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Galaxies
Astronomy 1-2
Lecture 24-1
Is Our Galaxy All There Is
Until the early twentieth century, the Milky Way
was thought to be the extent of the universe
All objects that were seen, such as nebulae, were
thought to be contained within the Milky Way
There were those who disagreed with this conclusion
It was not until 1924, that the true location of
many of these objects was determined
Astronomy 1-2
Lecture 24-2
Its Bigger Than we Thought
Edwin Hubble noticed that there were Cepheid
variable stars in the Andromeda “nebula”
Using the luminosity, period relation for Cepheids
and the measured luminosity, he showed that the
Andromeda system was far outside of the Milky
Way
Hubble did similar studies with respect to many
of the other “nebulae” and found that they also
were outside of the Milky Way
Thus there are many systems like the Milky Way
spread throughout the universe
Astronomy 1-2
Lecture 24-3
Hubble’s Galaxy Classification
This pair of
images shows the
Coma cluster of
galaxies
Almost every
object visible is a
galaxy
Astronomy 1-2
Lecture 24-4
Galaxy Classification
Hubble first classified the various types of galaxies
He classified the galaxies into four basic groups:
Astronomy 1-2
Elliptical Galaxies
Spiral Galaxies
Barred Spiral Galaxies
Irregular Galaxies
Lecture 24-5
Tuning Fork Description
As part of his classifying the galaxies, Hubble
produced a diagram that shows that transition
from elliptical galaxies to spiral galaxies
This diagram does not imply any evolutionary
progression in galaxy formation
Astronomy 1-2
Lecture 24-6
Spiral Galaxies
These are galaxies with lanes of stars and nebula
emanating from a central nucleus
Spiral galaxies range from
Tightly wound arms and Large central bulge
Loosely wound arms and Small central bulge
Direct relationship between the tightness of the
arms and size of the central bulge
Allows classification of the galaxy even when
viewed from on edge
Astronomy 1-2
Lecture 24-7
Spiral Galaxies
Spiral galaxies have
Flat, rotating disks
Central bulges
Gas and dusk in the disk
Star formation in the spiral arms
Spiral galaxies can range in size from 80,000 to
250,000 light years across
They can range in mass from 109 to 1012 solar masses
Astronomy 1-2
Lecture 24-8
Spiral Galaxies
Classified according to the size of their central
bulge:
Astronomy 1-2
Lecture 24-9
Spiral Galaxies
Type Sa has the largest central bulge, Type Sb is
smaller, and Type Sc is the smallest
Type Sa tends to have the most tightly bound spiral
arms with Types Sb and Sc progressively less tight,
although the correlation is not perfect
The components of spiral galaxies are the same as
in our own galaxy
disk, core, halo, bulge, and spiral arms
Astronomy 1-2
Lecture 24-10
Spiral Galaxies
The Sombrero galaxy, with its large central bulge, is
a type Sa
We cannot see the spiral arms, as they are edge-on
Infrared
Astronomy 1-2
Lecture 24-11
Barred Spiral Galaxies
Spiral arms emanate from the ends of a bar
Classified in exactly the same way as regular
spiral galaxies
By the size of the central bulge and the tightness of
the arms
In both regular and barred spiral galaxies, the
more tightly wound the arms - the less gas and
dust
There is mounting evidence for the Milky Way
being a Barred Spiral Galaxy with a short bar
Astronomy 1-2
Lecture 24-12
Barred Spiral Galaxies
Astronomy 1-2
Lecture 24-13
Barred Spiral Galaxies
Astronomy 1-2
Lecture 24-14
Elliptical Galaxies
Elliptical galaxies have no spiral arms
They range from being spherical to elliptical and
are classified according to the Eccentricity
Eccentricity = 10 (1 - b/a)
Sphere has eccentricity equal to 0
Remember the eccentricity we measure is as
seen from Earth
Astronomy 1-2
Lecture 24-15
Elliptical Galaxies
Elliptical galaxies come in many sizes, from giant
ellipticals of trillions of stars, down to dwarf
ellipticals of less than a million stars
Ellipticals also contain very little, if any, cool gas and
