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Galaxies
Galaxies
• Galactic Morphology
• Interacting Galaxies
• "Active" Galaxies
Galaxy Types
Hubble Classification
 Elliptical Galaxies
 Spiral Galaxies
 Lenticular
 Non-barred
 Barred
 Irregular Galaxies
Elliptical Galaxies
The most massive galaxies are ellipticals, and they
feature significantly in clusters of galaxies.
– a smooth, featureless appearance
– little gas and dust
– reddish color; low rate of star formation; no corecollapse supernovae.
– huge range of possible masses from 106 to 1013 Solar
Masses
– very elongated stellar orbits and little overall rotation.
– Classified as E0 through E7
Elliptical Galaxies
Elliptical galaxies, classed by E0 to E7
The E stands for elliptical (obviously)
The number indicates how egg-shaped the ellipse is
- 0 means a ball shape
- 5 a bit like a football
- 7 looks like a cigar
E0
E1
E5
E7
Spiral Galaxies
• Classified as S0, Sa, Sb, or Sc
• Barred Spirals: SBa, SBb SBc
• Have similar features
– Nucleus, Bulge, Disk and Halo
Lenticular Galaxies
S0 is the class for Lenticular galaxies
–Spiral galaxies without spiral arms
S0
ngc5866
Edge-On
Face-On
Spirals
All have spiral arms, and they are grouped by how tightly those arms
are wound and how large the central bulge is - the two happen to be
closely related.
The name is defined by the "S" and the lower case letter after
which indicates how wound up the arms are: from "a" to "c": Sa, Sb,
Sc
The lower branch of the tuning fork diagram is largely a copy of the
upper branch, but its occupants all have a line of stars through the
center - a bar. The B stands for barred: SBa, SBb, SBc
Sa
SBa
Sb
SBb
Sc
SBc
Spiral Galaxies
• The central bulge is similar to an elliptical galaxy (i.e. smooth
and reddish in color)
• a surrounding, highly flattened disk in circular orbital motion
about the spheroid
• large amounts of gas and dust in the disk where stars
actively form
• spiral arms within the disk
• haloes of stars and globular clusters and dark matter in
which the disk and spheroid are embedded
• masses: 1010 to 3x1011 Solar Masses
• in some spirals, central bulge has a bar-shape: `barred’
spirals.
Irregular Galaxies
• Some spirals are poorly
defined and merge with
a set classed as
`irregular'. Irregulars
feature
 very large dust and gas
fractions
 vigorous star formation
which gives a patchy
appearance.
 Masses: 106 to 1010
Solar Masses
Tuning Fork Diagram
Environmental effects
Unlike most stars, galaxies are heavily effected by their environment.
When discussing stars, collisions were barely mentioned except in the dense
cores of globular clusters.
In the Solar neighborhood, an average main-sequence star (excluding binary
stars) is separated by of order 107 times its size from its nearest neighbors
(1 Solar Radius vs. 1 pc).
Galaxies on the other hand have sizes ranging from 1 to 100 Kpc, but are
separated by of order 1 to 10 Mpc from their neighbors, only a factor of 100
to 1000. This means that almost all galaxies have probably had direct
interactions, collisions and mergers with others during their lives.
For an individual star, a galaxy collision would not mean much, however, gas clouds
are likely to collide and star formation affected considerably. The result may
be a much higher supernova rate and the birth of a young group of stars. It is
possible that the collisions of spirals disrupt their disks and lead to elliptical
galaxies.
Interacting Galaxies
Galactic Populations
• Population I
– Stars with heavy elements
– New star formation
– Found in
• Irregular galaxies
• Spiral galaxy disks
• Population II
– Stars with little or no heavy elements
– Old stars
– Found in
• Elliptical galaxies
• Spiral galaxy halos and bulge
Formation of Galaxies
• How do galaxies form?
– Structural differences
• Spiral versus Elliptical
– Seems to be largely a matter of the original
rotation
• No rotation of original gas cloud – Elliptical
• Rotation – Spiral
• Elliptical (E0 thru E7)
– Translation through the surrounding gas
• Leaves a ‘wake’
• Irregular -- Little translation or rotation
Spiral Arms
How do the spiral arms form?
