Transcript (From Seeds Ch. 12-14) Galaxies The Milky Way Almost everything we see in the night sky belongs to the Milky Way. We see most.

(From Seeds Ch. 12-14)
Galaxies
The Milky Way
Almost everything we see in the
night sky belongs to the Milky Way.
We see most of the Milky Way as a
faint band of light across the sky.
From outside, our Milky Way might
very much look like our cosmic
neighbor, the Andromeda Galaxy.
First Studies of the Galaxy
First attempt to unveil the
structure of the galaxy by
William Herschel (1785), based
on optical observations.
The shape of the Milky Way was
believed to resemble a grindstone,
with the sun close to the center
Strategies to explore the
structure of our Milky Way
I. Select bright objects that you can see
throughout the Milky Way and trace
their directions and distances.
II. Observe objects at wavelengths other than visible
(to circumvent the problem of optical obscuration),
and catalog their directions and distances.
III. Trace the orbital velocities of objects in different
directions relative to our position.
Determining the Structure
of the Milky Way
Galactic Plane
Galactic Center
The structure of our Milky Way is hard to determine because:
1) We are inside.
2) Distance measurements are difficult.
3) Our view towards the center is
obscured by gas and dust.
Measuring Distances:
The Cepheid Method
Instability Strip
The more
luminous a
Cepheid
variable, the
longer its
pulsation
period.
Observing the
period yields
a measure of
its luminosity
and thus its
distance!
The Cepheid
Method
Allows us to
measure the
distances to star
clusters
throughout the
Milky Way
Exploring the Galaxy
Using Clusters of Stars
Two types of clusters of stars:
1) Open clusters = young clusters of recently
formed stars; within the disk of the Galaxy
Open clusters h
and c Persei
Globular Cluster 2) Globular clusters = old, centrally concentrated
M 19
clusters of stars; mostly in a halo around the galaxy
Globular Clusters
Globular Cluster M80
• Dense clusters of 50,000 – a million stars
• Old (~ 11 billion years), lower-main-sequence stars
• Approx. 200 globular clusters in our Milky Way
Locating the Center of the Milky Way
Distribution of globular
clusters is not centered
on the sun,
but on a location which is
heavily obscured from
direct (visual) observation.
The Structure of the Milky Way
75,000 light years
Disk
Nuclear Bulge
Sun
Halo
Open Clusters,
O/B Associations
Globular Clusters
Infrared View of the Milky Way
Near-infrared image
Galactic plane
Interstellar dust
(absorbing optical light)
emits mostly infrared.
Nuclear bulge
Infrared emission is not
strongly absorbed and
provides a clear view
throughout the Milky Way
Far-infrared image
Orbital Motions in the Milky Way (I)
Disk stars:
Nearly circular
orbits in the disk
of the galaxy
Halo stars:
Highly elliptical
orbits; randomly
oriented
Orbital Motions in the Milky Way (II)
Differential Rotation
Sun orbits around
galactic center at
220 km/s
1 orbit takes approx.
240 million years.
Stars closer to the
galactic center
orbit faster.
Stars farther out orbit
more slowly.
Mass determination
from orbital velocity:
The more mass there is
inside the orbit, the faster
the sun has to orbit
around the galactic
center.
Combined mass:
4 billion
25
MMsun
M = 11
100
400
billion
billion
M
sun
sun
sun
The Mass of the Milky Way
If all mass was concentrated in the
center, Rotation curve would follow a
modified version of Kepler’s 3rd law.
Rotation Curve = orbital velocity
as function of radius.
The Mass of the Milky Way (II)
Total mass in the disk
of the Milky Way:
Approx. 200 billion
solar masses
Additional mass in an
extended halo:
Total: Approx. 1 trillion
solar masses
Most of the mass is not
emitting any radiation:
dark matter!
The History of the
Milky Way
The traditional theory:
Quasi-spherical gas cloud
fragments into smaller
pieces, forming the first,
metal-poor stars (pop. II);
Rotating cloud collapses
into a disk-like structure
Later populations of stars
(pop. I) are restricted to
the disk of the galaxy
Modifications of the
Traditional Theory
Ages of stellar population
may pose a problem to the
traditional theory of the
history of the Milky Way.
Possible solution: Later
accumulation of gas,
possibly due to mergers
with smaller galaxies.
Recently discovered ring of
stars around the Milky Way
may be the remnant of
such a merger.
Exploring the structure of the Milky
Way with O/B Associations
O/B Associations
Sun
O/B Associations trace out
3 spiral arms near the sun.
Distances to O/B Associations
determined using Cepheid variables
Radio Observations
21-cm radio observations reveal the distribution
of neutral hydrogen throughout the galaxy.
