A Universe of Galaxies

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A Universe of Galaxies

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Title: A Universe of Galaxies

1
A Universe of Galaxies
James J Marie, Astronomy 2005
2
Colliding Galaxies

Collisions between galaxies
are spectacular events which
happen on a time scale of
millions of years.
When galaxies collide there
is virtually no chance that any
of the stars belonging to
either galaxy will collide.
However gravitational shock
waves can ripple through the
interstellar medium of one or
both galaxies triggering the
birth of new stars.
The new stars can give rise to
new structures and shapes.

James J Marie, Astronomy 2005
3
Cartwheel Galaxy

In the direction of the constellation Sculptor
lies the aftermath of a galactic
collision between an originally spiral galaxy
and a smaller intruder galaxy.
The intruder galaxy has left the scene long ago
and the spiral galaxy has evolved
into an expanding ring like shape.

The cartwheel galaxy is 500
million light years away and
is 100,000 light years across.
Gravitational distortions of the
collision produced the ring like
structure.
The ring structure is composed
of newly formed massive
bright stars.

James J Marie, Astronomy 2005
4
NGC 6872

NGC 6872 is one of the
largest known barred spirals
measuring 750,000 light years
across.
The upper spiral arm shows
an unusual amount of star
formation.
This star formation is thought
to be associated with the
passage of galaxy IC 4970
through this part of NGC
6872.

James J Marie, Astronomy 2005
5
Future Collision

Because of local motion (also called peculiar
motion), the Andromeda Galaxy
is actually moving toward the Milky Way
Galaxy.
The Andromeda Galaxy and the Milky Way Galaxy
will collide in about
3 billion years.

? Andromeda Galaxy
James J Marie, Astronomy 2005
6
Galactic Mergers

Occasionally colliding galaxies merge into a
single galaxy.
Some elliptical galaxies are thought to have
formed from galactic collisions.

? NGC 1275

A spiral galaxy is seen
slicing through an
elliptical galaxy.

James J Marie, Astronomy 2005
7
Active Galaxies

The centers or nuclei of some galaxies are
tremendous sources of energy.
This energy is radiated in the form of radio
waves, visible light, x-rays, gamma
rays and subatomic particles.
The power radiated from the nuclei of some of
these galaxies varies in time.
Galactic nuclei have been observed to vary in
brightness by 15 billion Suns in
only a matter of 30 minutes!
Such a galactic nucleus is called an Active
Galactic Nucleus or AGN.
The host galaxy of an AGN is called an active
galaxy.

James J Marie, Astronomy 2005
8
A Strange Blue Radio

In the early 1960s Martin Schmidt discovered a
source of radio waves in the
sky called 3C273.
This radio source appeared as a blue star in a
telescope.

An image of 3C273 taken in
visible light from an optical
telescope.

The spectral lines of 3C273 seemed
very odd because they didnt
correspond the familiar spectral
lines of known elements.

James J Marie, Astronomy 2005
9
Quasars

After puzzling over the spectral lines for
several months, Schmidt realized
that the spectral lines were simply emission
lines from hydrogen but they
were greatly redshifted.
Upon measuring the redshift of 3C273, Schmidt
realized that 3C273 was
moving away from us at nearly 17 the speed of
light!
The luminosity-distance formula revealed that
3C273 had a luminosity more
than a trillion times that of the Sun!
Other objects similar to 3C273 were discovered
and called quasi-stellar radio
sources or quasars for short.
For several years quasars were very mysterious
because no one could explain
how so much energy could be radiated from a
single star-like object.

James J Marie, Astronomy 2005
10
Nature of Quasars

Most quasars lie more than halfway to the
cosmological horizon and are
the farthest known objects in the universe.
The light from these quasars was emitted when
the universe was less
than a third of its present age.
The most distant quasar known to humans was
detected in 2003. The light
from this quasar was emitted when the
universe was only 6 of its present
age!
Quasars radiate strongly across a wide portion
of the electromagnetic
spectrum with prominent spectral lines.
This implies that the matter that makes up a
quasar has a wide range
of temperatures.

