Download Galaxies

ASTR 113 – 003
Lecture 10 April 5, 2006
Spring 2006
Introduction To Modern Astronomy II
Review (Ch4-5): the Foundation
1.
2.
3.
4.
5.
6.
7.
Sun, Our star (Ch18)
Nature of Stars (Ch19)
Birth of Stars (Ch20)
After Main Sequence (Ch21)
Death of Stars (Ch22)
Neutron Stars (Ch23)
Black Holes (Ch24)
Star (Ch18-24)
Galaxy (Ch 25-27)
Cosmology (Ch28-29)
Extraterrestrial Life (Ch30)
1. Our Galaxy (Ch25)
2. Galaxies (Ch26)
3. Active Galaxies (Ch27)
1.
2.
Evolution of Universe (Ch28)
Early Universe (Ch29)
ASTR 113 – 003
Lecture 10 April 5, 2006
Spring 2006
Galaxies
Chapter Twenty-Six
Guiding Questions
• How did astronomers first discover other galaxies?
• How did astronomers first determine the distances to
galaxies?
• Do all galaxies have spiral arms, like the Milky Way?
• How do modern astronomers tell how far away galaxies
are?
• How do the spectra of galaxies tell astronomers that the
universe is expanding?
• Are galaxies isolated in space, or are they found near
other galaxies?
• What happens when galaxies collide with each other?
• Is dark matter found in galaxies beyond the Milky Way?
• How do astronomers think galaxies formed?
First Discovery of Other Galaxies
• Spiral “nebulae” were thought to be inside the Milky
way
Hubble proved that the spiral nebulae
are beyond the Milky Way
• Edwin Hubble used
Cepheid variables to
show that the
“nebula” were
actually immense
star systems far
beyond our Galaxy
• Cepheid variables
obey the periodluminosity law
Classifying Galaxies
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Hubble classification: classification is based on
appearance only
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Four major types of galaxies:
1. Spiral galaxies (S)
(Type 1+2: 77%)
2. Barred spiral galaxies (BS)
3. Elliptical galaxies (E) (20%)
4. Irregular galaxies (I) (3%)
Spiral galaxies (S)
• Spiral arms are active star-forming region
• The stars in the spiral arms are mainly metal-rich
population I star
• Sub-classification based on the smoothness of spiral
arms and size of bulge: Sa, Sb, Sc
– Sa: broad arms, relatively large bulge
– Sc: narrow arms, relatively small bulge
– Sb: intermediate
Barred Spiral galaxies (SB)
• A bar shaped region running through the nucleus.
• Spiral arms originate at the end of the bar rather than
the nucleus itself
• Sub-classifications: SBa, SBb, SBc (same as S
galaxies)
Elliptical galaxies (E)
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Elliptical shape, have no spiral arms
Devoid of gas and dues
Consists of old, red and metal-poor population II stars
Sub-classifications: E1, E2…E7 (based on flatness)
– E1: roundest
– E7: flattest
Irregular galaxies
• Irregular galaxies have ill-defined, asymmetrical shapes
• They are rich in interstellar gas and dust
They are often found associated with other galaxies
Hubble’s Tuning Fork Diagram
• E, S, Sb and I types
• Lenticular galaxies are intermediate between spiral and
elliptical galaxies
Summary Table of Classification
Determine Distances to Galaxies
•Use Standard Candle,
an object that lies
within that galaxy and
for which we know the
luminosity
•Standard candles
include Cepheid
variables, supernovae
(Type Ia).
The Distance Ladder
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Parallax: 500 pc
Spectroscopic parallax: 10 kpc
RR Lyrae variable: 100 kpc
Cepheid variable (104 Ls): 30 Mpc
Type 1a Supernovae (109 Ls): 1000 Mpc
Redshift of Galaxies
•Hubble found the spectrum of
galaxies have redshift
z = (λ – λ0)/λ0
z = (Δλ)/λ0
z: value of redshift
λ: wavelength of shifted spectral line
λ0: wavelength of unshifted spectral line
•According to Doppler’ law,
redshift means the galaxies are
receding from us
•e.g., z=0.20
V (velocity)=0.20 C (speed of light)
V = 75000 km/s
The Hubble law
•The Hubble law: the more distance a galaxy, the greater its
redshift and the more rapidly it is receding from us.
v = H0 d
V: velocity in unit of (km/s)
D: distance in init of Mpc
H0, Hubble constant, ~ 71 km/s/Mpc, but not certain
•The Hubble constant
indicates how fast our
universe is expanding
ASTR 113 – 003
Lecture 11 April 12, 2006
Spring 2006
Introduction To Modern Astronomy II
Review (Ch4-5): the Foundation
1.
