The Diverse Galaxies
© Sierra College Astronomy Department
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Lecture 14: The Diverse Galaxies
Galaxy Classification

Until the 1920s, we thought of our own galaxy as the
“Island Universe” and that everything we saw lay in our
galaxy

In 1924, Edwin Hubble found Cepheid variables in three
spiral nebulae, including one in Andromeda, proving that
they were actually spiral galaxies.

The proof that galaxies existed outside the Milky Way
expanded the scope of the universe.

Today it is estimated that over 100 billion galaxies exist in
the visible Universe.
The Hubble Classification

Hubble divided galaxies into three basic types: spiral,
elliptical, irregular.
The Diverse Galaxies
Galaxy Characteristics
Spiral Galaxies

Have relatively thin and flat whitish disks with disk
component stars (all ages and masses with
circular orbits in the disk).

Have yellowish central bulges with spheroidal
component stars (old and low mass with orbits of
all inclinations).

Bulges merge smoothly into halos that can extend
to a radius beyond 100,000 light-years.
The Diverse Galaxies
Galaxy Characteristics
Spiral Galaxies (continued)

Disks are filled with cool gas and dust
interspersed with hotter gas, and usually display
spiral arms, while bulges/halos exhibit very little
cool gas or dust.

Large bulge galaxies generally have less gas and
dust than small bulge galaxies.

Among the large galaxies in the Universe, 75-85%
are spiral or lenticular (“spiral” disks with no arms).
The Diverse Galaxies
Galaxy Classification
Spiral Galaxies – The Hubble Classification
 Hubble divided spiral galaxies into two
groups: normal spirals and barred spirals.

A barred spiral galaxy is a spiral galaxy in
which the spiral arms come from the ends of
a bar through the nucleus rather than from
the nucleus itself.

Spirals are designated with an S; barred
spirals are designated with an SB.
The Diverse Galaxies
Galaxy Classification
Spiral Galaxies (continued)

Each type of spiral galaxy is then further
subdivided into categories a, b, and c depending
on how tightly the spiral arms are wound around
the nucleus.

Galaxies with the most tightly wound arms are type a.

The size of a spiral galaxy’s bulge and the “dustiness”
of the disk can also be used to determine the
subdivision.
The Diverse Galaxies
Galaxy Characteristics
Elliptical Galaxies

Lack a significant disk component of stars.

Look like the bulges and halos of spiral galaxies (and
hence sometimes referred to as spheroidal galaxies).

Compared to spirals, ellipticals appear redder, rounder, and
often longer in one direction than in the other two.

Redder color infers ellipticals do not have hot, young, blue
stars.

Have very little cool gas or dust, though often contain very
hot, ionized gas.
The Diverse Galaxies
Galaxy Characteristics
Elliptical Galaxies (continued)
 Come in wide ranges of sizes.

Some of the most massive galaxies are giant
elliptical galaxies, some of which have 1013
stars and are thus larger than any spiral
galaxy.

Majority are small and are the most common in
the Universe.

Very small dwarf elliptical galaxies (one billion
solar masses or less) are often found near
larger spirals.
The Diverse Galaxies
Galaxy Classification
Elliptical Galaxies
 Elliptical
galaxies are classified from
round (E0) to very elongated (E7).
Lecture 14: The Diverse Galaxies
Galaxy Classification
Irregular Galaxies

Fewer than 20% of all galaxies fall in the category of
irregulars (designated as Irr) and is likely the rarest
type of galaxy at the present time.

Irregulars may have been more common in past

They are all small, normally having fewer than 25% of
the number of stars in the Milky Way.

Collisions between galaxies are not unusual because
on average galaxies are separated by distances only
about 20 times their diameter.

What Collides?
© Sierra College Astronomy Department
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Lecture 14: The Diverse Galaxies
Galaxy Classification
Hubble’s Tuning Fork Diagram

Hubble’s tuning fork diagram relates the
various types of galaxies.

Astronomers once also thought the
diagram represented an evolutionary
sequence, but this interpretation has been
discarded as old stars have been found in
all three types.
Lecture 14: The Diverse Galaxies
Galaxy Classification
Type
Designation
Elliptical
E0–E7
Spiral
Sa–Sc
Barred spiral SBa–SBc
S0
S0
Irregular
Irr
Description
Galaxies that appear
circular (E0) to very
elongated (E7).
Sa: large nuclei and tightly
wound arms. Sc: small
nuclei and open arms.
Spirals with elongated
nuclei.
Disklike; no spiral structure.
Do not fit into any other
category.
The Diverse Galaxies
Measuring Galaxies
The most important properties of a galaxy that
we can measure are its distance, mass, and
motion.
Distances Measured by a chain of interlocking
techniques:

Radar Ranging (Solar System)

Parallax (about 2000 ly max)

Main-Sequence Fitting (about 1 Mly max)

Cepheid Variables (about 100 Mly max)

Distant Standards (billions of light-years)
The Diverse Galaxies
Measuring Galaxies


Some Near-by Standard Candles (to a few 100 Mly)

Bright stars (giants, supergiants, novae) can be used as distance
indicators.

