Download Galaxies (Professor Powerpoint)

Galaxies
What is the Galaxy made of ?
StarsStellar remnants:
Clouds of gas (nebulae)found in
the interstellar medium.
Composed of :
70 % hydrogen
28 % helium
2 % ‘other stuff’ Heavy elements
Galaxy Formation
•We know much less about galaxies than
stars, because we can't see the universe
before galaxies formed. The evolution is
slower, more complex - but we can compare
cases.
< 1 Byr Galactic formation occurred in dense
areas of H & He gas in the early universe
~ 2 Byr Protogalactic clouds merge with a
high rate of star formation.
4.5-5 Byr Mature-looking galaxies appear.
Anatomy of Our Galaxy
1. Galactic Disk
2. Galactic Bulge
3. Galactic Halo
Disk
• young and old stars
• open star clusters
• gas & dust clouds (ISM)
100,000 lyrs
Bulge
• old stars
Halo
• old stars
• globular clusters
open cluster
globular cluster
The Disk
Thin Disk surrounds nuclear bulge
Most new star formation goes on in
the disk. Types of stars – all colors)
Stars are much younger (Population I,
younger metal rich stars).
Much free-floating gas & dust in the disk.
The Halo
A halo of stars and globular clusters
surrounds the entire galaxy.
Contains almost exclusively VERY old
stars (population II stars).
Population ll stars are low in heavy
elements, meaning that they’re as old as
the universe
Galactic Bulge in center
Contains much of the visible mass
in the galaxy.
Made of old (mostly yellow & red)
Population ll stars
Both the Galactic Bulge and the halo have
more or less 3-dimensional spherical
symmetry with the center.
Motion in the Galaxy
•The disk:
•Fairly ordered
motion
•The spiral arms do
not move with the
stars
•The halo
•Much more random
Population II stars are found in the halo, and
are metal poor and old. The spectra of these
stars are simple without many metals.
Population I stars are mainly found in the disk,
are metal rich and young. Their spectra contains
many more lines of metals.
Population II stars formed first,they have fewer
metals. Population I formed after more stars
exploded so that they have more metals.
Types of Galaxies
Elliptical galaxies are a big blob
Spiral Thin disk with lots of gas and dust
Central bulge and halo; Spiral arms extend
from most spiral galaxies
Barred spirals have extended central bulges
Lenticular galaxies ( lens shaped)
possess characteristics of spirals except that they
lack spiral arms
(len tik u lar)
Irregular Galaxies- no definite shape
Ellipticals
Spirals
Lenticular
Originally conceived
as an evolutionary
diagram
idea not correct, but
classification scheme
Barred
Spirals
The colors of late-type galaxies tend to be bluer.
“early type”
“late type”
Rounder appearance
Early-type galaxies turned
almost all of their gas into
stars, very quickly, very
early in their lives!
On
average
•Older Stars
Gas Poor
More Massive
Larger bulge, less dusty gas, tighter
spiral arms
Late-type galaxies turn
gas into stars slowly,
and have lots of gas left.
Presently forming stars.
•On-going Star Formation
Gas Rich
Less Massive
Elliptical Galaxies
No structure - just a big elliptical blob
Population II (older stars, yellow red, metal
poor) with very little free dust & gas and little
star formation.
Elliptical galaxies range from the biggest to
the smallest galaxies in the universe. The
smallest are called dwarf elliptical galaxies
which make up the most and contain only a
few million stars making them hard to see.
How did elliptical galaxies form? Rapidly
Elliptical Galaxies contain little gas
or dust, they’re just big balls of
stars
Giant Elliptical
Galaxies may
form by
Cannibalism
Ellipticals are classified according to how stretched
out they are…E0 – E7
Spirals
Two main types :
Those with bars &
Those without bars
In all except one spiral
galaxies,the arms trail
around behind as the
galaxy rotates.
NGC 1288
They are called trailing arm spirals.
