Galaxies

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.