Density Waves
Spiral Rotation
If the spiral galaxies were rigid, like a wheel or a disc
then we would expect to measure the speed as a
function of distance from the center as:
Spiral Rotation
On the other hand, if it was composed of independent
stars orbiting the great mass at the center, it would
follow Kepler's 3rd law and look like:
Rotation of the Milkyway
Even with the uncertainties in the data, it's clear that the Milkyway in
not a rigid body and is not following Kepler's 3rd law
Rotation of M31
Andromeda has similar behavior
Other Galaxies Rotation
Seems to be a feature, not an anomaly:
Galactic Rotations
• The odd speed distribution does have a
solution, but it adds to the mystery
• This type of speed distribution happens
when there is a lot more mass out in the
disk than toward the center.
• We can't see this mass. It is now called
"Dark Matter"
• Estimates of the Dark Matter imply that
the visible mass of the Universe is a very
small percentage of what is really there.
Active Galaxies
Active Galaxy Zoo
 Seyfert Galaxies
 Radio Galaxies
The Central Engine
 Energy generation efficiency of
accretion
 How big are the black-holes?
Seyfert Galaxies
• Bright, point-like
nuclei
– Seyfert I
• Broad emission line
spectra like a quasar
• Strong X-ray
• Low (compared to
quasars) luminosity
– Seyfert II
• Narrow emission lines
only
• Dust and Distance
Seyfert Galaxy
Depends on the View
Radio galaxies
• At radio wavelengths, most sources are galaxies; stars are
feeble emitters of radio waves in general. Some galaxies are
much more powerful at radio wavelengths than normal. They
can exceed the Milky Way by 103 to 107 times. These are
radio galaxies. When resolved many have a double-lobe
appearance in which two large lobes some hundred of
kiloparsecs apart emit radio waves.
• Further imaging revealed that these lobes are powered by
jets emanating from the nuclei of (usually) elliptical galaxies.
One can achieve remarkable resolution at radio wavelengths,
and yet it is never possible to resolve the source of these
jets. The jets contain material moving close to the speed of
light. The lobes are formed as these jets plough into the
intergalactic medium.
Radio Galaxies
Centaurius A
Radio Image
Optical Image
Anatomy of a Radio Source
Sagittarius A
Why the Double Lobe?
Blazars
A Blazar is a very compact and highly variable energy
source associated with a presumed supermassive black
hole at the center of a host galaxy.
Blazars are among the most violent phenomena in the
universe
Blazars are active galactic nuclei
(AGN) with a relativistic jet that is
pointing in the general direction of
the Earth. We observe "down" the
jet, or nearly so, and this accounts
for the rapid variability and compact
features
Peculiar Galaxies
The Cartwheel Galaxy
IRAS Galaxies
– Infrared Astronomy
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•
•
NASA's Spitzer Space Telescope has detected the building blocks of life in the distant
universe.
Training its eye on a faint object located at a distance of 3.2 billion light-years , Spitzer has
observed the presence of water and organic molecules in the galaxy IRAS F00183-7111.
With an active galactic nucleus, this is one of the most luminous galaxies in the universe,
rivaling the energy output of a quasar. Because it is heavily obscured by dust, most of its
luminosity is radiated at infrared wavelengths
The broad depression in the center of the
spectrum denotes the presence of silicates
(chemically similar to beach sand) in the galaxy.
An emission peak (red) within the bottom of the
trough is the chemical signature for molecular
hydrogen.
The hydrocarbons (orange) are organic molecules
comprised of carbon and hydrogen, two of the
most common elements on Earth.
Since it has taken more than three billion years
for the light from the galaxy to reach Earth, it is
intriguing to note the presence of organics in a
distant galaxy at a time when life is thought to
have started forming on our home planet.
