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The Andromeda Galaxy - nearest
galaxy similar to our own.
Only 2 million light years away!
• Galaxies are clouds of millions
to hundreds of billions of stars
held together by their mutual
• Often galaxies also contain
enormous clouds of gas and
dust from which new stars can
• Galaxies can have many
different shapes and sizes.
• The distribution of galaxies
across the Universe indicates
that they generally appear in
clusters with very large voids
separating clusters from one
Types of Galaxies
• Galaxies are classified based on
general characteristics and then
further subdivided based on more
specific characteristics.
• Spiral galaxies
– disk-shaped with spiral arms
winding out from the center.
– usually have clouds of gas and dust
– usually have both young (Pop. I)
and old (Pop. II) stars
– some spirals have a rectangularshaped bar through the central bulge
and are referred to as Barred Spirals
A Hubble Space Telescope view
of the Whirlpool Galaxy M51
Types of Galaxies
• Elliptical galaxies are always
smooth in appearance.
– can be spherical, egg-shaped or
flattened in shape
– usually have little or no gas and
– usually only contain old stars
(Pop. II)
• Irregular galaxies have a
random distribution of stars.
– often have large clouds of gas
and dust
– often contain young stars (Pop.
Galaxy Collisions
• galaxies were smaller in
the past
• collisions between
galaxies appear to have
been common in the
early Universe
• collisions can cause
bursts of star formation
as clouds of gas and dust
• galaxies may eventually
merge together forming
large elliptical galaxies
A Hubble Space Telescope image
of the Antennae Galaxies. Large streams
of stars and gas are trailing off the galaxies
while new stars are being formed near the center.
Active Galactic Nuclei (AGN)
• Spiral (or disk) galaxies with nuclei
that are more luminous than the rest
of the stars in galaxy.
– Spectrum of the nucleus is non-stellar
– Luminosity of the nucleus may change
over short (hours-months)
• Some elliptical galaxies show radiowave emitting jets on scales much
larger than the visible light size of
the galaxy
• Originally detected in images as
point-like (quasi-stellar) objects,
although spectra are non-stellar
• Underlying faint host galaxies
recently detected in some quasars
by the Hubble Space Telescope
• Largest redshifts of any
astronomical object
– Hubble law implies they are at
great distances (as much as 10
billion light-years away)
– To be visible at those distances,
they must be about 1000× more
luminous than the Milky Way
• Based on output fluctuations,
quasars resemble the AGNs of
radio galaxies and Seyfert galaxies
in that they are small (fractions of
a light-year in some cases)
Measuring the Diameter of Astronomical
Objects by Using Their Light Variability
• Technique makes three assumptions
– The rate at which light is emitted from an active region is the
same everywhere in that region
– The emitting region completely defines the object of interest
(there are no “dead” areas of significance)
– The speed of light is finite (a safe bet)
• The light variation then is just a measure of the time it
takes light to travel across the active surface
• Multiplying this time by the speed of light gives the
size of the emitting object
Measuring the Diameter of Astronomical
Objects by Using Their Light Variability
Active Galactic Nuclei (AGN)
• Large luminosity
• Variable on short timescales
– Implies small physical size
• Can not fit enough luminous stars (or
supernovae) into such a small region
• A supermassive black hole is thought to be
the source of energy for these AGNs.
• The black hole likely formed initially as the
remnant of a massive star supernova at the
center of a galaxy
• The black hole fed on the vast reservoir of
gas in the galaxy nucleus
• Eventually grows large enough to capture
• Galaxy-galaxy interactions may cause gas
infall within galaxy, supplying more fuel
• The variable, luminous source is the
accretion disk and associated gas clouds
surrounding the Black Hole
Black Holes and Galaxy Formation
•Black Holes may play a role in
galaxy formation
•Nearly all galaxy nuclei have a
Black Hole (active, or inactive)
•Black Hole mass is correlated
with galaxy bulge mass
106 < M● < 109 solar-masses
Distances to the Galaxies
Cepheid stars are very bright
and can be observed in nearby
galaxies. Other methods must
be used for more distant galaxies.
• Determining accurate distances to
galaxies requires knowledge of the
properties of stars.
• From the luminosity of a star and
its apparent brightness the star’s
distance can be found.
• Certain stars (called Cepheid
variables) show regular patterns of
variation in brightness.
• The period of these variations are
directly related to the stars
• So by measuring the time it takes
these stars to vary in brightness and
their apparent brightness their
distance can be found.