Distances to hydrogen
clouds determined
through radial-velocity
measurements
(Doppler effect!)
Sun
Galactic
center
Neutral
hydrogen
concentrated in
spiral arms
The Structure of the
Milky Way Revealed
Distribution of stars
and neutral hydrogen
Distribution of dust
Sun
Bar
Ring
Star Formation in Spiral Arms (I)
Shock waves from supernovae, ionization fronts
initiated by O and B stars, and the shock fronts
forming spiral arms trigger star formation.
Spiral arms
are stationary
shock waves,
initiating star
formation.
Star Formation in
Spiral Arms (II)
Spiral arms are basically
stationary shock waves.
Stars and gas clouds orbit
around the galactic center
and cross spiral arms.
Shocks initiate star formation.
Star formation selfsustaining through O/B
ionization fronts and
supernova shock waves.
Self-Sustained Star
Formation in Spiral Arms
Star forming regions get elongated
due to differential rotation.
Star formation is self-sustaining due to
ionization fronts and supernova shocks.
Grand-Design Spiral Galaxies
Grand-design spirals have
two dominant spiral arms.
M 100
Flocculent (woolly)
galaxies also have spiral
patterns, but no dominant
pair of spiral arms.
NGC 300
The Whirlpool Galaxy
Grand-design
galaxy M 51
(Whirlpool Galaxy):
Self-sustaining
star forming
regions along
spiral arm
patterns are
clearly visible.
The Galactic Center (I)
Our view (in visible light) towards the Galactic center (GC)
is heavily obscured by gas and dust:
Extinction by 30 magnitudes
 Only 1 out of 1012 optical photons makes its
way from the GC towards Earth!
galactic center
Wide-angle optical view of the GC region
Radio View of the Galactic Center
Many supernova remnants;
shells and filaments
Arc
Sgr A
Sgr A
Sgr A*: The center of our galaxy
The galactic center contains a supermassive
black hole of approx. 2.6 million solar masses.
Measuring the Mass of the Black
Hole in the Center of the Milky Way
By following the orbits of
individual stars near the center
of the Milky Way, the mass of
the central black hole could be
determined to be ~ 2.6 million
solar masses.
www.mpe.mpg.de/www_ir/GC
X-Ray View of the Galactic Center
Galactic center region contains many black-hole
and neutron-star X-ray binaries.
Supermassive black hole in the galactic
center is unusually faint in X rays,
compared to those in other galaxies.
Chandra X ray image of Sgr A*
Distance Measurements
to Other Galaxies (I)
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 in a binary system):
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.
The Extragalactic Distance Scale
Many galaxies are typically millions or billions
of parsecs from our galaxy.
Typical distance units:
Mpc = megaparsec = 1 million parsecs
Gpc = gigaparsec = 1 billion parsecs
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 (much
smaller and less
luminous than the
Milky Way) 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
Determining the
Masses of Galaxies
Based on rotation curves,
can use Newtonian Gravity
to determine the
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.
Clusters of Galaxies
Galaxies do not generally exist in
isolation, 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
Hot Gas in Clusters of Galaxies
Space between galaxies is not empty, but
filled with hot gas (observable in X rays)
That this gas remains gravitationally bound,
provides further evidence for dark matter.
Visible light
X rays
Coma Cluster of Galaxies
Gravitational Lensing
The huge mass of gas in a cluster of galaxies can
bend the light from a more distant galaxy.
Image of the galaxy is strongly distorted into arcs.
Our Galaxy Cluster:
The Local Group
Milky Way
Andromeda Galaxy
Small Magellanic Cloud
Large Magellanic Cloud
Starburst Galaxies
Starburst galaxies are often very rich in gas and dust;
bright in infrared:
Ultraluminous
infrared galaxies
Interacting Galaxies
Cartwheel Galaxy
Particularly in rich
clusters, galaxies can
collide and interact.
Galaxy collisions
can produce
ring galaxies and
NGC 4038/4039
tidal tails.
Often triggering active
star formation:
Starburst galaxies
Tidal Tails
Example for galaxy
interaction with tidal tails:
The Mice
Computer simulations
produce similar structures.
Simulations
of Galaxy
Interactions
Numerical
simulations of
galaxy interactions
have been very
successful in
reproducing tidal
interactions like
bridges, tidal tails,
and rings.
Galactic Interaction Simulations
• Joshua Barnes:
http://www.ifa.hawaii.edu/faculty/barnes/transform.ht
ml
• John Dubinksi:
• http://www.cita.utoronto.ca/~dubinski/nbody/
• Chris Mihos:
• http://burro.astr.cwru.edu/models/models.html
• Bob Berrington (Wyoming):
• http://physics.uwyo.edu/~rberring/
• There are others…
Mergers of Galaxies
NGC 7252:
Probably
result of
merger of two
galaxies, ~ a
billion years
ago:
Small galaxy
remnant in the
center is rotating
backwards!
