The Universe of Galaxies

A Brief History

• 1610 - Galileo

• 1610 - Galileo resolves the Milky Way into stars

• 1784 – Hershel maps the Milky Way

• 1784 – Hershel maps the Milky Way

• 1912 – Leavitt discovers the period-luminosity relation for Cepheid variable stars.

• This eventually became the key to measuring distances to galaxies

• 1917 – Shapley measures the Galaxy using globular star clusters..

Using star clusters

• Instead of 1000 ly across, it is 100,000 ly and the sun is not at the center.

The Galactic system

The First Extragalactic Nebula

The Distance to NGC 6822

• 1925 Hubble measures the distance to NGC 6822 and finds that it is a separate, distant galaxy, 500,000 ly away (the correct distance is 2 million ly)

M31

The Hubble Deep Field

Types of galaxies

Ellipticals

• Elliptical in shape • Smooth light

distribution – no structure

• Mostly old stars (red)• Predominate in large

clusters

Ellipticals

• Elliptical in shape • Smooth light

distribution – no structure

• Mostly old stars (red)• Predominate in large

clusters

A cluster of elliptical galaxies

Spiral galaxies

• Sa galaxies– Large central bulge– Tightly wound arms– Arms smooth, dusty

Spiral galaxies

• Sb galaxies– smaller central bulge– Less tightly wound

arms– Arms blue, patchy

Spiral galaxies

• Sc galaxies– small central bulge– Loosely wound arms– Arms blue, very patchy

Barred spiral galaxies

• SB galaxies are divided into SBa, SBb, SBc, with similar characteristics to regular spirals, except for a centrally-oriented bar

Barred spiral galaxies

• SB galaxies are divided into SBa, SBb, SBc, with similar characteristics to regular spirals, except for a centrally-oriented bar

Barred spiral galaxies

• SB galaxies are divided into SBa, SBb, SBc, with similar characteristics to regular spirals, except for a centrally-oriented bar

Irregular galaxies

• Irr galaxies – No spiral structure– Blue, many young

stars– Patchy light

distribution

Irregular galaxies

Dwarf galaxies

• Dwarf galaxies – Low surface

brightness– Less than 5000 ly

across– Some are irregular,

some are elliptical– Various star formation

histories

NGC 4414

Interacting galaxies

• Merging galaxies are found in the nearby universe and may have been very common in the early universe.

• This is NGC 520

• “The Mice”

Centaurus A

Cen A in radio

Cen A in x-rays

Milky Way mergers

• The MW galaxy is tidally interacting with two small galaxies (the MCs) and recently has swallowed two dwarf galaxies.

• This is a map of the Sagittarius dwarf, which is falling into the MW.

Milky Way mergers

Galaxies with active galactic nucleiAGNs

An AGN and a normal galaxy

Seyfert galaxies

The extreme case: quasars

3C273, the first quasar

Identified by Schmidt in 1961

Quasars are at cosmological distances

Studied in detail by Burbidge and others

HST showed that quasars are the nuclei of galaxies

The Local Group

MCs, Sculptor, Fornax, Ursa Minor, Draco, Sagittarius are too close to be identified here

Spiral Members

• The Milky Way

MWG

• Type: Sbc

• Absolute magnitude: -21.0

• Diameter: 40 kpc

• Disk Thickness: ~200 pc, depends on spectral type of stars; thin for massive stars, thick for old, low mass stars

• Mass: 2 x 1011 solar masses

M31, the Andromeda Galaxy

M31 = NGC 224

• Type: Sb• Asolute magnitude: -21.8• Diameter: 50 kpc +• Active star formation, especially at ~10 kpc • 2 close but separate companions: M32&NGC

205• Evidence for past mergers• Mass: 4 x 1011 solar masses

M31, M32 and NGC 205

M33

M33 = NGC 598

• Type: Sc

• Absolute magnitude: -18.87

• Tilted by 57 degrees

• High rate of star formation

• Mass: 1.1 x1010 solar masses

• Mass of gas: 2.6 x 109 solar masses

NGC 604 in M33

The Magellanic Clouds

LMC

The Magellanic Clouds

SMC

The Magellanic Clouds

• Type: Irregular

• Absolute magnitudes: -18.7, -17.8

• Orbiting MWG

• Bursts of star formation

NGC 6822

WLM

Phoenix

Leo II

Sculptor

Sextans I