James J Marie, Astronomy 2005
11
Quasars Revealed

Quasars were initially thought to be strange
new phenomena and new
laws of physics would have to be discovered
before we could understand
them.
The first clues about what quasars might be
came from the spectra nearby
galaxies with active nuclei.
The spectra of quasars were very similar to
active galaxies.
This led to suspicion that quasars are actually
galaxies with very energetic
nuclei.
The biggest difference between quasars and
active galaxies is that quasars
are far more luminous.

Quasars are associated with tremendously active
galactic nuclei seen from an early era of the
universe.
James J Marie, Astronomy 2005
12
Compact Energy Sources

The total luminosity or power output from
quasars can vary rapidly in time.
The rapid variation of luminosity
implies that the radiated energy
comes from a very small region
of space.
This region of space is no larger
than the size of the solar system.
Since quasars are so bright, the
compact region (nucleus) is an
unbelievable source of energy!
The most powerful quasars
have a luminosity of 2 trillion
times that of the Sun (100 times
more luminous than the entire
Milky Way Galaxy)!

James J Marie, Astronomy 2005
13
Radio Galaxies

Radio galaxies are galaxies that emit unusually
powerful radio waves
from huge pairs of lobes on either side of the
galaxy.
The radio waves radiate from electrons and
protons spiraling around
magnetic fields at nearly the speed of light.

James J Marie, Astronomy 2005
14
Radio Galaxies

Radio galaxies are closely related to quasars
because they also have
very powerful active nuclei.
Two incredibly energetic jets blast in opposite
directions from the nuclei
of radio galaxies.
The jets carry blobs of plasma moving at nearly
the speed of light!
These blow torches feed energy to the radio
lobes which are far outside
of the visible galaxy.
All of the energy that powers the jets and
radio lobes comes from the tiny
galactic nucleus!
The total power output of a radio galaxy,
implies that the nucleus of a radio
galaxy is as powerful as the nucleus of a
quasar.

James J Marie, Astronomy 2005
15
Radio Galaxy Structure

The lobes of a radio galaxy may lie
as far away as a million light years
away from the core.
Many quasars also have a radio
core-jet-lobe structure very similar
to radio galaxies.
The nuclei of many radio galaxies are
hidden behind donut shaped rings
of molecular clouds.
If the orientation of the radio galaxy is
such that the nucleus is visible from
the donut hole, then the galaxy
may appear very similar to a quasar.

James J Marie, Astronomy 2005
16
Supermassive Black Holes

The only known mechanism that can supply such a
prodigious amount
of energy from a volume of space as small as
the nuclei of quasars
and radio galaxies are supermassive black
holes.

Accretion disk feeding infalling
matter into a black hole.

This is an image of the AGN of
NGC 4261 taken by the
Hubble Space Telescope.
The disk is 300 light years
across and radio jets emanate
from the plane of the disk.

James J Marie, Astronomy 2005
17
An Awesome Powerhouse

The potential energy of matter falling into the
black hole is converted into kinetic
energy.
The added kinetic energy promotes violent
collisions between atoms and the
gas becomes super hot.

The gas is ionized into a
plasma which radiates
energy in the form of
ultraviolet radiation and
x-rays.
This is the most
efficient process in
the universe for
generating energy!

James J Marie, Astronomy 2005
18
Relativity at Work

The amount of gravitational potential energy
that gets converted into kinetic
energy for a falling chunk of matter is
equivalent to its mass energy, E mc2.

Between 10 and 40
of this energy is radiated
away before it crosses
the event horizon.
This is far more efficient
than energy production
from nuclear fusion.
The typical supermassive
black hole in a quasar
consumes about 17
solar masses a year!

James J Marie, Astronomy 2005
19
Primordial Quasar

An artists impression of a primordial quasar
that formed after the first few billion
years of the universe is shown below.
Primordial quasars might have been surrounded
by sheets of gas, dust, stars and
early star clusters.

It is thought that most
galaxies may have had
a quasar phase when they
were much younger.

