Galaxies
Chapter 13:
Galaxies
• Contain a few thousand to tens of billions of stars,
• Large variety of shapes and sizes
• Star systems like our Milky Way
as well as varying amounts of gas and dust
Spiral galaxy
Bulge and Halo:
old stars, few gas clouds
Disk Component:
stars of all ages,
many gas clouds
M31: Andromeda Galaxy
Pinwheel Galaxy
M81
NGC 7742
M104: Sombrero Galaxy
M51: Whirlpool Galaxy
Elliptical
Galaxy:
All spheroidal
component,
virtually no
disk
component
Elliptical
Galaxy:
All spheroidal
component,
virtually no
disk
component
Red-yellow
color
indicates
older star
population.
M49
M87
Irregular Galaxy: Neither spiral nor elliptical.
Blue-white color indicates ongoing star formation.
NGC 1427
M82
Barred Spiral Galaxy: Has a bar of stars
across the bulge.
NGC 1672
NGC 1097
Hubble’s galaxy classes Spheroid
dominates
Disk
dominates
Galaxy Classification
Sa
Sb
Sc
Elliptical Galaxies Spiral Galaxies
E0 =
Spherical
Small nucleus;
loosely wound
arms
E1
E6
E0, …, E7
Large nucleus;
tightly wound
arms
E7 =
Highly
elliptical
Barred Spirals
Sequence:
SBa, …, SBc,
analogous to
regular spirals.
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
Clusters of Galaxies
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
Our Galaxy Cluster:
The Local Group
Milky Way Andromeda galaxy
Small Magellanic Cloud
Large Magellanic Cloud
Some galaxies of our local group are difficult to
observe because they are located behind the
center of our Milky Way, from our view point.
The Local Group
The Canis Major Galaxy is
being ripped apart by the
Milky Way’s tidal forces.
Galaxies that get too close to each
other can join together. Systems in the
process of joining together are called
galaxy mergers.
Interacting Galaxies Cartwheel Galaxy
Particularly 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
Tidal Tails
Example for galaxy interaction with tidal tails:
The Mice
Simulations of
Galaxy Interactions
Numerical simulations of
galaxy interactions have been
very successful in reproducing
tidal interactions like bridges,
tidal tails, and rings.
Tadpole Galaxy
NGC 4676: The Mice
Antennae Galaxies
Antennae Galaxies (zoomed out)
Rose Galaxy
Irregular Galaxies 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)
Large
Magellanic
Cloud NGC 4038/4039
The Cocoon Galaxy
Starburst Galaxies
Ultraluminous
Infrared Galaxies
Starburst galaxies are often very rich in gas
and dust; bright in infrared:
The Puzzle of “Spiral Nebulae”
• Before Hubble, some scientists argued that “spiral nebulae” were entire galaxies like our Milky Way, whereas other scientists maintained they were smaller collections of stars within the Milky Way.
• The debate remained unsettled until someone finally measured the distances of spiral nebulae.
Hubble settled the debate by measuring the distance
to the Andromeda Galaxy using Cepheid variables
as standard candles.
Cepheid Variable Stars
The light curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50-day period.
Cepheid variable stars with longer periods have
greater luminosities.
Distance Measurements
to Other Galaxies
a) Cepheid Method: Using Period – Luminosity relation for
classical Cepheids:
Measure Cepheid’s Period → Find its luminosity → Compare to
apparent magnitude → Find its distance
b) Type Ia Supernovae
(collapse of an accreting
white dwarf):
Type Ia Supernovae have
well known standard
luminosities → Compare to
apparent magnitudes →
Find its distances
Both are “Standard-candle” methods:
Know absolute magnitude (luminosity) → compare to
apparent magnitude → find distance
Cepheid Distance Measurement
Repeated
Brightness
measurements
of a cepheid
allow the
determination
of the period
and thus the
absolute
magnitude.
→ Distance
Distance Measurements to Other
Galaxies (II): 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
Hubble also knew that the spectral features of virtually
all galaxies are redshifted they’re all moving
away from us.
By measuring
distances to
galaxies,
Hubble found
that redshift
and distance
are related in a
special way.
Discovering Hubble's Law
Hubble’s law: velocity = H0 distance
The Extragalactic Distance Scale
Many galaxies are typically millions or billions
of parsecs from our Galaxy.
The light we see has left the Galaxy
millions or billions of years ago!!
“Look-back times” of millions or billions of years
The Furthest Galaxies
The most distant galaxies visible by HST are seen at a
time when the Universe was only ~ 1 billion years old.
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
Determining the
Masses of Galaxies
Based on rotation curves,
use Kepler’s 3rd law to infer
masses of galaxies
Supermassive Black Holes From the measurement of stellar velocities
near the center of a galaxy:
Infer mass in the
very center →
Central black
holes!
Several million,
up to more than
a billion solar
masses!
→ Supermassive black holes
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 at this time.
Some ideas:
Brown dwarfs, small black holes,
exotic elementary particles
Gravitational Lensing
The huge mass of gas in a
cluster of galaxies can bend the
light from a more distant galaxy.
The galaxy’s image is strongly distorted into arcs.
Active Galaxies
Galaxies with extremely violent energy
release in their nuclei (pl. of nucleus)
→ “Active Galactic Nuclei” (= AGN)
Up to many thousand times more
luminous than the entire Milky Way;
the energy is released within a region
approx. the size of our solar system!
Seyfert Galaxies
NGC 1566
Circinus Galaxy
Unusual spiral galaxies:
• Very bright cores
• Emission line spectra.
• Variability: ~ 50 % in a few
months
Most likely power source:
Accretion onto a supermassive
black hole (~107 – 108 Msun)
NGC 7742
Interacting Galaxies
Seyfert galaxy NGC 7674
Active galaxies are
often associated with
interacting galaxies,
possibly result of
recent galaxy mergers.
NGC 1275, in the center of the
Perseus galaxy cluster
Often: gas outflowing at high velocities, in opposite directions
Cosmic Jets and Radio Lobes
Many active galaxies show powerful radio jets.
Radio image of
Cygnus A
Material in the jets
moves with almost the
speed of light
(“Relativistic jets”).
Hot spots:
Energy in the jets is
released in interaction
with surrounding
material
Radio Galaxies
Centaurus A (“Cen A” = NGC 5128): the nearest AGN
Quasars
Active nuclei in
elliptical galaxies
with even more
powerful central
sources than
Seyfert galaxies
Also show very strong, broad emission
lines in their spectra
Also show strong
variability over
time scales of a
few months.
The Spectra of Quasars
The Quasar 3C 273
Spectral lines show
a large red shift of
z = Dl / l0 = 0.158
Model for AGNs
Accretion disk
Dense dust torus
Gas clouds
UV, X-rays
Emission lines
Supermassive
black hole
Seyfert I:
Strong, broad emission lines from
rapidly moving gas clouds near the BH
Seyfert II:
Weaker,
narrow
emission
lines from
more slowly
moving gas
clouds far
from the BH
Formation of Radio Jets Jets are powered by accretion of matter
onto a supermassive black hole.
Black Hole
Accretion Disk
Twisted magnetic fields help to confine the material
in the jet and to produce synchrotron radiation.
Radio Galaxy:
Powerful “radio lobes”
at the end points of the
jets, where power in the
jets is dissipated
Cyg A (radio emission)
AGN Unification
Emission from the jet pointing
towards us is enhanced
(“Doppler boosting”) compared
to the jet moving in the other
direction (“counter jet”).
AGN Unification
Quasars: