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Galaxies

• Contain a few thousand to tens of billions of stars,

• Large variety of shapes and sizes

• Star systems like our Milky Way

• as well as varying amounts of gas and dust

The Family of GalaxiesEven seemingly empty

regions of the sky containthousands of very faint, very

distant galaxies

Large variety ofgalaxy

morphologies:

Spirals

Ellipticals

Irregular(some

interacting)

Galaxy Classification

Sa

Sb

Sc

Elliptical Galaxies Spiral Galaxies

E0 = Spherical

Small nucleus;loosely wound

arms

E1

E6

E0, …, E7

Large nucleus;tightly wound

arms

E7 = Highlyelliptical

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old stars

young stars

Gas and Dust in GalaxiesSpirals are rich in

gas and dustEllipticals are almost

devoid of gas and dust

Galaxies with disk and bulge,but no dust are termed S0

Barred Spirals

Some spirals show apronounced bar structure

in the center.

They are termedbarred spirals:

Sequence:

SBa, …, SBc,

analogous toregular spirals.

Irregular GalaxiesOften: result of galaxy

collisions / mergers

Often: Very active star formation(“Starburst galaxies”)

Some: Small (“Dwarf galaxies”)satellites of larger galaxies

(e.g., Magellanic Clouds)

LargeMagellanicCloudNGC 4038/4039

The Cocoon Galaxy

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Formation of Spiral Galaxies

Matthias Steinmetz

Structure of Spiral Arms• density wave model

– arms are higher density regions– rotate at velocities lower than gas and stars– like traffic slow-down

• moving 60 mph• encounter a few cars moving 40 mph• slow down to 40 mph, pass, and then resume 60 mph

• Density waves produced by perturbation -interaction?

• Spiral structure is stable

Formation of Spiral Arms Formation of Spiral Arms?

http://antwrp.gsfc.nasa.gov/apod/ap020710.html

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Properties of Elliptical Galaxies

• oblate spheroid– oblate, prolate (symmetric egg), triaxial?

• Smooth distribution of light– log I(r) ~ r-1/4

• ellipticity = 10(a - b)/1– a = semi-major axis– b = semi-minor axis

• stars follow random orbits• Formed from merger of two spirals

Formation of Elliptical Galaxy

Matthias Steimetz

107 - 109108 - 1010108 - 1010105 - 1011Lumin. (L)

fieldfield &group

clusters &groups

rich clusterLocation

all disk?disk > bulgebulge >disk

all bulgeDisk/Bulgemuchmuchvery littlevery littleDust

>201011Gas(%mass)

mainly II & IIIIIIPopulation

O-MO-MG-MK-MStars

1-105-505-501-200Diam (kpc)

107 - 109109 - 1012109 - 1012105 - 1013Mass (M)IrregularSpiralS0EllipticalProperty

Galaxy Properties Galaxy Environments

Abell 2218 (HST)

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Clusters

Coma (NOAO)

Abell 1689 (WFPC2)

z=0.183

●Morphology variations●clusters ~55% E/S0s●field ~20% E/S0s

●Two observables●Morphology●Star formation