James J Marie, Astronomy 2005
20
Supermassive Black Holes and Galaxies

It is presently believed that all galaxies
probably harbor a supermassive black
hole at their center.
However it is not known how supermassive black
holes are born.
Nor is it known if the supermassive
black holes formed before or after
their host galaxies formed.
This is one of the deepest
mysteries in astronomy.

James J Marie, Astronomy 2005
21
Galactic Neighborhood

The nearest galaxies to the Milky Way are shown
below in a 3-D map.

James J Marie, Astronomy 2005
22
Local Group

The local group is a cluster of galaxies which
includes the Milky Way galaxy.
Over 30 galaxies are included in the local
group which spans 10 million light
years across.

The center of mass of
the local group lies between
the Milky Way Galaxy and
the Andromeda Galaxy.
The two largest galaxies of
the local group are the Milky
Way Galaxy and the
Andromeda Galaxy.
The total number of galaxies
in the local group is not yet
known.

James J Marie, Astronomy 2005
23
Three Dimensional Map of the Local Group
James J Marie, Astronomy 2005
24
Virgo Cluster

The Virgo Cluster is the closest cluster of
galaxies to our Milky Way Galaxy.
The Virgo Cluster contains between 1300 and
2000 galaxies.
This cluster is so massive that it pulls the
Local Group toward it.

An image of the Virgo Cluster
taken by the Palomar
Observatory.

All types of galaxies are seen
including spiral, elliptical and
irregular galaxies.

James J Marie, Astronomy 2005
25
Closest Galactic Clusters

Shown below is a three dimensional map of the
closest galactic clusters.
The Virgo Cluster is 62 million light years
away from the Milky Way.

James J Marie, Astronomy 2005
26
Virgo Supercluster

The Virgo Supercluster is a
supercluster of galaxies that
contains the local group.
It spans about 200 million
light years and contains about
100 clusters and groups of
galaxies dominated by the
Virgo Cluster near the center.
The total mass of the Virgo
Supercluster is about one
quadrillion solar masses.
A gravitational anomaly called
the great attractor is drawing
in galaxies over a region
hundreds of millions of light
years across.

James J Marie, Astronomy 2005
27
Large Scale Structure of the Universe

In 1989, astronomers M. Geller and J. Huchra
set out to map the large scale
structure of the universe within a narrow
slice.
The distances to the galaxies were obtained by
measuring the redshifts of
very far galaxies.
They found that galaxies are arranged into
sheets and filaments.
Clusters of galaxies are located at the
intersections of these sheets.
The sheets of galaxies surround giant
bubble-like voids.
An enormous sheet of galaxies called the Great
Wall was discovered.
The Great Wall is 500 million light years long
and 200 million light years wide.
Most astronomers believe that these
large-scale structures grew from
slight density enhancements in the early
universe, just as galaxies did.

James J Marie, Astronomy 2005
28
Slice of the Universe

A thin slice of the universe extending to a
distance of 700 million light years
mapped out by Geller and Huchra is shown
below.
The tip of the slice coincides with the
location of the Earth and each dot
represents a galaxy.

Large voids are
seen under the
arms of the stickman
figure.

James J Marie, Astronomy 2005
29
North and South Slices

Large scale structure of slices
of the universe as seen from the
northern and southern
hemispheres.
Each of the 9,325 points
represents a galaxy.
The existence of the walls and
voids are surprising since the
matter distribution in the
universe was originally thought
to be uniform.
Many theoretical ideas exist
about the formation of these
structures.

James J Marie, Astronomy 2005
30
Deeper Slices

A map of the galaxies has been extended into
deeper slices measuring
4 billion light years in length.
There is evidence that large structures in the
universe may still be growing!

James J Marie, Astronomy 2005
31
Galactic Map

Shown below is a map of galaxies from a 15?
patch of the sky.
The positions of over 3 million galaxies are
depicted by each point.
It is seen that the universe appears more
uniform at very large scales.

Bright areas contain
more galaxies than
dark areas.
The black rectangles
are regions for which
there are no data.