The Eye of the Beholder:
What we see depends on how we see it
Radio Galaxy / Seyfert 2
Quasar / Seyfert 1
Blazar
Gamma Ray Burst (GRB)
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•
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Gamma-ray bursts (GRBs) are the most luminous electromagnetic events occurring
in the universe since the Big Bang.
They are flashes of gamma rays emanating from seemingly random places in deep
space at random times.
The duration of a gamma-ray burst is typically a few seconds, but can range from a
few milliseconds to several minutes, and the initial burst is usually followed by a
longer-lived "afterglow" emitting at longer wavelengths
Most observed GRBs appear to be caused by the collapse of the core of a rapidly
rotating, high-mass star into a black hole.
Magnetar
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A magnetar is a neutron star with an extremely powerful magnetic field, the
decay of which powers the emission of copious amounts of high-energy
electromagnetic radiation, particularly X-rays and gamma-rays.
Magnetars are somewhere around 20 kilometers in diameter. Despite this, they
are substantially more massive than our Sun. Magnetars are so compressed that a
thimbleful of its material is estimated to weigh over 100 million tons.
Most magnetars recorded rotate very rapidly, at least several times per second.
The active life of a magnetar is short.
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Their strong magnetic fields decay after about 10,000 years, after which point activity and strong Xray emission cease.
Given the number of magnetars observable today, one estimate puts the number
of "dead" magnetars in the Milky Way at 30 million or more.
Quakes triggered on the surface of the magnetar cause great volatility in the star
and the magnetic field which encompasses it, often leading to extremely powerful
gamma ray flare emissions which have been recorded on Earth in 1979, 1998 and
2004.[
The Power Source
It is now widely believed that all active galaxies are
powered by the same phenomenon: accretion onto
supermassive black-holes. The various types
reflect differences in viewing angle and jet
activity. The evidence that suggests this model
can be summarized by:
 high-velocity gas ( 10,000 Km/s) and relativistic jets imply a
deep potential.
 the tiny size of the energy generation region is impossible
for stable star clusters
 accreting black-holes are efficient 1014 Solar Luminosities.
e.g. implies 4x1024 Kg/s at 10% conversion efficiency, or 70
solar masses per year.
 Any stellar source would use up material at 10 times the
rate
Energy generation efficiency of accretion
Accretion is a source of power. In fact, other than
matter/anti-matter annihilation (which does not
play a significant role in astronomical energy
generation), it is by some way the most efficient
source of power.
– For a Neutron Star, this is about 30x more efficient
than nuclear fusion
– Black-holes are also efficient although less so than
neutron stars
• This is because black-holes have no surface so much of the energy
is never released but is swallowed up by the black-hole directly and
also orbits are unstable within three times the Schwarschild radius
and little energy is returned inside this distance.
• These factors lead to an efficiency of about 10%
How big are the black-holes?
There is an interesting physical limit that allows us
to estimate a minimum mass for the black-holes
that power active galaxies, if indeed they do. It is
based upon the balance of gravity with radiation
pressure.
– Material coming into the black-hole is hot and ionized.
Photons radiated by the black-hole interact mostly with
electrons and exert an outward force on them. The
electrons are electrostatically coupled to protons which
are gravitationally attracted to the black-hole.
How big are the black-holes?
• If the accretion rate and corresponding luminosity
are too high, the radiation pressure will exceed
gravity and mass will be pushed away from the
black-hole. The higher the mass of the black-hole,
the larger luminosity will be required for this to
take place, but in the end we conclude that
 for a given black-hole there is a maximum accretion rate
and luminosity that it can sustain.
 the limiting luminosity scales linearly with the black-hole
mass
 This is known as the Eddington limit after its discoverer.
It applies equally to neutron stars and white dwarfs as to
black-holes
How big are the black-holes?
•
It is now simple to estimate what mass we need to produce 1014 L.
It comes out to be 3x109 M . Such a black-hole has a
Schwarschild radius of 1010 Km, comparable to the radius of Pluto's
orbit around the Sun. Although large, this satisfies the restriction
upon the size of the energy generation region.
•
There is now good evidence that most galaxies, active or not,
contain large black-holes, if not always as large as a billion solar
masses.