Large globular clusters and supernovae are of consistent
brightness so they, too, can be used to determine distances to
more distant galaxies.
Standard Candles and Indicators Beyond 100 Mly



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Brightest galaxies
Brightest galaxies in a galaxy cluster
Supernovae, especially Type Ia
Tully-Fisher relation
The Diverse Galaxies
Hubble Law
Some History

In 1921, Slipher found that spiral
nebulae had redshifted spectra
indicating that they were moving away
from us at tremendous velocity.

In 1929, Hubble showed that there is
a linear relationship between the
recessional velocities of galaxies and
their distances.
Lecture 14: The Diverse Galaxies
Hubble Law

The Hubble law:
v = H0d
Cosmic Calculations 16.2
where v is radial velocity, d is distance, and H0 is the
Hubble constant (the 0-subscript refers to its value
today, and not the past).

The Hubble constant is the proportionality constant
in the Hubble law; the ratio of recessional velocities of
galaxies to their distances.

Modern day measurements of the Hubble constant
place it about 73 km/s per megaparsec (Mpc) or 22
km/s per Mly.

The Hubble law is not ideal:
 It does not apply to nearby galaxies where gravity dominates
 It relies on a accurate measurement of Hubble’s constant
The Diverse Galaxies
Hubble Law


Practical Use of Hubble Law

The spectral shift seen in distant galaxy spectra can be
translated into the recession velocities of the galaxies.

For these distant galaxies, the Hubble law can then be used
to determine their distances
Cosmological Implications

The Hubble Law shows that the universe is expanding, and it
is the foundation for today’s theories of cosmology - the study
of the nature and evolution of the universe as a whole

Order of magnitude age of the Universe using Hubble Law is
1/Ho and this gives a range of 12-15 billion years.
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
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Lecture 14: The Diverse Galaxies
Active Galaxies
Active Galaxies

An active galaxy is a galaxy with an
unusually luminous nucleus.

Three main types of active galaxies:

Radio galaxies

Have greatest luminosity at radio wavelengths with a
double-lobed radio source.

Radio galaxies often exhibit unusual jets in visible light.

Generally, they are elliptical galaxies.
Lecture 14: The Diverse Galaxies
Active Galaxies - Quasars

Seyfert galaxies

A class of spiral galaxies having abnormally luminous nuclei.

The immense luminosity is spread over all wavelengths are fluctuates
rapidly.

Contain very fast moving gas clouds in some instances being ejected in
small jets.

BL Lacertae objects is another type of active galaxy which have
their jets point right at us

Quasars (Quasi-stellar objects or QSO)

A small, intense celestial source of radiation with a very large redshift
(implying speeds close to c and at very large cosmological distances).

Some are powerful radio sources and others eject hot gas from their
centers.

Often appear to lie within ordinary galaxies.
Lecture 14: The Diverse Galaxies
Active Galaxies - Quasars

From the Hubble Law, a large redshift
implies large distances and existence in the
past (era of the quasars).

Due to their large distances, quasars provide
an excellent testing ground for general
relativity through observations of
gravitational lensing and microlensing.

Quasars are compact objects the size of a
solar system
Lecture 14: The Diverse Galaxies
The Nature of Active Galaxies

What Makes Some Galaxies Active?

Current explanation: An accretion disk feeding
material into a supermassive black hole at the
galactic center.

A supermassive black hole is created in the early years of a
galaxy’s growth.

As long as there is enough material in the disk to feed to black
hole, the galaxy remains active.

All galaxies appear to have supermassive black holes
at their centers.

The different types of active galaxies may be the same
basic object simply seen from different vantage points.
Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Two Important Ingredients

The Structure and Fate of the Universe
Depend on Two Important Ingredients
Dark Matter: Discovered in galactic halos, it is
the dominant form of mass in the Universe, but
other than the shape of galactic rotation curves, it
has not been detected, and hence identified, by
any other means. (Possible causes: MACHOs,
black holes, WIMPs, neutrinos)
 Dark Energy: An unidentified form of energy that
is causing the Universe to increase its rate of
expansion with time, contrary to all expectations
before its discovery. (Possible causes: vacuum
energy, cosmological constant, quintessence)

© Sierra College Astronomy Department
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Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter

Evidence for Dark Matter in Galaxies

(continued)
Other Spiral Galaxies
 Rotation
curves determined by measuring 21-cm
radiation from atomic hydrogen clouds since this
radiation can be detected at large distances from
the galactic centers.
 Rotation curve used to determine a galaxy’s total
mass.
 Total luminosity of galaxy used to determine
galaxy’s mass due to stars and this is subtracted
from the total mass to obtain the dark matter mass.
 Typical spiral galaxies are 90% dark matter and
10% stellar matter.
© Sierra College Astronomy Department
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Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter

Evidence for Dark Matter in Galaxies

(continued)
Elliptical Galaxies
Rotation curves are not possible due to random motion
of stars and 21-cm radiation analysis not possible due
to the lack of gas.
 The random motion of the stars creates a broadened
spectral line and the broader the line the faster the
stellar motion.
 Spectral lines maintain a fairly constant width, and
hence the star velocities remain fairly constant as we
look at greater distances from the galaxy center.
 Consequently, ellipticals show evidence of dark matter.
 Globular clusters orbiting around ellipticals also lend
evidence to the dark matter being present.