Spiral Galaxies
Hubble Sequence (Sa , Sb , Sc) according to :
• Size of nuclear bulge vs disk & Tightness
of spiral arms
•
Sa tightest pattern & largest bulge
•
Sb medium pattern & medium bulge
Sc open pattern & smallest bulge
•
S0 Lenticular (lens shapped) -Have disk
but no arms
Spiral galaxies
Spiral Galaxies formed more slowly.
Spiral galaxies have a nuclear bulge, and at
least 2 spiral shaped arms, with a halo of,
stars, and globular clusters.
Population I stars are found in the disk.
Population II stars are found mostly in the
halo and the bulge. Disks of spiral galaxies
include young stars
Spiral Galaxies
The arms of spiral & barred spiral galaxies
are sites of active star formation.
Differential rotation of a galaxy stretches the
star forming regions into long arches of
stars and nebulae, that we see as spiral
arms.
NGC 1232
S0 Spiral Galaxies have
dust lanes but no arms
Sa Spirals have tightly
wound arms and a
large nucleus
M104
Sa because of large nuclear bulge.
Sb Spirals have looser
arms & a smaller nucleus
NGC 891 is an Sb,
because of the small
nucleus
Sc Spirals have very
loose arms and a
small nucleus
M-31 Andromeda Galaxy & satellite galaxies
2.4 million ly away & 100,000 ly in diameter
M-51 Galaxy & satellite galaxy
Barred Spirals
Barred Spirals are like normal spirals except
with a bar across their center
Barred Spirals
There is some evidence to suggest our galaxy
might be a barred spiral
Bar across central region is
made of stars, gas, and dust
Spiral arms begin at both ends
of the bar
M- 61
The bar is funneling material into the hub,
which triggers star formation and feeds the
bulge
Barred Spiral galaxies (SB) are classified
just like ordinary spiral galaxies: SBa, SBb,
SBc.
The bar is very noticeable in this SBc type.
Sba Galaxies
Sbb Galaxies
M91
SBc Galaxies
M109
NGC 1097
NGC 4123
Irregular Galaxies
Irregular galaxies have no definite shape.
These galaxies are full of regions of new star
formation, dusty.
Irregular galaxies are often the result of
collisions between galaxies.
Collisions between galaxies are common
•Galaxy collisions can produce:
•Elliptical galaxies
•Irregular galaxies
•Intense activity in galaxies
Every now and then galaxies collide especially
those in large groups. Mergers are devastating
events, they change the galaxy.
Mergers can transform two spirals into an elliptical
galaxy.
Some astronomers think that galaxies are always born as
spirals, and they can become ellipticals only via mergers.
Two galaxies in a cluster or from nearby
clusters can occasionally pass through
one another.
There is so much space that the chances
of stars colliding is extremely small, but
huge clouds of gas and dust do collide.
Collisions can merge galaxies together or
hurl stars and gases out into space.
These collisions can produce strong
shock waves and new star formation.
Starburst galaxies
Stars are forming in our galaxy
Star formation rate: R* ~ 1 star / year
In some galaxies R* ~ 100 stars /
year- Known as starburst galaxies
Often have double nuclei
Likely explanation: A collision of two
spiral galaxies
Why spirals?
Need gas / dust for star formation
Cosmic Capitalism: Mergers and acquisitions
Galaxy Clusters
Galaxies occur in clumps called clusters
and these are in clumps called
superclusters.
Typical superclusters contain dozens of
individual clusters. Most Clusters in
superclusters are drifting away.
Between superclusters are voids, areas
of few galaxies.
