Galaxies

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Galaxies
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Galaxies
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• Star systems like our Milky Way

• Contain a few thousand to tens of billions of
  stars.

• Large variety of shapes and sizes

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Even seemingly empty regions of the sky contain
thousands of very faint, very distant galaxies
Large variety of galaxy morphologies
Spirals
Ellipticals
Irregular (some interacting)
The Hubble Deep Field 10-day exposure on an
apparently empty field in the sky
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Shapes of Galaxies Spirals
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NGC 1201
NGC 2841
NGC 5236
Type S0
Type Sb
M 81
NGC 2811
Type SBb
Barred Spirals
Classical Spirals
Type Sa
Type Sb
NGC 1530
NGC 488
M 74
Type Sab
Type Sc
Type SBc
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Shapes of Galaxies Ellipticals/Irregulars
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Small Magellanic Cloud
M 87
Type E1
Elliptical Galaxies
Irregular Galaxies
NGC 205
NGC 6822
Type E6
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What type of galaxy is our Milky Way?

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1. Elliptical.
2. Spiral.
3. Barred Spiral.
4. Irregular.
5. None of the above.

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Galaxy Classification
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Sa
Sb
Sc
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Gas and Dust in Galaxies
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Spirals are rich in gas and dust
Ellipticals are almost devoid of gas and dust
Galaxies with disk and bulge, but no dust are
termed S0
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In which type of galaxy do you expect that stars
are being formed at a higher rate?

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1. Elliptical.
2. Spiral.
3. There should be no difference.

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Remember Spiral arms are density waves that
trigger self-sustaining star formation!
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The Hubble Sequence of Galaxies
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Late Types
Early Types
Almost devoid of gas and dust little or no star
formation
Rich in gas and dust active star formation
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Irregular Galaxies
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Often result of galaxy collisions / mergers
Often Very active star formation (Starburst
galaxies)
Large Magellanic Cloud
Some Small (Dwarf galaxies) satellites of
larger galaxies (e.g., Magellanic Clouds)
The Cocoon Galaxy
NGC 4038/4039
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What is the type of this galaxy?

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1. E0
2. S0
3. Sa
4. Sc
5. Irregular

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IC 342
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What is the type of this galaxy?

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1. E0
2. S0
3. Sa
4. Sc
5. Irregular

M 82
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What is the type of this galaxy?

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M 89

1. E0
2. E4
3. E7
4. SBc
5. Irregular

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How could astronomers, for the first time,
measure the distance to other galaxies?

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1. Using the trigonometric parallax.
2. Using light travel time arguments.
3. Using Cepheid Variables.
4. Using cosmological redshift.
5. Measuring the time required for extragalactic
   space travel.

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Distances to Other Galaxies
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• Cepheid Method
• Using Period Luminosity relation for Cepheid
  variables

b) Type Ia Supernovae (collapse of an accreting
white dwarf in a binary system) Type Ia
Supernova have well known standard luminosity ?
Compare to apparent magnitude ? Find its distance
Both are Standard-candle methods Know absolute
magnitude (luminosity) ? compare to apparent
magnitude ? find distance.
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The Doppler effect provides a relation between
the and the of a light source.

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1. brightness distance
2. temperature wavelength of maximum energy output
3. radial velocity distance
4. radial velocity frequency shift
5. distance frequency shift

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Expansion VelocityThe Doppler Effect
Blueshift (shorter wavelength)
Redshift (shorter wavelength)
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Distance Measurements to Other Galaxies The
Hubble Law
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E. Hubble (1913) Distant galaxies are moving
away from our Milky way, with a recession
velocity, vr, proportional to their distance d
vr H0d
H0 70 km/s/Mpc is the Hubble Constant.
gt Measure vr through the Doppler effect ? Infer
the distance.
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The Universe is 14 billion years old. How far
away is, theoretically, the most distant galaxy
that you could possibly observe?

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1. 14 billion parsec.
2. 14 billion light years.
3. 7 billion parsec.
4. 7 billion light years.
5. There is no theoretical limit it just depends on
   the size of your telescope.

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The Extragalactic Distance Scale
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Many galaxies are typically millions or billions
of parsecs from our Galaxy.
Typical distance units Mpc Megaparsec 1
million parsec Gpc Gigaparsec 1 billion parsec
Distances of Mpc or even Gpc ? The light we see
has left the Galaxy millions or billions of years
ago!!
? Look-back times of millions or billions of
years
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Galaxy Sizes and Luminosities
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Vastly different sizes and luminosities
From small, low-luminosity irregular galaxies to
giant Ellipticals and large spirals, a few times
the Milky Ways size and luminosity
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Rotation Curves of Galaxies
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From blue / red shift of spectral lines across
the galaxy ? infer rotational velocity
Plot of rotational velocity vs. distance from the
center of the galaxy Rotation Curve
Observe frequency of spectral lines across a
galaxy.
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What can we infer from the rotation curve of a
galaxy?

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1. Its distance.
2. Its mass.
3. Its luminosity.
4. Its morphhological type.
5. Its radius.

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Rotation Curves of Galaxies
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From blue / red shift of spectral lines across
the galaxy ? infer rotational velocity
Plot of rotational velocity vs. distance from the
center of the galaxy Rotation Curve
Observe frequency of spectral lines across a
galaxy.
? Infer the mass of the galaxy!
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Masses and Other Properties of Galaxies
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What makes up most of the mass in the Milky Way?

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1. The central supermassive black hole.
2. Globular clusters.
3. Population I stars in the disk and spiral arms.
4. Neutral hydrogen gas, invisible in the optical,
   but visible in the 21 cm radio line.
5. Dark matter.

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Dark Matter
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Adding visible mass in stars, interstellar
gas, dust, etc., we find that most of the mass
is invisible!
The nature of this dark matter is not
understood.
Some ideas Brown dwarfs, small black holes,
exotic elementary particles.
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Clusters of Galaxies
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Galaxies do generally not exist isolated, but
form larger clusters of galaxies.
Rich clusters 1,000 or more galaxies, diameter
of 3 Mpc, condensed around a large, central
galaxy
Poor clusters Less than 1,000 galaxies (often
just a few), diameter of a few Mpc, generally not
condensed towards the center
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Our Galaxy Cluster The Local Group
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Milky Way
Andromeda galaxy
Small Magellanic Cloud
Large Magellanic Cloud
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Is the Local Group a rich or a poor cluster?

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1. A poor cluster.
2. An intermediate rich/poor cluster.
3. A rich cluster.
4. Impossible to say without a bank statement.

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Large Scale Structure
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Superclusters clusters of clusters of galaxies
Superclusters appear aligned along walls and
filaments.
Vast regions of space are completely empty
Voids
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The Furthest Galaxies
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The most distant galaxies visible by HST are seen
at a time when the Universe was only 1 billion
years old.
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Interacting Galaxies
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Especially in rich clusters, galaxies can collide
and interact.
Cartwheel Galaxy
Galaxy collisions can produce ring galaxies and
tidal tails.
Often triggering active star formation Starburst
galaxies
NGC 4038/4039
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Starburst Galaxies
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M 82
Starburst galaxies are often very rich in gas and
dust bright in infrared Ultraluminous Infrared
Galaxies
Cocoon Galaxy
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What type of object / phenomenon should be
associated with starburst galaxies (if the
currently favored model is correct)?

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1. White dwarfs.
2. Supermassive black holes.
3. Nova explosions.
4. Gamma-ray bursts.
5. Type Ia supernovae.

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If the hypernova model is correct, then GRBs
should be associated with galaxies with very
active star formation.
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