Galaxies

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Galaxies

• Star systems like our Milky Way

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

• Large variety of shapes and sizes

The Hubble Deep Field:

10-day exposure on an apparently empty field in the sky

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)

Shapes of Galaxies: SpiralsNGC 5236

NGC 1530

NGC 1201 NGC 2841

NGC 2811 M 81

NGC 488 M 74

“Classical” Spirals Barred Spirals

Type SBb

Type SBc

Type Sa Type Sb

Type Sab Type Sc

Type S0 Type Sb

Shapes of Galaxies: Ellipticals/Irregulars Small Magellanic CloudM 87

NGC 205 NGC 6822

Type E1

Type E6

Elliptical Galaxies Irregular Galaxies

What type of galaxy is our Milky Way?

1. Elliptical.

2. Spiral.

3. Barred Spiral.

4. Irregular.

5. None of the above.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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Galaxy Classification

Sa

Sb

Sc

Gas and Dust in GalaxiesSpirals are rich in

gas and dustEllipticals are almost

devoid of gas and dust

Galaxies with disk and bulge, but no dust are termed S0

In which type of galaxy do you expect that stars are being

formed at a higher rate?

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!

The Hubble Sequence of Galaxies

“Late Types”“Early Types”

Rich in gas and dust; active star

formation

Almost devoid of gas and dust; little or no

star formation

Irregular GalaxiesOften: result of galaxy

collisions / mergers

Often: Very active star formation (“Starburst galaxies”)

Some: Small (“Dwarf galaxies”) satellites of larger galaxies

(e.g., Magellanic Clouds)

Large Magellanic Cloud

NGC 4038/4039

The Cocoon Galaxy

What is the type of this

galaxy?

1. E0

2. S0

3. Sa

4. Sc

5. Irregular1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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41 42 43 44 45 46 47 48 49 50 IC 342

What is the type of this

galaxy?

1. E0

2. S0

3. Sa

4. Sc

5. Irregular

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M 82

What is the type of this

galaxy?

1. E0

2. E4

3. E7

4. SBc

5. Irregular1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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M 89

How could astronomers, for the first time, measure the distance

to other galaxies?

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.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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Distances to Other Galaxies

a) 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.

The Doppler effect provides a relation between the … and the

… of a light source.

1. brightness; distance

2. temperature; wavelength of maximum energy output

3. radial velocity; distance

4. radial velocity; frequency shift

5. distance; frequency shift1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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Expansion Velocity:The Doppler Effect

Blueshift (shorter wavelength)

Redshift (shorter wavelength)

Distance Measurements to Other Galaxies: The Hubble Law

E. Hubble (1913):

Distant galaxies are moving away from our Milky way, with

a recession velocity, vr, proportional to their distance d:

vr = H0*d

H0 ≈ 70 km/s/Mpc is the Hubble Constant.

=> Measure vr through the Doppler effect →

Infer the distance.

The Universe is 14 billion years old. How far away is, theoretically, the most distant galaxy that you could

possibly observe?

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. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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The Extragalactic Distance Scale

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

Galaxy Sizes and Luminosities

Vastly different sizes and luminosities:

From small, low-luminosity irregular

galaxies to giant Ellipticals and large spirals, a few times

the Milky Way’s size and luminosity

Rotation Curves of Galaxies

Observe frequency of spectral lines across a galaxy.

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

What can we infer from the rotation curve of a galaxy?

1. Its distance.

2. Its mass.

3. Its luminosity.

4. Its morphhological type.

5. Its radius.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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Rotation Curves of Galaxies

Observe frequency of spectral lines across a galaxy.

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

→ Infer the mass of the galaxy!

Masses and Other Properties of Galaxies

What makes up most of the mass in the Milky Way?

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 MatterAdding “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.

Clusters of GalaxiesGalaxies 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

Our Galaxy Cluster: The Local Group

Milky Way Andromeda galaxy

Small Magellanic Cloud

Large Magellanic Cloud

Is the Local Group a rich

or a poor cluster?

1. A poor cluster.

2. An intermediate rich/poor cluster.

3. A rich cluster.

4. Impossible to say without a bank statement.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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Large Scale Structure

Superclusters = clusters of clusters of galaxies

Superclusters appear

aligned along walls and filaments.

Vast regions of space are completely

empty:

“Voids”

The Furthest Galaxies

The most distant galaxies visible by HST are seen at a time when the Universe was only ~ 1 billion years old.

Interacting GalaxiesCartwheel Galaxy Especially in rich clusters, galaxies

can collide and interact.

Galaxy collisions can produce ring galaxies and

tidal tails.

Often triggering active star formation: Starburst galaxies

NGC 4038/4039

Starburst Galaxies

Starburst galaxies are often very rich in gas

and dust; bright in infrared:

Ultraluminous Infrared Galaxies

M 82

Cocoon Galaxy

What type of object / phenomenon should be associated with starburst

galaxies (if the currently favored model is correct)?

1. White dwarfs.

2. Supermassive black holes.

3. Nova explosions.

4. Gamma-ray bursts.

5. Type Ia supernovae.1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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If the hypernova model is correct, then GRBs should be associated with galaxies with

very active star formation.