Radio image of M64: Central
regions rotating backwards!
Multiple nuclei
in giant elliptical
galaxies
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;
energy released within a region
approx. the size of our solar system!
Line Spectra of
Galaxies
Taking a spectrum of the
light from a normal galaxy:
The light from the galaxy should be mostly star
light, and should thus contain many absorption
lines from the individual stellar spectra.
Seyfert Galaxies
Unusual spiral galaxies:
• Very bright cores
• Emission line spectra.
• Variability: ~ 50 % in a few months
NGC 1566
Most likely power source:
Accretion onto a supermassive
black hole (~107 – 108 Msun)
Circinus Galaxy
NGC 7742
Interacting Galaxies
Seyfert galaxy NGC 7674
Seyfert galaxy 3C219
Active galaxies are often
associated with interacting
galaxies, possibly result of
recent galaxy mergers.
Often: gas outflowing at high
velocities, in opposite directions
Cosmic Jets and Radio Lobes
Many active galaxies show powerful radio jets
Hot spots:
Radio image of
Cygnus A
Material in the jets moves
with almost the speed of
light (“relativistic jets”).
Energy in the jets is
released in interaction
with surrounding
material
Radio Galaxies
Centaurus A (“Cen A” = NGC 5128): the closest AGN to us.
Jet visible in radio and Xrays; show bright spots in
similar locations.
Radio image superposed
on optical image
Infrared image
reveals warm gas
near the nucleus.
Visual + radio
image of 3C31
Radio Galaxies (II)
Radio image
of 3C75
Radio image
of NGC 1265
Evidence for
the galaxy
moving
through
intergalactic
material
3C75: Evidence for two
nuclei  recent galaxy
merger
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.
The Jets of M87
M87 = Central, giant elliptical galaxy in
the Virgo cluster of galaxies
Optical and radio observations detect
a jet with velocities up to ~ 1/2 c.
The Dust Torus in NGC4261
Dust torus is directly visible with Hubble Space Telescope
Model for Seyfert Galaxies
Seyfert I:
Gas clouds
Strong, broad emission
lines from rapidly
moving gas clouds near
the black hole
Emission lines
UV, X-rays
Seyfert II:
Accretion disk
dense dust torus
Supermassive
black hole
Weaker, narrow
emission lines
from more slowly
moving gas clouds
far from the black
hole
Other Types of AGN and
AGN Unification
Cyg A (radio emission)
Radio Galaxy:
Powerful “radio lobes”
at the end points of the
jets, where power in the
jets is dissipated.
Other Types of AGN and
AGN Unification
Quasar or BL Lac object
(properties very similar to
quasars, but no emission lines)
Emission from the jet pointing
towards us is enhanced
(“Doppler boosting”) compared
to the jet moving in the other
direction (“counter jet”).
The Origin of
Supermassive
Black Holes
Most galaxies seem to
harbor supermassive black
holes in their centers.
Fed and fueled by stars
and gas from the nearcentral environment
Galaxy interactions may
enhance the flow of matter
onto central black holes
Quasars
Active nuclei in
elliptical galaxies with
even more powerful
central sources than
Seyfert galaxies.
Also show strong
variability over time
scales of a few months.
Also show very strong,
broad emission lines in
their spectra.
The Spectra of Quasars
The Quasar 3C273
Spectral lines show
a large redshift of
z = Dl / l0 = 0.158
Quasar Red Shifts
z=0
z = 0.178
z = 0.240
z = 0.302
Quasars have been
detected at the highest
redshifts, up to
z~6
z = Dl/l0
Our old formula
Dl/l0 = vr/c
z = 0.389
is only valid in the
limit of low speed,
vr << c
Studying Quasars
The study of high-redshift quasars allows
astronomers to investigate questions of
1) Large scale structure of the universe
2) Early history of the universe
3) Galaxy evolution
4) Dark matter
Observing quasars at high redshifts
 distances of several Gpc
 Look-back times of many billions of years
 Universe was only a few billion years old!
Probing Dark Matter with High-z
Quasars:
Gravitational Lensing
Light from a distant quasar is bent
around a foreground galaxy
 two images of the same quasar!
Light from a quasar behind a galaxy
cluster is bent by the mass in the cluster.
Use to probe the distribution of
matter in the cluster.
Gravitational Lensing of
Quasars
Gallery of Quasar Host Galaxies
Elliptical galaxies; often merging / interacting galaxies