dust, and they show no evidence of ongoing star
formation
Many do, however, have large clouds of hot gas,
extending far beyond the visible boundaries of the
galaxy
Astronomy 1-2
Lecture 24-16
Elliptical Galaxies
Classification according to their shape
From E0 (almost spherical) to E7 (the most elongated)
Astronomy 1-2
Lecture 24-17
Elliptical Galaxies
Astronomy 1-2
Lecture 24-18
Lenticular Galaxies
This is a subspecies of galaxy that is related to
spiral galaxies
Lenticular galaxies have
Flat rotating disks
Central bulges
Very little gas and dust
No spiral arms
It may be that it is necessary a galaxy to have
interstellar gas for spiral arms to form
Astronomy 1-2
Lecture 24-19
Lenticular Galaxies
S0 (lenticular) and SB0 galaxies have a disk and
bulge, but no spiral arms and no interstellar gas
Astronomy 1-2
Lecture 24-20
Irregular Galaxies
Irregular galaxies tend to contain lots of gas and
interstellar dust
Contain lots of star formation
Star formation is not uniformly spread over the galaxy
Gas and interstellar dust not uniformly
spread through galaxy
Astronomy 1-2
Lecture 24-21
Irregular Galaxies
Astronomy 1-2
Lecture 24-22
Irregular Galaxies
The small and large
Magellanic Clouds are
close neighbors to our own
Milky Way
Astronomy 1-2
Lecture 24-23
Irregular Galaxies
NGC 4485 and NGC 4490 on the left, and M82 on the
right
Astronomy 1-2
Lecture 24-24
Summary of Galaxy Properties
Astronomy 1-2
Lecture 24-25
Question
Is the central bulge of a spiral galaxy in any way
related to elliptical galaxies?
Probably not
Spiral galaxies rotate, whereas elliptical
galaxies rotate very little, if at all
Astronomy 1-2
Lecture 24-26
Two Questions
1) Is there a connection between the types of galaxies?
2) Does one type of galaxy evolve from the other?
All galaxies are of about the same age
Spiral galaxies have stars just as old as the
stars in elliptical galaxies
Hubble tried to devise an evolutionary scheme for
galaxies
There is no evolutionary staging of galaxies
Astronomy 1-2
Lecture 24-27
Clusters of Clusters
Clusters of galaxies also seem to be clustered
into Superclusters
Clusters of galaxies are not uniformly spread
throughout the universe
Astronomy 1-2
Lecture 24-28
Bound Together
As galaxies are bound systems of stars, clusters
are also bound systems of galaxies
Measured masses within the clusters insufficient
to generate the necessary gravitational forces to
hold the cluster together
The missing mass problem again
Dark Matter
Astronomy 1-2
Lecture 24-29
Collision Course
Galaxies can and do pass through each other
There are gravitational effects, tidal effects,
between the galaxies
Gases and dusts create shock waves
Gases are stopped even though galaxies continue to
move
Gases are heated to high temperatures
When gases cool, protostar formation begins
and star formation can be prolific - Star-Burst
Galaxies
Astronomy 1-2
Lecture 24-30
Collision Course
Gravitational forces seriously can distort the
shape of both the interacting galaxies
Galaxies can "combine", with the resulting
galaxy being larger still
Merger of spiral galaxies produces elliptical galaxies
If the resultant galaxy is in a rich cluster, this
larger galaxy can continue to gobble up other
galaxies - Galactic Cannibalism
Astronomy 1-2
Lecture 24-31
Collisions
Whirlpool Galaxy
Astronomy 1-2
Lecture 24-32
Collisions
The “Mice”
For a simulation of colliding galaxies look here
Astronomy 1-2
Lecture 24-33
Collisions
Cartwheel Galaxy
Astronomy 1-2
Lecture 24-34
Mergers
Antennae Galaxies
Astronomy 1-2
Lecture 24-35
A Collision Coming Soon
The Milky Way Galaxy and the Andromeda
Galaxy are falling towards each other
Tidal effects should start in about 4 to 5 billion years
The two spiral galaxies will become a combined
elliptical galaxy
Astronomy 1-2
Lecture 24-36
A Collision Coming Soon
M81 versus M82
Astronomy 1-2
Lecture 24-37
Distribution of Galaxies
How are galaxies distributed throughout the Universe?
