– Sun’s trip around galactic center is ~ 2.2 . 108 yrs
• Basic conclusion
– Most of the Milky Way’s mass is beyond the Sun
• The visible mass cannot account for this mass
• Much of the mass beyond the Sun is “dark matter”
Possible Forms of Dark Matter
• Massive compact halo objects
MACHOs
– Very dim stars between 0.01 & 1.0 M☉
– Gravitational bending of light has been observed
• Tentatively, MACHOs account for < 40% of dark matter
• Known subatomic particles
– Neutrinos, now known to have mass
• Weakly interacting massive particles
WIMPs
– Predicted mathematically but not yet observed
• Masses 10 to 10,000 times the mass of a neutron
The Milky Way’s Rotation Curve
Microlensing by Halo Dark Matter
Density Waves Produce Spiral Arms
• The winding dilemma
– No spiral galaxies revolve like a solid disk
• This is not too far from the case
• Any difference in rotation rate tends to destroy arms
– All spiral galaxies have persistent arms
• Density waves
One possible explanation
– Waves are similar to those on ocean surfaces
• Propagation in slightly different directions
• Constructive & destructive wave interference
– Constructive interference prone to extensive star formation
– Destructive interference prone to minimal star formation
– Waves are
relatively short-lived
• Many stars in spiral arms are OB associations
– Very massive & short-lived at 3 to 15 million years
» Only ~ 5% the rotation period of the Milky Way
– Very prone to inducing additional compression & star formation
Problems With the Density Wave Model
• A driving mechanism to continue density waves
– Basic issues
• A matter of space
• A matter of time
Huge distances are involved
Huge time periods are involved
– One possibility
• Barred spirals have asymmetrical gravitational fields
– One problem
• Most spiral galaxies are not barred spirals
– Another possibility
• Tidal influences of neighboring galaxies
• Types of spiral galaxies
– Grand design spirals
Classic spiral forms
• The density wave model fits these galaxies well
– Flocculent
spirals
Fuzzy
spiral forms
• The density wave model fits these galaxies poorly
Grand-Design & Flocculent Spirals
Infrared & Radio Observations
• The Milky Way’s nucleus
– Extremely crowded with stars
• One million stars as bright as Sirius
• As bright as 200 full moons
– Dominated by a feature named Sagittarius A
• Powerful source of synchrotron radiation
– Relativistic electrons spiraling in intense magnetic fields
– Contains
a feature named Sagittarius A*
• Thought to be the galactic center
– Brightest radio source in its vicinity
• The mysterious identity of Sagittarius A*
– Not a star
Too energetic
– Not a pulsar
Too energetic
– Not a supernova remnant
Not expanding
– Maybe a small supermassive black hole ~ 106 M☉
Nucleus of the Milky Way Galaxy
Stars Orbiting the Galactic Center
Important Concepts
•
An historic perspective
•
– William Herschel’s Milky Way map
– Rotation curve stays nearly constant
• Definite disk shape
• Same number of stars in all directions
• Much unseen mass lies beyond Sun
– Three major possibilities
– The problem: Interstellar extinction
• MACHOs
• Neutrinos & other known particles
• WIMPs
• Dust clouds scatter & absorb light
– The solution: Globular clusters
• Cepheid variables give distance
•
•
A modern perspective
– A thin disk
• Sun
• Constructive & destructive interference
– Causal mechanism is unclear
8 kpc from center
– A central bulge ~ 0.6 kpc high
– A halo dominated by globular clusters
•
Non-visible telescopic observations
– Radio l’s
– Infrared
•
Production of the spiral arms
– The density wave model
~ 50 kpc in diameter
~
Dark matter
21 cm spin-flip line
Near- & Far-IR l’s
Spiral arms
– Confirmed by 21 cm observations
– 4 major & several minor arms
• Asymmetry in barred spirals
• Tidal effects from nearby galaxies
•
The Milky Way’s nucleus
– Viewed in radio & IR l’s
– Sagittarius A & Sagittarius A*
• Small supermassive black hole