Recessional Velocity of Galaxies
• In 1911, it was discovered
that all galaxies (with but a
few exceptions) were
moving away from the Milky
• Edwin Hubble found that
these radial speeds,
calculated by a Doppler shift
analysis and called a
recessional velocity,
increased with a galaxy’s
Insert Figure 17.18a here
The Expansion of the Universe
• Astronomers in the early 20th century
found that Doppler shifts seen in the
spectra of galaxies indicated that almost
all galaxies are moving rapidly away
from us.
• Edwin Hubble, using careful
determinations of galactic distances,
showed that the farther a galaxy is from
us the faster it appears to be moving
• He showed that the velocity of the
galaxy was simply equal to its distance
times a constant.
The exact value for Hubble’s
• This is known as Hubble’s Law and the
constant has been one of the
constant is Hubble’s constant.
great problems in astronomy.
• The recessional velocity is a
It was one of the reasons for
consequence of the expansion of space.
building the Hubble Space Telescope.
Ho = 71 +/- 4 km/sec/Mpc
Galaxy Mass and Dark Matter
If you know the distance to a
galaxy and measure its angular
size, you can find its actual, linear
Now measure the rotation curve:
the velocity of stars as a function of
distance from the center of the
galaxy (x-symbols on plot)
Use Newton’s modification of
Kepler’s third law to estimate a
rotation curve for the lightemitting the mass in the galaxy
(stars, gas, dust) (dot-symbols on
the plot)
The observed rotation curve and
the estimated curve do not match.
The mass derived from the
observed rotation curve more than
the estimated mass of the light
emitting mass (stars, gas, dust)
– Dark matter mass is estimated to
be 10x mass of light emitting
Galaxy Clusters
• Galaxies do not
uniformly fill the
universe but rather
collect into clusters and
superclusters spanning
millions of light years.
• The Milky Way is a
member of a cluster
called the Local
Galaxy Clusters and Dark Matter:
Galaxy Velocity
• Astronomers find that
galaxies often are moving in
these clusters with very high
speeds. The clusters should be
flying apart but there appears
to be enough mass to hold the
cluster together.
• This is more mass than can be
accounted for just by gas,
dust, and stars.
• This invisible mass holding
the cluster together may be
100x larger than all the
visible mass and is another
example of “dark matter”.17
Galaxy Clusters and Dark Matter:
Hot Gas in Clusters
• The particle velocity of hot gas in galaxy clusters exceeds
the escape velocity of the light-emitting matter
– Infer that dark matter binds the observed hot gas to the cluster
Gravitational Lenses
Galaxy Clusters and Gravitational Lenses
• Light can be bent as it
passes close to massive
objects. This is what
happens in black holes and
also happens in galaxy
• Light from very distant
galaxies can be bent
around nearby galaxy
clusters and appear as arcs
or mirror images of the
original galaxy.
• Astronomers use these
gravitational lenses to
Hubble Space Telescope image of
study galaxies that may
a gravitational lens formed by a galaxy
have been too far to
observe any other way.
Galaxy Clusters, Gravitational Lenses and
Dark Matter
• The lensing
morphology depends on
the mass distribution in
the cluster
• Lensing studies show
that the mass of the
cluster exceeds the
mass of the lightemitting matter within
the cluster  more
evidence for dark
matter in the universe
Dark Matter in Clusters
The figure shows two clusters that
The red regions shows the X-ray
emitting shocked gas stripped out of the
clusters by the collision
The blue regions show the estimated
dark matter distribution of each cluster
Note that the dark matter seems to be
disturbed less than the hot gas
– Dark matter does not seem to
interact strongly with itself, or has
a faster relaxation time than normal
Dark Matter Candidates
• Dark matter cannot be:
– Ordinary dim stars because they would show up in infrared
– Cold gas because this gas would be detectable at radio
– Hot gas would be detectable in the optical, radio, and x-ray
regions of the spectrum
• Objects that cannot be ruled out:
– Tiny planetesimal-sized bodies, extremely low-mass cool
stars, dead white dwarfs, neutron stars, and black holes
• MACHOS: Massive Astrophysical Compact Halo Objects
– Subatomic particles like neutrinos
– Theoretically predicted, but not yet observed, particles
• WIMPS: Weakly Interacting Massive Particles
Large Scale Structure of the
•Two 2-D slices mapping
angular position and
distances of galaxies
relative to the Earth
•High redshift galaxy
surveys show galaxies
distributed in a pattern of
filaments and voids
Large Scale Structure and Dark
•3-D Snapshots of a large scale structure simulation as the universe
•Simulation input: expansion of the universe, mass of galaxies,
•Simulations that include dark matter result in large scale structure
maps that show the observed filament + void morphology