2.
3.
4.
5.
6.
7.
Sun, Our star (Ch18)
Nature of Stars (Ch19)
Birth of Stars (Ch20)
After Main Sequence (Ch21)
Death of Stars (Ch22)
Neutron Stars (Ch23)
Black Holes (Ch24)
Star (Ch18-24)
Galaxy (Ch 25-27)
Cosmology (Ch28-29)
Extraterrestrial Life (Ch30)
1. Our Galaxy (Ch25)
2. Galaxies (Ch26)
3. Active Galaxies (Ch27)
1.
2.
Evolution of Universe (Ch28)
Early Universe (Ch29)
Galaxies are grouped into clusters and
superclusters
• Galaxies do not evenly or randomly distributed
throughout the Universe
• Galaxies tend to be grouped into clusters
• Rich cluster has far more number of galaxies than
poor cluster
• A poor cluster is often called group
• Local Group is the galaxy cluster containing the Milky
Way Galaxy; Local Group is a poor cluster of about 40
galaxies
Local Group
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A poor, irregular cluster of about 40 galaxies
The diameter is about 1 Mpc (mega parsec)
The largest is M31, the Andromeda Galaxy
The Milky Way is in the second place
Both Milky Way and M31 are surrounded by a number of
small satellite galaxies
An example of Rich Cluster of Galaxies
Coma Cluster of Galaxies
• A rich cluster contains
hundreds or even
thousands of galaxies
• The Coma cluster, a
rich and regular
cluster is about 90
Mpc (300 million light
year) from the Earth
• It has as many as
10000 galaxies
Supercluster of Galaxies
• A supercluster of galaxies is a huge association of
clusters of galaxies
• A typical supercluster contains a dozen of individual
clusters
• It spans up to 50 Mpc
Distribution of Galaxies in the Universe
• This map shows 1.6 million galaxies from the 2MASS
(Two-Micron All-Sky Survey) survey
• Supercluster of Galaxies lie along filaments
• There are large dark voids that contain few galaxies
Distribution of Galaxies in the Universe
• This map shows 60000 galaxies in two wedges extending up
to redshift z=0.25 from 2dfGRS (Two Degree Field Galactic
Redshift Survey)
• It also show filements and voids
• The voids are roughly spherical, 30 to 120 Mpc in diameter
Galaxy Collision
The gravitational tidal force deforms the galaxies: stars are
hurled into intergalactic space along arching streams
Galaxy Collision
•Two galaxies can merge into a bigger galaxy
•Galactic cannibalism
•Interstellar gas can be compressed, triggering star formation
Dark Matter inferred from Rotation Curve
• If mass distribution follows the luminosity distribution, the
rotation curve would fall off according to Neuton’s and/or
Kepler’s Law
• The flat rotation curve at large distance indicates the presence
of extended halo of no-luminous matter, or dark matter
Dark Matter inferred from Gravitational Lensing
•Gravitational Lensing: a massive galaxy deflects light rays
like a lens so that an observer sees multiple distorted images
of a more distant galaxy
•Gravitational lensing is predicted by Einstein’s general theory
of relativity: space is curved due to gravity
Dark Matter inferred from Gravitational Lensing
• Examples of Gravitational lensing
•The mass of galaxies calculated from gravitational lensing is
much larger than the visible mass; again, 90% dark matter
One More Example of Gravitational Lensing
Dark Matter Candidates
• Massive neutrons, called WIMPs (weakly interacting
massive particles)
• MACHOS (massive compact halo objects), e.g., small black
holes or brown dwarfs
Galaxies formation
• A full-size galaxy is
formed by the merger of
smaller objects (or subgalactic unit)
• These small objects
(less than 1 kpc)
(numbered in the figure)
are seen when the
Universe is young (3400
Mpc away, or 11 billion
lys ago)
Formation of Spiral or Elliptical Galaxies
• It depends how fast the gas is used up to form galaxies
• If star formation is fast, no gas is left  elliptical galaxy
• If star formation is slow, gas forms disk  spiral galaxy
Key Words
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anisotropic
barred spiral galaxy
clusters (of galaxies)
dark-matter problem
distance ladder
dwarf elliptical galaxy
elliptical galaxy
fundamental plane
galactic cannibalism
giant elliptical galaxy
gravitational lens
groups (of galaxies)
Hubble classification
Hubble constant
Hubble flow
Hubble law
intracluster gas
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irregular cluster
irregular galaxy
isotropic
lenticular galaxy
Local Group
maser
poor cluster
redshift
regular cluster
rich cluster
spiral galaxy
standard candle
starburst galaxy
supercluster
Tully-Fisher relation
tuning fork diagram
void