© Sierra College Astronomy Department
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Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter

Evidence for Dark Matter in Galaxy Clusters

Orbits of Galaxies in Clusters

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

The recessional velocities of a cluster of galaxies are found
and these velocities are used to determine the cluster center.
The radial velocity of each cluster relative to the cluster
center is then determined, and through an averaging
process, the average orbital velocity of the cluster galaxies is
determined.
The average orbital velocity of the galaxies then gives
the cluster mass and this mass is compared to the
cluster luminosity.
Cluster luminosities are found to be far too low for the
amount of mass present.
The amount of dark matter is found to be up to 50 times more
than the mass in the stars, a multiplication factor significantly
greater than what is found for individual galaxies.
© Sierra College Astronomy Department
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Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter

Evidence for Dark Matter in Galaxy Clusters

(continued)
Hot Gas in Clusters



Gas within a cluster (intercluster medium) has been found
from X-ray studies to be very hot (10s to 100s of millions of
degrees) and have 7 times as much mass as the stars in the
cluster.
Since most clusters are is a state of gravitational equilibrium,
the temperature of a cluster’s intercluster medium is
dependent on the cluster’s total mass.
Specifically, the speed vH (in m/s) at which hydrogen nuclei
move around the center of a cluster given a gas temperature
of T (in Kelvin) is:
m
v H  140

s
x T
The amount of dark matter is found to be up to 50 times more
than the mass in the stars, the same amount as found with
the orbiting galaxies technique, and is the “glue” that holds
clusters together.
© Sierra College Astronomy Department
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Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Evidence for Dark Matter

Evidence for Dark Matter in Galaxy Clusters (continued)

Gravitational Lensing





All methods to this point are based on Newton’s laws to measure
galaxy and cluster masses.
General Relativity predicts that mass can “bend” spacetime and this
prediction has been confirmed by numerous observations.
A consequence of this bending of spacetime is that a concentration
of mass can act like a lens and bend light beams by a process
known as gravitational lensing.
The amount of bending is directly related to the amount of mass
causing the bending.
From the analysis of photos showing galaxy clusters (acting as the
mass lenses) and the multiple images of very distant galaxies, the
amount, it is once again found that dark matter exceeds the mass of
the stars by almost 50 times.
© Sierra College Astronomy Department
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Cosmology: Dark Matter, Dark Energy, Fate of the Universe
Is Dark Matter for Real?

Does Dark Matter Really Exist?




All evidence for dark matter rests on our
understanding of gravity, either from the Newtonian
view or the Einsteinian view.
It is possible that our theory of gravity is wrong on
cosmological scales?
Perhaps, but all attempts to explain dark matter with
alternate theories of gravity, do not succeed in
explaining all the other systems involving mass which
are explained by Newton/Einstein.
Thus, until some other solid idea comes along to the
contrary, dark matter needs to be taken seriously.
© Sierra College Astronomy Department
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Cosmology: Dark Matter, Dark Energy, Fate of the Universe
The Stuff of Dark Matter

Dark Matter Composition

“Ordinary” Dark Matter


Most ordinary matter is made of protons and neutrons, which in
turn belong to a group of particles called baryons.
Objects like you, the planets, brown dwarfs, and faint red mainsequence stars, all baryonic matter, will appear as dark matter
since our technology cannot detect the radiation from them.




They do not emit enough light even if they were a few light years
away.
These objects are called MACHOs = MAssive Compact Halo
Objects.
Lone black holes would also go undetected from observations of
radiation.
Back-of-the-envelope calculations and probing the Milky Way’s
halo with gravitational lensing techniques (more properly microlensing), shows that MACHOs and black holes cannot account
for all of the dark matter mass.
© Sierra College Astronomy Department
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Cosmology: Dark Matter, Dark Energy, Fate of the Universe
The Stuff of Dark Matter

Dark Matter Composition

(continued)
“Extraordinary” (Non-Baryonic) Dark Matter

Neutrinos

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
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
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Produced in large quantities by stellar fusion.
No electrical charge and give off no radiation.
A neutrino has very low mass and is only subject to weak and
gravitation forces.
 Known as a weakly interacting particle.
Very high speeds do not allow a galaxy to trap them gravitationally.
Calculations show they can only make up a small part of the dark
matter outside galaxies.
Other Particles




What about more massive and weakly interacting particles?
These types of particles, called Weakly Interacting Massive
Particles (WIMPs) are theoretical.
Believed to constitute most of the dark matter in the Universe.
Explains why dark matter in halo and not disk (WIMPs lack a
cooling and collisional mechanisms to bring them into disk).
© Sierra College Astronomy Department
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The End