Download Galaxies

Galaxies
Please press “1” to test your
transmitter.
Galaxies
• Star systems like our Milky Way
• Contain a few thousand to tens of billions of stars.
• Large variety of shapes and sizes
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
Shapes of Galaxies: Spirals
NGC 1201
NGC 2841
Type S0
NGC 2811
Type Sb
M 81
Type SBb
“Classical” Spirals
Type Sa
NGC 488
Type Sab
NGC 5236
Type Sb
Barred Spirals
NGC 1530
M 74
Type Sc
Type SBc
Shapes of Galaxies: Ellipticals/Irregulars
M 87
Small Magellanic Cloud
Type E1
Elliptical Galaxies
NGC 205
Type E6
Irregular Galaxies
NGC 6822
What type of galaxy is
our Milky Way?
1.
2.
3.
4.
5.
Elliptical.
Spiral.
Barred Spiral.
Irregular.
None of the above.
Galaxy Classification
Sa
Sb
Sc
Gas and Dust in Galaxies
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
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.
Remember: Spiral arms are
density waves that trigger selfsustaining star formation!
The Hubble Sequence
of Galaxies
“Early Types”
Almost devoid of gas
and dust; little or no
star formation
“Late Types”
Rich in gas and
dust; active star
formation
Irregular Galaxies
Large
Magellanic
Cloud
NGC 4038/4039
The Cocoon
Galaxy
Often: result of galaxy
collisions / mergers
Often: Very active star formation
(“Starburst galaxies”)
Some: Small (“Dwarf galaxies”)
satellites of larger galaxies
(e.g., Magellanic Clouds)
What is the
type of this
galaxy?
1.
2.
3.
4.
5.
E0
S0
Sa
Sc
Irregular
IC 342
What is
the type
of this
galaxy?
1.
2.
3.
4.
5.
E0
S0
Sa
Sc
Irregular
M 82
What is the
type of this
galaxy?
1.
2.
3.
4.
5.
E0
E4
E7
SBc
Irregular
M 89
How could astronomers, for the
first time, measure the distance
to other galaxies?
1.
2.
3.
4.
5.
Using the trigonometric parallax.
Using light travel time arguments.
Using Cepheid Variables.
Using cosmological redshift.
Measuring the time required for
extragalactic space travel.
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.
2.
3.
4.
5.
brightness; distance
temperature; wavelength of maximum
energy output
radial velocity; distance
radial velocity; frequency shift
distance; frequency shift
Expansion Velocity:
The Doppler Effect
Redshift
(shorter wavelength)
Blueshift
(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.
2.
3.
4.
5.
14 billion parsec.
14 billion light years.
7 billion parsec.
7 billion light years.
There is no theoretical limit; it just
depends on the size of your telescope.
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, lowluminosity irregular
galaxies to giant
Ellipticals and large
spirals, a few times
the Milky Way’s
size and luminosity
Rotation Curves of Galaxies
From blue / red shift of spectral
lines across the galaxy
→ infer rotational velocity
Observe frequency
of spectral lines
across a galaxy.
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.
2.
3.
4.
5.
Its distance.
Its mass.
Its luminosity.
Its morphhological type.
Its radius.
Rotation Curves of Galaxies
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:
Observe frequency
of spectral lines
across a 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.
2.
3.
4.
5.
The central supermassive black hole.
Globular clusters.
Population I stars in the disk and spiral arms.
Neutral hydrogen gas, invisible in the optical, but
visible in the 21 cm radio line.
Dark matter.
Dark Matter
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.
Clusters of Galaxies
Galaxies do generally not exist isolated,
but form larger clusters of galaxies.
Rich clusters:
Poor clusters:
1,000 or more galaxies,
diameter of ~ 3 Mpc,
condensed around a large,
central galaxy
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.
2.
3.
4.
A poor cluster.
An intermediate rich/poor cluster.
A rich cluster.
Impossible to say without a bank statement.
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 Galaxies
Cartwheel 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
M 82
Starburst galaxies are
often very rich in gas
and dust; bright in
infrared:
Ultraluminous
Infrared Galaxies
Cocoon Galaxy
What type of object / phenomenon
should be associated with starburst
galaxies (if the currently favored
model is correct)?
1.
2.
3.
4.
5.
White dwarfs.
Supermassive black
holes.
Nova explosions.
Gamma-ray bursts.
Type Ia supernovae.
If the hypernova model is
correct, then GRBs should be
associated with galaxies with
very active star formation.