Galaxy Clusters
Poor Clusters
10-100 galaxies
Spirals, Irregulars and lastly dwarf
ellipticals
Local Group - our galaxy cluster
Rich Clusters
Thousand + galaxies
Ellipticals, SO, with spirals at edges only
Giant Ellipticals at the center
Galaxies
come in
groups
The Coma
cluster is a rich,
regular cluster
The Virgo
Cluster is a
rich,
regular
cluster
A Poor,
Irregular
Group
Measuring distance :The Tully-Fisher Relationship
First, more massive galaxies rotate faster than less
massive ones. Tully and Fisher then made the
plausible assumption that brighter galaxies are
more massive than dimmer ones.
The relationship works like
this:
As a galaxy rotates, some
of the stars move toward
us and the light is blue
shifted.
Other stars are moving
away the light is red
shifted
Taken together, the the 21 cm
emission line is spread out. The
wider the emission line, the
faster the galaxy rotates, so
the more massive it must be.
Because the line widths can
be measured accurately, the
luminosities ( absolute
magnitudes) of spiral
galaxies can be measured
and we can calculate the
distance.
Hubble Space
Telescope
image of
nearly
“empty” part
of sky on
1/30 of the
size of the
moon.
Over 1000
galaxies seen
Likely around
100 billion
galaxies in
observable
universe!!!
The Hubble Deep Field
Dark Matter
Rotation Curve
Now consider the Milky Way galaxy. Using radio
observations, it is possible to measure the orbital speed
as a function of the distance from the center.
This is not a single gravitational source like the Sun, so the
speed near the center will be small and rise quickly. As you
get to the edge of the visible galaxy, the velocity should drop
down.
But, in our galaxy the orbital speeds continue to climb well
above the visible edge of the galactic disk, so there must be
more gravitational force acting on the stars & clouds.
Rotation Curve
Compare the two
graphs:
The rotation curve
for the Solar
system is falling
with radius.
The rotation curve
for the Milky Way
is flat or even
rising with radius.
The mass
distribution of the
Milky Way is not a
simple “point
mass” distribution.
Stars in the outer parts are orbiting at a much higher velocity than
expected, based on the amount of visible matter (e.g. stars). There
must be dark matter in order to account for this.
IfWhat
we trust
theory
of gravity... there may
isour
Dark
Matter?
be 10 times more dark than luminous matter
in our Galaxy
Dark matter is found in the halo and far
beyond the luminous disk
There must be sufficient
matter to provide
gravitational force to
bind galaxies together in
clusters. No clusters of
galaxies contain enough
visible matter to keep
them bound together.
Dark Matter can be anything that gives off no light.
Possibilities :
1. Ordinary matter is called baryonic and consists of
MACHOS (Massive Compact Halo Objects). These
objects could be Brown Dwarfs, White Dwarfs,
Jupiters , protons and neutrons. Protons and
neutrons are composed of baryons.
2. Non-baryonic matter called WIMPS (Weakly
Interacting Massive Particles). Neutrinos are now
thought to have some mass, so they are a
possibility. There may be some sub-atomic
particles yet to be discovered that could be the
answer.
3.The answer could be a combination of both.
4. Maybe our gravity works differently for
massive galaxies.
The other question about the nature of
dark matter is, the matter:
1. “hot” fast moving like neurtinos
2. “cold” slow moving
The type of dark matter determines when structure
could actually form. It is hard to form structures in
a hot Universe and easier in a cold Universe. So we
need to look at when structure was formed.
  0
z
0
v   0
Z 
c
0
If z=1 , then v = c , if z = 3 then v=3c . Can’t be
moving faster than light. Yet, there are objects
with z = 7 or 8. The answer is you must use the
Relativistic form below.
If Z > .1, you need to use the Relativistic form.
cv
z
1
cv
v  z  1  1

2
c  z  1  1
2
Or more
useful
Redshift
z
0
Recessional velocity
v/c
0
Distance
Bly
0
0.2
0.180
2.41
0.4
0.324
4.26
0.75
0.508
6.57
1
0.6
7.73
2
0.8
10.3
3
0.882
11.5
4
0.923
12.1
5
0.946
12.5
10
0.984
13.2
Infinite
1
13.7