Galaxies Live in Clusters

Hickson

Fornax

Coma

Virgo

Abell 2218

Galaxy clusters join in larger groupings, called superclusters. This is a 3-D map of the superclusters nearest us; we are part of the Virgo supercluster.

The Universe on Larger Scales

This plot shows the locations of individual galaxies within the Virgo Supercluster.

Large Scale Structure

This slice of a larger galactic survey shows that, on the scale of 100-200 Mpc, there is structure in the Universe – walls and voids.

This survey, extending out even farther, shows structure on the scale of 100-200 Mpc, but no sign of structure on a larger scale than that.

The decreasing density of galaxies at the farthest distances is due to the difficulty of observing them.

Superclusters and Voids

Interacting Galaxies

Gravitational forces from one galaxy can act on nearby galaxies:

Sometimes they collide, merge etc.

Galactic Mergers

• When two roughly equal size galaxies collide and eventually form a single galaxy.

• So much space between the stars that they rarely collide.

• Gas between stars does collide and get compressed.

• Compressed gas triggers new star formation.

This galaxy collision has led to bursts of star formation in both galaxies; ultimately they will probably merge.

Starbursts

The Antennae galaxies collided fairly recently, sparking stellar formation. The plot on the right is the result of a computer simulation of this kind of collision.

The Antennae

A Merger

This Hubble Deep Field view shows some extremely distant galaxies. The most distant appear

irregular, supporting the theory of galaxy formation by merger.

Galactic Cannibalism

When two unequal size galaxies collide and merge.

The bigger galaxy “eats” the smaller galaxy.

Giant ellipticals form this way (and keep getting bigger).

Cannibalism

This appears to be an instance of galactic cannibalism – the large galaxy has three cores.

Galactic Cannibalism

Sometimes even when galaxies do not look like they are interacting – they really are.

Active Galaxies (aka Quasars)

Most big galaxies have supermassive black holes in their centers.

During a merger, fuel (stars, gas, etc.) is fed into the black hole.

This results in a hot disk of material that spirals into the black hole.

This disk shines very brightly (brighter than the galaxy itself).

The energy source in a quasar is a black hole, surrounded by an accretion disk. The central black hole may be billions of solar masses. Often strong magnetic field lines around the black hole channel particles into jets perpendicular to the magnetic axis.

A Quasar’s Central Engine

The Discovery of Quasars

• 1940’s – the birth of radio astronomy in Grote Reber’s backyard.

• 1950’s – the Third Cambridge Catalog (3C Catalog) of radio sources.

• Some of these 3C radio sources had no optical counterparts.

1960 – Allan Sandage discovers an optical counterpart to 3C 48 (the 48th object in the 3C catalog). It looks like a blue star.

3C 273

1962 – a spectrum of 3C 273 (another source like 3C 48) is taken. It has strong emission lines, but nobody can identify the element that causes them.

1963 – Maarten Schmidt at Cal Tech realizes that the lines have the same spacing as Hydrogen lines, but Doppler shifted by 15% of the speed of light!

“Quasars” are Born

Using Hubble’s Law a Doppler shift of 15% of the speed of light corresponds to 2 billion light years. 3C 273 is well outside of our galaxy!

These objects were called quasi-stellar radio sources – shortened to quasars. Some did not emit radio waves and were called quasi-stellar objects (QSOs).

Quasar RedshiftsThe redshift (z) of a quasar is a measure of how far away it is.

3C 273 has z=0.15, which corresponds to a distance of about 2 billion light years.

We are seeing the object as it looked 2 billion years ago!

The highest redshift quasar discovered to date has z>6, or over 13 billion light years!

Quasar Energy Output

• Very distant galaxies have faint apparent magnitudes and are very hard to see.

• Yet, we see quasars quite easily.

• So, quasars must emit a lot of light.

• Typically 100x brighter than the Milky Way.

Even more impressive is that quasars are only about as big as the Solar System!

Quasar Central EnginesHow do quasars emit so much light from so little space?

• They are powered by supermassive black holes.

• Mass spiraling into the black hole heats up and gives off light.

• In some quasars, huge jets are shot out at the poles.

• These jets are strong radio sources.

A Quasar

The jets emerging from an active galaxy can be quite spectacular:

The Central Engine of an Active Galaxy

The Active Galactic Nuclei “Zoo”

How we classify an AGN depends on how big the monster is and what angle we view it at.

They may have enormous lobes, invisible to optical telescopes, perpendicular to the plane of the galaxy:

Radio galaxies emit very strongly in the radio portion of the spectrum.

Radio Galaxies

Radio galaxies may also be core-dominated:

Core-dominated and radio-lobe galaxies are probably the same phenomenon viewed from different angles:

Active galactic nuclei have some or all of the following properties:

• high luminosity

• nonstellar energy emission

• variable energy output, indicating small nucleus

• jets and other signs of explosive activity

• broad emission lines, indicating rapid rotation

Active Galaxy Summary