GALAXIES
Review The distance of Andromeda Island Universes Galactic Coordinate System The location of the sun in Milky Way
(Shapely) Oort’s method and mapping the Milky Way Proper motion
The rotation curve of Milky Way Metallicity Stellar populations The distribution of stars in Milky Way The Formation of a galaxy Classification of galaxies
Test
Where are these objects located in sky/Milky Way?
NGC 6946 and NGC 6939Face on spiral galaxy
Open Cluster
In Cepheous
Messier Marathon
Supernova in NGC 6946
Test
What type this supernova is?What population the originating star was?
Normal Galaxies
Spirals Ellipticals
Morphology Disk + bulge, Spiral arms
All bulge
Stellar population
Young and old stars Old stars only
Interstellar material
Present Virtually none
Star Formation Present None
Kinematics Disk rotating, Bulge and halo have random 3-D orbits
Little rotation, mostly random 3-D orbits
Spiral ArmsWinding Problem
Spiral Pattern 1960 Lin-Shu theory of density waves Spiral arms are waves of excess density Stars are crowded together temporarily Cosmic traffic jam Stars enter and exit the wave just as cars
enter and exit a jam
Precession of orbits
No Wave
Fixed frame Rotating frame
Aligned Orbits
Bar Wave Spiral Wave
Resonances Spiral arms are
instance representation of waves
Corotation circle ωorbital* = ωwave
Lindblad resonances Particular point in
orbit
Bars
Computer simulations predicts: Bars are formed besides spiral arms Resonance Bars transfer the lost angular
momentum Bars destroy themselves
Question We know bars are formed necessarily Bars destroy themselves
And 75% of spiral galaxies are barred spirals. How?
Summary
Galaxies are not born with a given shape (barred or unbarred)
A typical spiral galaxy spend ¾ of its life time barred
Galactic Center Evidences of stellar formation in the last
50 million years ISM orbits the center in a orbit with inner
radius of 2pc Strong magnetic field (milli-Gauss) Compact radio source (Sgr A) High radial velocities and proper motion
Existence of a large unseen, compact object
Problem
We can observe the center of the Milky Way in infrared light
We predict there are hot, massive stars there
We cannot distinguish the exact spectral type of the stars in the center of the Milky Way
Why?
Problem Find the mass of Sgr A A star identified rotating Sgr A, with
orbital velocity = 1000 km/sec that lies 0.01 pc from Sgr A.
Problem
Is galaxy a Keplerian system? VLBA measurements of SgrA set
limits of ~3AU for the size of SgrA: What is SgrA?
VLBA
Click icon to add picture
Vary Long Baseline Array
Rotation in Elliptical Galaxies and Bulges of Spirals Stars have random
velocities in 3D
Δλ = Observed size of a spectral line σ = Velocity dispersion
Faber-Jackson RelationGravitational binding energy (potential of a mass distribution of radius R and mass M)Kinetic energy
Virial theorem
Faber-Jackson RelationAssumption
Luminosity and the velocity dispersion in a elliptical galaxy are related.
Faber-Jackson RelationAssumption
Luminosity and the velocity dispersion in a elliptical galaxy are related.
Tully-Fisher Relation
Luminosity of spiral galaxies are related to their velocity width.
Standard candle
Tully-Fisher Relationas Standard Candles
Luminosity
Elliptical galaxies:
Ie : surface brightness at re
re : radius enclosing 50% of flux
Luminosity
Spiral galaxies:
I0 : central surface brightness
r0 : disk scale length
Problem
A star is orbiting around a galaxy.Orbital velocity = vDistance from the center of the galaxy = RFind the mass within R.
Most of
our univ
erse is
missing
Evidences
1933: Fritz Zwicky studied the motions of 7 galaxies in a group in Coma Cluster
Dynamic Mass: mass calculated using gravitation laws and the velocity dispersions
Dynamic Mass / Luminosity Mass > 400
Clusters: temporal structures
Evidences 1970: Vera Rubin noted rotation curve of
spiral galaxies (dynamic vs. luminous mass)
Dynamic mass is measured using gravitational influences
Gravitation laws are false! Or luminous mass is not accurate
There are large amount of hidden mass
Dark Matter
Dark matter is not significant in Solar System
Dark matter surrounds spirals and ellipticals’
Dark matter is significant in galaxy clusters
Dark matter was not required if Large structures (eg. Galaxies) weren’t
bound systems(galaxies are bound systems at least in a time equals to the age of the universe)
Gravity laws were wrong in large scales
Candidates for Dark Matter Neutrinos or other exotic sub-atomic
particles
Byronic matter
Sub-atomic particles
Widely distributed No interaction with regular (baryonic)
matter Absent in solar neighborhood
Axions: required to explain some aspects of the strong nuclear force
Neutrinos Supersymmetric particles WIMPS (Weakly Interacting Massive
Particles), CHAMPS, etc.
Baryonic Matter: not luminous.
ancient white dwarfs, brown dwarfs, chunks of cold matter significantly larger than the wavelength of visible light, small black holes
What have been found No axions or WIMP was found. Neutrinos may have non-zero mass. But the large amount of massive
neutrinos arise other problems. MACHO: Massive Compact Halo Objects
brown dwarfs or dim white dwarfs or other low mass stars
Milky Way halo has 50% MACHOs. With masses around 0.1 to 0.5% of the mass of the sun.
Top Related