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Galaxies
And the Foundation of Modern Cosmology
What are the three major types of
galaxies?
Hubble
Ultra
Deep
Field
Hubble
Ultra
Deep
Field
Hubble
Ultra
Deep
Field
Spiral Galaxy
Hubble
Ultra
Deep
Field
Spiral Galaxy
Hubble
Ultra
Deep
Field
EllipticalGalaxy
Galaxy
Elliptical
Spiral Galaxy
Hubble
Ultra
Deep
Field
EllipticalGalaxy
Galaxy
Elliptical
Spiral Galaxy
Hubble
Ultra
Deep
Field
EllipticalGalaxy
Galaxy
Elliptical
Irregular Galaxies
Spiral Galaxy
halo
disk
bulge
Spiral Galaxy
Disk Component:
stars of all ages,
many gas clouds
Spheroidal Component:
bulge & halo, old stars,
few gas clouds
Type Sa Galaxy
Sa Galaxies
Sa Galaxies:
• Dominant nuclear bulge
• Tightly wound spiral pattern
• Few (but some) newly formed stars, HII regions or other
evidence of active star formation
Sb Galaxies
• Moderate nuclear bulge
• Intermediate spiral
pattern
• Some evidence for
massive young stars, HII
regions, star formation
Type Sc
Galaxy
Disk
Component:
stars of all
ages,
many gas
clouds
Spheroidal
Component:
bulge & halo,
old stars,
few gas
clouds
Blue-white color
indicates ongoing
star formation
Red-yellow color
indicates older star
population
Sc Galaxies
(Some classify Messier as as Type Sd)
• Small to nearly non-existent nuclear bulge
• Open spiral pattern
• Active star-formation
Disk
Component:
stars of all
ages,
many gas
clouds
Spheroidal
Component:
bulge & halo,
old stars,
few gas
clouds
Blue-white color
indicates ongoing
star formation
Red-yellow color
indicates older star
population
Barred Spiral Galaxy
Has a bar of stars across the bulge
Barred Spiral Types
SBa
SBb
SBc
S0 Lenticular Galaxy
Has a disk like
a spiral galaxy
but very little
dust or gas
(intermediate
between spiral
and elliptical)
S0 Edge-on
Note the clear
presence of a
disk, but absence
of dust band in
this S0 galaxy:
NGC 3115
Elliptical Galaxy:
All spheroidal
(bulge)
component,
no disk
Elliptical
Galaxy:
All spheroidal
component,
virtually no
disk
component
Red-yellow
color indicates
older star
population
Irregular Galaxies
Irregular I Galaxy
Blue-white color
indicates ongoing
star formation
Irr II Galaxy - Messier 82
Spheroid
Dominates
Hubble’s Galaxy Classes
Disk
Dominates
How are galaxies grouped
together?
Spiral
galaxies are
often found
in groups of
galaxies
(up to a few
dozen
galaxies)
Our Galaxy
&
Andromeda
belong to a
small “Local
Group”
of about 20
or so
galaxies
Elliptical
galaxies are
much more
common in
huge clusters
of galaxies
(hundreds to
thousands of
galaxies)
How do we observe the life
histories of galaxies?
Deep
observations
show us very
distant
galaxies as
they were
much earlier
in time
(Old light
from young
galaxies)
Denser regions
contracted, forming
protogalactic clouds
H and He gases in
these clouds formed
the first stars
Supernova
explosions from
first stars kept
much of the gas
from forming stars
Leftover gas settled
into spinning disk
Conservation of
angular
momentum
Why do galaxies differ?
NGC 4414
M87
But why do some galaxies end up looking so different?
Why don’t all galaxies have similar disks?
Nature: Conditions in Protogalactic
Cloud?
Spin: Initial angular momentum of protogalactic cloud could
determine size of resulting disk
Conditions in Protogalactic Cloud?
