AST101

The Evolution of Galaxies

Virgo Cluster

Collisions of Galaxies

• Outside of Clusters (the field), most galaxies are spiral or irregular

• In dense clusters, the most massive galaxies are ellipticals

• Spirals in clusters tend to inhabit the periphery

Formation of Galaxies

• Initially: masses ~ 106 M

• Protogalaxies collided and merged to form larger galaxies.

• If there is a net angular momentum, a disk forms, giving a spiral galaxy

Arp 220the Antennae

NGC 2207

Collisions Today

The Sagittarius Dwarf galaxy

Active Galactic Nuclei

• The center of the Milky Way is a 2x106 M black hole.

• Bigger galaxies have more massive central black holes (up to 109 M)

• Some just sit there

• Others are actively accreting matter (gas, stars)

Active Galactic Nuclei

• Accretion proceeds through a disk

• Accreting gas orbits near the Schwarzchild radius (event horizon) of the black hole

• The gas heats up as potential energy is converted to kinetic energy (heat)

• We see bright X-ray sources

M87Virgo Cluster

Relativistic Jets

• Accreting black holes emit jets of radiation at velocities near the speed of light

Radio Jet: M87

Radio Galaxies

Cygnus A Centaurus A

Quasars• In 1963 Martin Schmidt puzzled over

the spectrum of a 12th magnitude star-like radio source (a quasi-stellar object, or QSO)

3C 273• Redshift z = 0.16; Distance 2.4 x 109 ly

• It is one of the most luminous objects in the universe

Active Galactic Nuclei

• Quasars

• Blazars

• Liners

• Seyfert galaxies

All involve matter infall into massive black holes.