James J Marie, Astronomy 2005
32
Gravitation and Structures

Large structures in the universe are held
together by the force of gravity.
These structures include galaxies, clusters of
galaxies, superclusters,
enormous filaments and sheets of galaxies.
The birth, evolution and fate of these
structures are determined by the
gravitational pull of mass that makes up the
structures.
The total mass contained in these structures is
essential to understanding the
evolution of the universe.
To understand the evolution of large structures
and the fate of the
universe it is necessary to ask

How much mass is in the universe?
James J Marie, Astronomy 2005
33
The Masses of Galaxies and Galactic Clusters

To measure the mass in the universe we
must survey the amount of mass in the
galaxies.
With galactic rotation curves we can weigh
the galaxies.
These curves suggest that galaxies
contain far more mass than all the stars,
gas and dust combined.
This additional mass is not visible and is
therefore called dark matter.
The existence of dark matter implies that
most of the universe is hidden from us!

James J Marie, Astronomy 2005
34
Dark Matter

This illustration depicts the
the large spherical
distribution of dark matter
around a spiral galaxy.

The amount of dark matter
far exceeds the luminous
matter in the galaxy.
The radius of the dark matter
sphere is typically 10 times
the radius of the galactic
halo.

James J Marie, Astronomy 2005
35
Mystery of Dark Matter
What is dark matter?

This is one of the great mysteries of
cosmology.
However, there are several ideas and we may
already know a little about what
makes up dark matter.
We speculate that there are two types of dark
matter

Ordinary Matter
Extraordinary Matter

James J Marie, Astronomy 2005
36
Ordinary Dark Matter

It is possible that at least part of the dark
matter could be ordinary matter
made up of protons, neutrons and electrons.
Since protons and neutrons are called baryons,
another name for ordinary
matter is baryonic matter.
In this case, dark matter simply refers to
matter which doesnt glow as brightly
as stars or hot gases.
This implies that the Solar System, the Earth
and human beings are dark
matter.
Since the universe is so vast, baryonic matter
cannot be seen from great
distances.

James J Marie, Astronomy 2005
37
MACHOs

Ordinary dark matter includes planets, brown
dwarfs, Jupiter sized objects left
over from the formation of the Milky Way and
very dim low mass red stars.
Such objects are called MAssive Compact Halo
Objects.
It is possible that trillions of MACHOs drift
through galactic halos contributing
to the masses of galaxies.
MACHOs could also include black holes.
We can only detect MACHOs with indirect methods
such as gravitational
lensing.
A lensing event occurs when a MACHO drifts
across our line of sight to a distant
star.
Indeed, if the galaxy is filled with trillions
of MACHOs we should occasionally
witness a rare lensing event involving a
distant star.

James J Marie, Astronomy 2005
38
Lensing of MACHOS

If trillions of MACHOS really exist, a lensing
event involving one star out of
a million should occur every year.
A lensing event is signaled by a
star which temporarily brightens.
The duration of the lensing event
reveals the mass of the MACHO.
Intensive large-scale projects to
monitor lensing events have
revealed the presence of MACHOs.
However, there are not nearly
enough MACHOs to account
for all of the dark matter.
Solar mass sized black holes
have been eliminated as dark

James J Marie, Astronomy 2005
39
Extraordinary Dark Matter

Extraordinary dark matter is also known as
nonbaryonic dark matter.
Exotic particles of matter that interact very
weakly with ordinary matter are
predicted by some theories of high energy
physics.
These hypothetical Weakly Interacting Massive
Particles are called WIMPs.
WIMPs which are moving slow enough to collect
in the halos of galaxies are
referred to as cold dark matter.
However, fast moving WIMPs that can escape the
gravitational pull of galaxies
are referred to as hot dark matter.
Hot dark matter includes neutrinos which are
moving at nearly the speed of
light.
It is possible that cold dark matter could make
up most of the mass of the
galaxies, but it is completely invisible in
all wavelengths of light.

