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Galaxies Live in Clusters
Fornax
Hickson
Coma
Virgo
Abell 2218
The Universe on Larger Scales
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.
Large Scale Structure
This plot shows
the locations of
individual galaxies
within the Virgo
Supercluster.
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.
Starbursts
This galaxy collision has led to bursts of star formation
in both galaxies; ultimately they will probably merge.
The Antennae
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.
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
Galactic Cannibalism
This appears to be an instance of galactic
cannibalism – the large galaxy has three cores.
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).
A Quasar’s Central Engine
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.
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 quasistellar objects (QSOs).
Quasar Redshifts
The 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 Engines
How 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 Central Engine of an Active Galaxy
The jets emerging from an active galaxy can be
quite spectacular:
The Active Galactic Nuclei “Zoo”
How we classify an AGN depends
on how big the monster is and what
angle we view it at.
Radio Galaxies
Radio galaxies
emit very
strongly in the
radio portion of
the spectrum.
They may have
enormous lobes,
invisible to optical
telescopes,
perpendicular to the
plane of the galaxy:
Radio galaxies may also be core-dominated:
Core-dominated and radio-lobe galaxies are probably
the same phenomenon viewed from different angles:
Active Galaxy Summary
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