23. the milky way galaxy the sun’s location in the milky way galaxy nonvisible milky way galaxy...
TRANSCRIPT
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 Wavelengths relatively close to the visible
– Far infrared 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 WayFar-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
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 MSun
– 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 Huge distances are involved• A matter of time 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 MSun
Nucleus of the Milky Way Galaxy
Stars Orbiting the Galactic Center
• An historic perspective– William Herschel’s Milky Way map
• Definite disk shape• Same number of stars in all directions
– The problem: Interstellar extinction• Dust clouds scatter & absorb light
– The solution: Globular clusters• Cepheid variables give distance
• A modern perspective– A thin disk ~ 50 kpc in
diameter• Sun ~ 8 kpc
from center– A central bulge ~ 0.6 kpc
high– A halo dominated by globular clusters
• Non-visible telescopic observations– Radio l’s 21 cm spin-flip
line– Infrared Near- & Far-IR
l’s• Spiral arms
– Confirmed by 21 cm observations– 4 major & several minor arms
• Dark matter– Rotation curve stays nearly constant
• Much unseen mass lies beyond Sun– Three major possibilities
• MACHOs• Neutrinos & other known particles• WIMPs
• Production of the spiral arms– The density wave model
• Constructive & destructive interference– Causal mechanism is unclear
• 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
Important Concepts