Download Galaxies

Galaxies
Galaxy Classification
Distances to Galaxies
Galaxy Mass
Galaxy Clusters
Interacting Galaxies
Large Scale Structure of the Universe
Galaxies
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Star systems, some like our Milky Way
They contain a few thousand to tens of billions of stars
They contain varying amounts of gas and dust
They come in a large variety of shapes and sizes
The Family of Galaxies
Even seemingly empty
regions of the sky contain
thousands of very faint,
very distant galaxies
Galaxy
morphologies:
Spirals
Ellipticals
Irregular
(some
interacting)
Galaxy Classification
Elliptical Galaxies
E0, …, E7
Spiral Galaxies
Sa
E0 = Spherical
E1
Large
nucleus;
tightly wound
arms
Sb
Sc
E7 = Highly
elliptical
E6
Small
nucleus;
loosely
wound arms
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
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.
Barred Spirals
Some spirals show a
pronounced bar
structure in the center.
They are termed barred
spirals:
Sequence:
SBa, …, SBc,
analogous to regular
spirals.
Is the Milky Way a Barred
Spiral?
Distribution of stars
and neutral hydrogen
Distribution of dust
Sun
Bar
Ring
Irregular Galaxies
The Cocoon Galaxy
NGC 4038/4039
 Often: result of galaxy
collisions or mergers
 Often: Very active star
formation (―Starburst
galaxies‖)
 Some: Small (―Dwarf
galaxies‖) satellites of larger
galaxies (e.g., Magellanic Clouds)
Large
Magellanic
Cloud
A summary of galaxy properties by type
Distance Measurements
to Other Galaxies
a) Cepheid method: Using period – luminosity
relation for classical Cepheids:
1. Measure Cepheid’s period
2. Find its luminosity from graph
3. Measure apparent magnitude
4. Calculate its distance
Cepheid variables allow measurement of galaxies
to about 25 Mpc away.
However, some galaxies have no Cepheids and
most galaxies are farther away than 25 Mpc.
Distance Measurements to Other
Galaxies
b) Type Ia supernovae (collapse of an accreting white
dwarf in a binary system):
1. Type 1a supernovae have well known standard luminosities
2. Measure apparent magnitude
3. Calculate its distance
c) Tully–Fisher relation: correlates a galaxy’s rotation
speed (which can be measured using the Doppler
effect) to its luminosity.
All are ―Standard-candle‖ methods:
1. Determine the absolute magnitude (luminosity)
2. Measure apparent magnitude
3. Calculate its distance
Measuring Distances in Space
With these
additions, the
cosmic distance
ladder has been
extended to about
1 Gpc.
Distance Measurements
to Other Galaxies:
Hubble’s Law
Edwin Hubble (1913)
found that 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.
1. Measure vr using the
Doppler effect
2. Calculate the distance.
Measuring Distances in Space
This puts the
final step on
our cosmic
distance
ladder
The last step
cannot be used
to justify
Hubble’s Law
and its
cosmological
implications.
We will look at
that later.
Type Ia
Supernovae
Tully-Fisher
Method
Cepheid
Variables
Spectroscopic
Parallax
Trigonometric
Parallax
Radar
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 (d) of Mpc or even Gpc
 The light we see left the galaxy millions
or billions of years ago!!
 ―Look-back times‖ (t = d/c)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, use Kepler’s 3rd law to infer
masses of galaxies
Adding ―visible‖ mass in stars, interstellar gas, dust, etc.,
we find that most of the mass is ―invisible,‖ just as we did
for the Milky Way.
 Dark Matter
Determining the Masses of Galaxies
Another way to measure the average mass of galaxies in
a cluster is to calculate how much mass is required to
keep the cluster gravitationally bound.
Dark Matter in the Universe
Galaxy mass measurements show that galaxies need
between 3 and 10 times more mass than can be observed
to explain their rotation curves.
The discrepancy is even larger in galaxy clusters, which
need 10 to 100 times more mass. The total needed is more
than the sum of the dark matter associated with each
galaxy.
Dark Matter in the Universe
 There is evidence for intracluster
superhot gas (about 106 K)
throughout clusters, densest in
the center
 This head–tail radio galaxy’s
lobes are being swept back,
probably because of collisions
with intracluster gas
 It is believed this gas is
primordial—dating from the very
early days of the Universe.
 There is not nearly enough of it
to account for most of the matter
in galaxy clusters.
Dark Matter in the Universe
This map of dark matter in and near a small galaxy
cluster was created by measuring distortions in the
images of background objects
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!
 Super massive black holes
like we found for the Milky Way
The Origin of
Supermassive
Black Holes
 Most galaxies seem to harbor
supermassive black holes in
their centers.
 They are fed and fueled by
stars and gas from the nearcentral environment
 Galaxy interactions may
enhance the flow of matter
onto central black holes
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
Gravitational Lensing
The huge mass of gas in a cluster of galaxies can bend the light
from a more distant galaxy. This is an effect of the General
Theory of Relativity.
Image of the galaxy is strongly distorted into arcs.
Gravitational Lensing
This is what appeared
at first to be a double
quasar, but on closer
inspection the two
quasars turned out to
be not just similar, but
identical—down to their
luminosity variations.
This is not two quasars
at all—it is two images
of the same quasar.
Gravitational Lensing
This could happen by gravitational lensing. From this
we can learn about the quasar itself, as there is
usually a time difference between the two paths. We
can also learn about the lensing galaxy by analyzing
the bending of the light.
Gravitational Lensing
Here, an intervening galaxy has made four images of a
distant quasar.
Gravitational Lensing
Here are two spectacular images of gravitational
lensing:
Distant galaxies being
imaged by a whole
cluster
A cluster with images of
what is probably a
single galaxy.
Our Galaxy Cluster:
The Local Group
The Local Group
Some galaxies of our local group are difficult to
observe because they are located behind the center
of the Milky Way, which obscures our view.
Dwingaloo 1
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
Starburst Galaxies
Starburst galaxies are often very rich in gas and dust;
bright in infrared
Ultraluminous
infrared galaxies
Simulations of
Galaxy
Interactions
Numerical
simulations of
galaxy interactions
have been very
successful in
reproducing tidal
interactions like
bridges, tidal tails,
and rings.
Tidal Tails
Example for galaxy
interaction with tidal
tails:
The Mice
Computer simulations
produce similar
structures.
Mergers of Galaxies
NGC 7252 is
probably the
result of the
merger of two
galaxies, ~109
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
Interactions of Galaxies with
Intergalactic Matter
Galaxies may not only
interact with each
other directly, but also
with the gas between
them.
Gas within a galaxy is
stripped off the galaxy
by such an interaction.
The Furthest Galaxies
The most distant galaxies visible by HST are seen at
a time when the universe was only ~109 years old.
The Universe on Large Scales
Galaxy clusters join in
larger groupings,
called superclusters.
This is a 3-D map of the
Local Supercluster, of
which our Local Group
is a part. It contains
tens of thousands of
galaxies.
The Universe on Large Scales
This slice of a larger galactic survey shows that,
on the scale of 100–200 Mpc, there is structure in
the universe – walls and voids.
The Universe on Large Scales
This survey, extending
out even farther, shows
structure on the scale
of 100–200 Mpc, but no
sign of structure on a
larger scale than that.
The decreasing density
of galaxies at the
farthest distance
comes from the
difficulty of observing
them.
Large-Scale Structure
A large survey of distant galaxies
shows the largest structures in the
universe
Filaments and walls of galaxy
superclusters and voids
(basically empty space).