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 ~ 10 6 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 ~ pcin diameter –Earth is ~ pcfrom 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 Shortvisiblewavelengths are scattered most LongIR & radiowavelengths are scattered least –Development of non-optical telescopes Radiotelescopes Thermal IRtelescopes –NearinfraredWavelengths relativelyclose tothe visible –Far infraredWavelengths relativelyfar fromthe 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 Numberof spiral arms varies Distinctnessof 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 VisibleNear-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 KeplerianorbitsSpeed decreases with distance –Farthest planets in the Solar System have slowest orbital speeds Non-KeplerianorbitsSpeed is almost constant –Milky Way’s rotation curve is nearly constant –Sun’sspeedaround galactic center is ~ km. hr –1 –Sun’striparound galactic center is ~ 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 objectsMACHOs –Very dim stars between 0.01 & 1.0 M Sun –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 particlesWIMPs –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 wavesOne possible explanation –Waves aresimilar 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 arerelatively 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 spaceHuge distances are involved A matter of timeHuge 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 designspiralsClassicspiral forms The density wave model fits these galaxies well –FlocculentspiralsFuzzyspiral 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 bya feature named Sagittarius A Powerful source of synchrotron radiation –Relativistic electrons spiraling in intense magnetic fields –Containsa feature named Sagittarius A* Thought to be the galactic center –Brightest radio source in its vicinity The mysterious identity of Sagittarius A* –Not a starToo energetic –Not a pulsarToo energetic –Not a supernova remnantNot expanding –Maybe a small supermassive black hole ~ 10 6 M Sun

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 ’s21 cm spin-flip line –InfraredNear- & Far-IR ’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 ’s – Sagittarius A & Sagittarius A* Small supermassive black hole Important Concepts