Density: Elliptical galaxies could come from dense
protogalactic clouds that were able to cool and form stars
before gas settled into a disk
Distant Red Ellipticals
• Observations of
some distant red
elliptical galaxies
support the idea that
most of their stars
formed very early in
the history of the
universe
We must also consider the effects of collisions
Collisions were much more likely early in time, because
galaxies were closer together
Many of the galaxies we see at great distances (and early
times) indeed look violently disturbed
The collisions we observe nearby trigger bursts of star
formation
Modeling such collisions on a computer shows that two
spiral galaxies can merge to make an elliptical
Modeling such collisions on a computer shows that two
spiral galaxies can merge to make an elliptical
Shells of stars
observed
around some
elliptical
galaxies are
probably the
remains of past
collisions
Collisions
may explain
why elliptical
galaxies tend
to be found
where
galaxies are
closer
together
Giant elliptical
galaxies at the
centers of
clusters seem
to have
consumed a
number of
smaller
galaxies
What is the evidence for dark
matter in galaxies?
We measure the
mass of the solar
system using the
orbits of planets
• Orb. Period
• Avg. Distance
Or for circles:
• Orb. Velocity
• Orbital Radius
Rotation curve
A plot of orbital
velocity versus
orbital radius
Solar system’s
rotation curve
declines because
Sun has almost
all the mass
Rotation
curve of
merry-goround rises
with radius
Rotation curve
of Milky Way
stays flat with
distance
Mass must be
more spread
out than in
solar system
Mass in
Milky Way is
spread out
over a larger
region than
the stars
Most of the
Milky Way’s
mass seems to
be dark
matter!
Mass within Sun’s
orbit:
1.0 x 1011 MSun
Total mass:
~1012 MSun
The visible
portion of a
galaxy lies
deep in the
heart of a
large halo of
dark matter
We can
measure
rotation
curves of
other spiral
galaxies
using the
Doppler
shift of the
21-cm line
of atomic H
Spiral galaxies all tend to have flat rotation curves
indicating large amounts of dark matter
Broadening of
spectral lines in
elliptical galaxies
tells us how fast
the stars are
orbiting
These galaxies also
have dark matter
Clusters of Galaxies
We can
measure the
velocities of
galaxies in a
cluster from
their Doppler
shifts
The mass we
find from
galaxy
motions in a
cluster is
about
50 times
larger than
the mass in
stars!
Clusters contain
large amounts of Xray emitting hot gas
Temperature of hot
gas (particle
motions) tells us
cluster mass:
85% dark matter
13% hot gas
2% stars
Gravitational lensing, the bending of light rays by
gravity, can also tell us a cluster’s mass
All three methods of measuring cluster mass indicate
similar amounts of dark matter
Does dark matter really exist?
Either:
1.
2.
Dark matter really exists, and we are observing the effects of its gravitational
attraction
Something is wrong with our understanding of gravity, causing us to
mistakenly infer the existence of dark matter
Bottom Line:
• What is the evidence for dark matter in
galaxies?
– Rotation curves of galaxies are flat, indicating
that most of their matter lies outside their
visible regions
• What is the evidence for dark matter in
clusters of galaxies?
– Masses measured from galaxy motions,
temperature of hot gas, and gravitational
lensing all indicate that the vast majority of
matter in clusters is dark
Our Options
1. Dark matter really exists, and we are observing
the effects of its gravitational attraction
2. Something is wrong with our understanding of
gravity, causing us to mistakenly infer the
existence of dark matter
Because gravity is so well tested, most astronomers
prefer option #1
Two Basic Options
• Baryonic (Ordinary) Dark Matter (MACHOS)
– Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
– Weakly Interacting Massive Particles:
mysterious neutrino-like particles
MACHOs
occasionally
make other
stars appear
brighter
through
lensing
MACHOs
occasionally
make other
stars appear
brighter
through
lensing
… but not
enough
lensing
events to
explain all
the dark
matter
Why Believe in WIMPs?
• There’s not enough ordinary matter
• WIMPs could be left over from Big Bang
• Models involving WIMPs explain how galaxy
formation works