Cepheid variables allow distance measurements of galaxies
Astronomy 1-2
Lecture 24-38
Distribution of Galaxies
However, some galaxies have no Cepheids, and most are
farther away then 25 Mpc
New distance measures are needed
Tully–Fisher relation correlates a galaxy’s rotation speed
(which can be measured using the Doppler effect) to its
luminosity
Astronomy 1-2
Lecture 24-39
Distribution of Galaxies
Type I supernovae can also be used
They all have about the same luminosity, as the
process by which they happen doesn’t allow for much
variation
They can be used as “standard candles”
Objects whose absolute magnitude is known, and
which can therefore be used to determine distance
using their apparent magnitude
Astronomy 1-2
Lecture 24-40
Distribution of Galaxies
With these additions,
the cosmic distance
ladder has been
extended to about 1 Gpc
Astronomy 1-2
Lecture 24-41
Clustering of Galaxies
Galaxies are not uniformly distributed throughout
the universe
Galaxies are clustered
Two types of clusters:
Rich Clusters - These contain many galaxies
Poor Clusters - These contain few galaxies
Astronomy 1-2
Lecture 24-42
Local Group
The Milky Way is part of a cluster of about 45 galaxies
within about 1 Mpc of it
This is known as the Local Group
Astronomy 1-2
Lecture 24-43
Local Group
There are three spirals in this group—the Milky
Way, Andromeda, and M33
These and their satellites—about 45 galaxies in
all—form the Local Group
Such a group of galaxies, held together by its
own gravity, is called a galaxy cluster
Astronomy 1-2
Lecture 24-44
Clustering of Galaxies
A nearby galaxy cluster is the Virgo cluster
It is much larger than the Local Group, containing
about 3500 galaxies
Astronomy 1-2
Lecture 24-45
Clustering of Galaxies
This image shows the Abell 1689 cluster of galaxies,
a very large cluster almost 1 billion parsecs away
Astronomy 1-2
Lecture 24-46
Cluster of Galaxies
Perseus Cluster > 1000 Galaxies
Astronomy 1-2
Lecture 24-47
Characterization of Clusters
Clusters of galaxies are also characterized by shape
Regular Clusters
Many elliptical galaxies, but few spiral galaxies
Distinctly spherical shape
Marked concentration of galaxies at the center
Acting in concert for a long time
Dominated by at least one giant elliptical
galaxy near the center of cluster
Irregular Clusters
Marked by an asymmetrical distribution of
the galaxies
Astronomy 1-2
Lecture 24-48
Regular Cluster
Coma Cluster ~ 10,000 Galaxies
Astronomy 1-2
Lecture 24-49
Correlations in Clusters
There is a strong correlation between
Rich and Poor clusters and
Regular and Irregular clusters
Rich ==> Regular
Poor ==> Irregular
Astronomy 1-2
Lecture 24-50
Hubble’s Law
Universal recession
All galaxies (with a couple
of nearby exceptions) seem
to be moving away from
us
With the redshift of their
motion correlated with
their distance
Astronomy 1-2
Lecture 24-51
Motion of Galaxies
An examination of the spectra of other galaxies
has shown that the spectra are red shifted
A red shift in a spectrum tells us that the object
that is emitting the spectra is moving away from
us
Therefore the other galaxies are moving away
from us
Astronomy 1-2
Lecture 24-52
Galactic Motion
Hubble and his colleagues found a correlation
between a galaxy's velocity and its distance from
the Milky Way
v = H0 r
with H0 being Hubble's
constant, the expansion
rate of the universe
The more distant the galaxy, the faster it was moving!