James J Marie, Astronomy 2005
40
Balance of the Universe

All of the mass in the entire universe
collectively generates an enormous
gravitational field.
Since gravity is attractive, the universe is
self attractive.
In other words, gravity of the entire universe
should slow the rate of
expansion of the universe.
The rate at which the expansion of the universe
is slowed down is called the
deceleration parameter.
The amount of mass in the universe determines
the rate at which the expansion
slows down.
The more mass there is, the more the expansion
should slow down.

James J Marie, Astronomy 2005
41
Geometry of the Universe

Since gravity is the curvature of spacetime,
the mass of the entire universe
plays a role in determining the global
geometry of the universe.
The geometry of the universe can tell us
whether the universe is finite or
infinite.
The amount of mass in the universe controls the
rate at which the expansion
slows down and the geometry of the universe.

What are the possibilities?
James J Marie, Astronomy 2005
42
Zero Curvature

One possibility is that the universe has just
enough mass to slow the rate
of expansion down after an infinite amount of
time.
If this is the case, the geometry of the
universe is flat.

A flat universe is destined to expand forever
with the rate of expansion
gradually slowing down to zero after an
infinite amount of time.
A flat universe is also unbounded and infinite.

James J Marie, Astronomy 2005
43
Positive Curvature

Another possibility is that the universe has
more than enough mass to slow
the rate of expansion down.
At some time in the distant future, the
expansion of the universe could stop
and then reverse.

In this case, the universe would be finite and
unbounded.
Such a universe is called a closed universe.

James J Marie, Astronomy 2005
44
Negative Curvature

A third possibility is that the universe does
not have enough mass stop the
expansion.
In this case, the universe would have no bounds
and would continue to expand
forever.

Such a universe is infinite and is called an
open universe.

James J Marie, Astronomy 2005
45
Critical Density

The critical density of the universe is the
mean density at which the universe
would balance between collapse and eternal
expansion.
If the density of the universe is less than the
critical density, then the universe
will expand forever.
If the density universe of the universe is more
than the critical density, then the
expansion will eventually stop and the
universe will collapse upon itself.
Present calculations tell us that the critical
density is about 10-29 grams per
cubic centimeter.
This is roughly equivalent to a few hydrogen
atoms in a volume the size of a
closet.

Does the universe have the critical density?
James J Marie, Astronomy 2005
46
Density of the Universe

All of the luminous matter in the universe
appears to add up to only about
1 of the critical density.
So is there enough dark matter in the universe
to stop the expansion?
Our best estimates from clusters of galaxies
and large scale structures is that
all of the dark matter seems to add up to only
about 30 of the critical density.
Based on our present knowledge, it appears that
the universe is infinite and will
continue to expand forever.

James J Marie, Astronomy 2005
47
A Shocking Discovery

In 1998, a ten year study called the Supernova
Cosmology Project measured
distances to very distant galaxies using type
Ia supernova as standard candles.
The primary goal of the project was to measure
changes in the rate of
expansion of the universe (deceleration
parameter).
It was discovered that the expansion of the
universe is not slowing down but
instead, is accelerating!
This discovery is shocking because it has no
known explanation.

What could cause the expansion of the universe to
accelerate?
James J Marie, Astronomy 2005
48
Dark Energy

The mysterious nature the drives the
acceleration of the expansion of the
universe is called dark energy.
It has the nature of energy but no one knows
for sure what it is.
Strong negative pressure is associated with
dark energy.
The negative pressure effectively acts as
repulsive gravity!
Two sources of dark energy have been proposed

Cosmological Constant A constant uniform energy
density filling the
vacuum of space. The vacuum is filled with
the creation and annihilation
of virtual matter antimatter particles.
Quintessence A dynamic field whose energy
density can vary in space
and time.

James J Marie, Astronomy 2005
49
What Kind of Universe Do We Live In?

The accelerating universe adds uncertainty
about the kind of universe we are
living in.

? Dark energy could propel a closed universe to
eternal expansion.
James J Marie, Astronomy 2005
50
A Negatively Curved Universe?

It is possible, although not likely, that
systematic distortions such as a novel
form of light absorption by intergalactic dust
may cause distant supernova
to appear deceptively fainter.