Introductory Astronomy 101 Prof. Buta Properties of Galaxies

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Introductory Astronomy 101Prof. Buta
Properties of Galaxies
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Dark Matter in the Milky Way
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What is dark matter?

• - a form of matter that is detectable only
  through gravity
• emits no detectable electromagnetic radiation (or
  simply is undetectable in such radiation)
• - dark matter is always detected as higher than
  expected speeds in galaxies or groups of galaxies

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Evidence for Dark Matter in the Milky Way
lt------orbital velocities continue to rise where
little matter is seen
lt------------------------------- ---- apparent
edge of disk
lt-------orbital velocities should decline
like this outside visible disk
Galactic rotation curve plot of orbital speed
versus distance from Galactic center
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Where is dark matter found?
- since the Galactic rotation curve rises where
little or no visible matter is seen, we conclude
the dark matter is mostly in the halo of our
Galaxy, outside the visible disk
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What types of objects are candidates for dark
matter?

• MACHOS massive compact halo objects
• EXOTIC ELEMENTARY PARTICLES (Wimps, axions)
• -extremely cold molecular hydrogen
• Of these, MACHOS and extremely cold hydrogen are
  least likely!

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How much dark matter is there in the Milky Way?

• because the halo occupies a very large volume,
  DARK MATTER IS THE DOMINANT FORM OF MATTER IN THE
  MILKY WAY!
• It is also the dominant form of matter in the
  Universe

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The Galactic Nucleus
Mysterious area because - completely obscured in
visible light, so not seen directly - can be seen
in infrared, microwaves, and radio waves - lots
of activity (fast gas motions, high star
density) - supermassive black hole (2.5 million
solar masses)
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On what evidence is supermassive black
holededuced?
lt------ these stars move very fast, as if they
are orbiting a massive central object
Infrared image of bright stars near Galactic
center
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Stellar Populations in Galaxies
Broadly contrasting star types characterized
by - spatial distributions - orbital
characteristics - metal content
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Discovery of stellar populations
Walter Baade, 1944 two stellar populations
identified in the Andromeda Galaxy
lt-------------------------------population II
stars in bulge region and halo
lt---------population I stars in disk
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Characteristics of stellar population types
Population I stars
Population II stars - young, recently
formed - all very old -
occupy the disk
-occupy outer bulge and halo - metal rich
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metal poor - circular orbits
- orbits not circular -
includes Sun
- includes globular clusters
These two population types are found in any
galaxy with a disk shape!
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Example of extreme pop. I stars an open cluster
(NGC 2264) 3 million years old
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Example of an extreme pop. II system the
globular Cluster M13, more than 10 billion years
old
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Introduction to Galaxy Morphology and
Classification
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Types of Galaxies

• -Galaxies come in a variety of forms, or
  morphologies
• - elliptical galaxies (E)
• - spiral galaxies (S)
• - irregular galaxies (I)
• armless disk galaxies (S0)
• The reasons for the different forms are still
  under investigation!

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Elliptical galaxies (E)
- featureless, round or elliptical-shaped
galaxies made mostly of old stars - extremely
wide range of sizes, from giants to dwarfs
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Giant elliptical galaxies
Rare and extremely massive
lt---------------two giants in the Coma galaxy
cluster
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Dwarf elliptical galaxies
Small and of low mass
Most common type of galaxy in the Universe!
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Spiral Galaxies
- disk includes a spiral pattern most easily seen
in face-on view - edge-on they appear highly
elongated, with planar dust lane Two kinds of
spirals
lt-------------------------bar
Normal spirals, where arms break from a round
central bulge
Barred spirals, where arms break from ends of a
straight bar
Type S
Type SB
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Edge-on Spiral Galaxies
NGC 4565
The Sombrero Galaxy M104
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Mass of a typical spiral galaxy

• 100 billion solar masses or more
• spiral category includes the Milky Way

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Irregular galaxies (I)

• galaxies having asymmetric, less regular shapes
• usually small and very common

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Armless Disk Galaxies (S0)
Disk- shaped galaxies without spiral arms -
little dust and gas, made entirely of old stars
Edge-on example (NGC 3115)
Tilted example (NGC 1553)
The nature of these systems is still under debate!
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Hubble Tuning Fork classification
E. Hubble 1926, 1936 proposed this system - two
parallel sequences of spirals nonbarred and
barred - bulge shrinks, arms more open
prominent from Sa to Sc and SBa to SBc
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Digital Hubble Tuning Fork
S0
Sa
Sb
Sc
E0
E3
SBa
SBb
SBc
SB0
The de Vaucouleurs Atlas of Galaxies, Buta,
Corwin, and Odewahn, 2005
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Galaxy morphology ad infinitum!
The de Vaucouleurs Atlas of Galaxies
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The Distribution of Galaxies
- individual galaxies are rarely isolated - most
galaxies are members of pairs, groups, and
clusters - sometimes clusters group into
superclusters - thus, the cosmic patchwork is
nonuniform
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Double or Binary Galaxies
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Small Groups of Galaxies
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The Local Group
- small group of galaxies to which our Milky Way
belongs - includes about 20 galaxies, most of
which are dwarfs - Andromeda spiral M31 and our
Galaxy are the dominant members - includes the
Magellanic Clouds
Small Magellanic Cloud (SMC)
Large Magellanic Cloud (LMC)
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Clusters of Galaxies hundreds or thousands of
members
Two types of clusters
Coma Cluster regular type - dominated by E, S0
galaxies
Hercules Cluster irregular type -dominated by
spiral galaxies
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Missing Mass Problem in Clusters
Two ways to measure mass of a galaxy cluster 1.
Sum light of all members assume light traces
mass 2. Measure random speeds of member
galaxies Method 2 gives about 10 times as much
mass as method 1! -----gt Dark matter exists in
galaxy clusters
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Collisions and interactions between galaxies
Galaxies often interact or collide because they
are large compared to their typical
separations What happens when two galaxies
collide? - INDIVIDUAL STARS NEVER COLLIDE - GAS
AND DUST CLOUDS DO COLLIDE The result is -
distorted galaxy shapes - enhanced infrared and
radio emission - starbursts
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The Mice
Ring Galaxy
Merger of two galaxies
The Antennae
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Merging galaxies Centaurus A
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Starburst galaxy
- a galaxy that is forming large numbers of new
stars in a very short time period
?----------------------------new stars
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Movies of colliding galaxies
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The redshift of galaxies and the Hubble Law
Spectra of galaxies can show - absorption lines
due to unresolved starlight - emission lines due
to interstellar gas In most galaxies, these lines
are REDSHIFTED --------gt velocities of
RECESSION Recession velocity INCREASES WITH
INCREASING DISTANCE
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Recession velocity increases with increasing
distance
These galaxies all show redshifted absorption
lines due to ionized calcium (HK lines)
lt-------------comparison laboratory lines
53,000,000 light years
lt----------- galaxy spectrum
660,000,000 light years
970,000,000 light years
1,700,000,000 light years
2,700,000,000 light years
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A plot of recession velocity V versus distance D
is called the Hubble Law
VH0 D
H0 Hubble Constant 74 km/sec/Mpc where
1Mpc1,000,000 parsecs
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Implication of the Hubble Law
The Universe is expanding!