Astronomy 1-2
Lecture 24-53
Hubble’s Law
The relationship (slope of the line) is characterized
by Hubble’s constant H0
recessional velocity = H0 × distance
The currently accepted value for Hubble’s
constant:
H0 = 70 km/s/Mpc
Measuring distances using Hubble’s law actually
works better on farther away objects
Random motions are overwhelmed by the recessional
velocity
Astronomy 1-2
Lecture 24-54
Distance Scales
This puts the final step
on our distance ladder
Astronomy 1-2
Lecture 24-55
Distribution of Galaxies
Astronomy 1-2
Lecture 24-56
Active Galactic Nuclei
About 20–25% of galaxies don’t fit well into the
Hubble scheme — they are far too luminous
Such galaxies are called
active galaxies
They differ from
normal galaxies in both
the luminosity and type
of radiation they emit
Astronomy 1-2
Lecture 24-57
Active Galactic Nuclei
The radiation from these galaxies is called
nonstellar radiation
Many luminous galaxies are experiencing an
outburst of star formation, probably due to
interactions with a neighbor
These galaxies are called starburst galaxies, and we
will discuss them later
The galaxies we will discuss now are those whose
activity is due to events occurring in and around
the galactic center
Astronomy 1-2
Lecture 24-58
Active Galactic Nuclei
This active galaxy has star-formation rings
surrounding a very luminous core
Astronomy 1-2
Lecture 24-59
Active Galactic Nuclei
Active galaxies are classified into three types
Seyfert galaxies, radio galaxies, and quasars
Seyfert galaxies resemble
normal spiral galaxies,
but their cores are
thousands of times more
luminous:
Astronomy 1-2
Lecture 24-60
Active Galactic Nuclei
The rapid variations in the luminosity of Seyfert
galaxies indicate that the core must be extremely
compact
Astronomy 1-2
Lecture 24-61
Active Galactic Nuclei
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
Astronomy 1-2
Lecture 24-62
Active Galactic Nuclei
Radio galaxies may also be core dominated
Astronomy 1-2
Lecture 24-63
Active Galactic Nuclei
Core-dominated and radio-lobe galaxies are probably
the same phenomenon viewed from different angles
Astronomy 1-2
Lecture 24-64
Active Galactic Nuclei
Many active galaxies have jets
Most show signs of
interactions with other
galaxies
Astronomy 1-2
Lecture 24-65
Active Galactic Nuclei
Quasars — quasi-stellar objects
Starlike in appearance
Have very unusual
spectral lines
Astronomy 1-2
Lecture 24-66
Active Galactic Nuclei
Eventually it was realized that quasar spectra were
normal, but enormously redshifted
Astronomy 1-2
Lecture 24-67
Active Galactic Nuclei
Solving the spectral problem introduces a new
problem
Quasars must be among
the most luminous objects
in the galaxy, to be visible
over such enormous
distances.
Astronomy 1-2
Lecture 24-68
Relativistic Redshifts and Look-Back Time
The redshift of a beam of light is its fractional
increase in wavelength
Redshifts are measured directly
Distances are calculated from them using Hubble’s
constant, which is uncertain
Astronomers therefore prefer to quote redshifts
rather than distances
The look-back time is the time when light was
emitted from a distant object
For very distant objects it is less than the redshift would
indicate, as the object has receded in the meantime
Astronomy 1-2
Lecture 24-69
The Central Engine of an Active Galaxy
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
Astronomy 1-2
Lecture 24-70
The Central Engine of an Active Galaxy
This is the leading theory
for the energy source in an
active galactic nucleus
A black hole is surrounded
by an accretion disk
The strong magnetic field
lines around the black hole
channel particles into jets
perpendicular to the
magnetic axis
Astronomy 1-2
Lecture 24-71
The Central Engine of an Active Galaxy
In an active galaxy, the central black hole may be
billions of solar masses
The accretion disk is whole clouds of interstellar gas
and dust
They may radiate away as much as 10–20% of their
mass before disappearing
Astronomy 1-2
Lecture 24-72
The Central Engine of an Active Galaxy
This pair of images shows evidence for a black hole
at the center of NGC 4261
Astronomy 1-2
Lecture 24-73
The Central Engine of an Active Galaxy
The central portion of M87 shows rapid motion and
jets characteristic of material surrounding a black
hole
Astronomy 1-2
Lecture 24-74
The Central Engine of an Active Galaxy
One might expect the
radiation to be mostly XRays and gamma-rays
But apparently this
radiation is often
“reprocessed” in the dense
clouds around the black
hole and re-emitted at
longer wavelengths
Astronomy 1-2
Lecture 24-75
The Central Engine of an Active Galaxy
Particles will emit synchrotron radiation as they spiral
along the magnetic field lines
This radiation is decidedly nonstellar.
Astronomy 1-2
Lecture 24-76