Download Lecture 21 (pdf from the powerpoint)

Dark Matter
•Dark matter may be:
•MACHOS (Massive
Compact Halo Objects),
dwarf stars, planets
•WIMPS (Weakly
Interacting Massive
Particles), or other new
sub-atomic particles
•Primordial black holes
•Non-luminous gas
32-Mpc simulation cold-dark matter Universe. Courtesy R. Cen, Princeton University.
A History of Galactic Discovery
• In the early 20th century, the existence
of other galaxies was unknown
– The Milky way was the Universe!
– Other galaxies were called nebulae
• Light from galaxies always appears
fuzzy and diffuse, due to the vast
separation between the Sun and the
observed galaxy, as well as the
separation between the stars of that
galaxy!
– The paleness of visible light from distant
galaxies is called the surface brightness.
• Galaxies are therefore difficult to
observe, even with good telescopes.
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A History of Galactic Discovery
• In the 1700’s, Charles Messier was
observing comets, and kept finding objects
that while fuzzy, were not comets
– He made a list (or catalog) of these undesired
objects, so he could avoid seeing them
– They became known as Messier Objects, a
number preceded by an M.
– M31 (the Andromeda galaxy) is one such
object
• William and Caroline Herschel (1800’s)
developed a catalog of faint objects in the
heavens
– Now known as the New General Catalog
– Objects are known by a number preceded by
the letters NGC
– Objects can appear in both the Messier and
NGC catalogs!
M31
A Sky Full of Galaxies
• Technology has
advanced to the point
where we have found
as many galaxies as
there are stars in the
Milky Way!
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A Sky Full of Galaxies
•Diameter of the known universe ~ 8 x 10^10 LY
•Avg distance between galaxies ~ 1 million LY
(10 to 30 big galaxy diameters)
•If lake Mendota was the observable universe
then each galaxy would be about 25 mm across
and separated by about 1/2 meter away.
•In contrast if the galaxy was the size of Mendota
the solar system would be 25 microns in
diameter and the nearest star would be 1/2 meter
Our Galactic Neighborhood
•
•
•
The smallest organization of
galaxies are called galaxy groups
Our local group is called the Local
Group
The Local Group contains 40 known
members, including the Andromeda
Galaxy and the Large and Small
Magellanic clouds, dwarf satellite
galaxies of the Milky Way
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Distances to other galaxies
• We can use Cepheid variable
stars to measure the distance
to other galaxies
• A Cepheid’s luminosity is
proportional to its period, so if
we know how rapidly it
brightens and dims, we know
much energy it emits
• If we see a Cepheid in another
galaxy, we measure its period,
determine its luminosity, and
calculate its distance!
• Distance between galaxies is
huge!
– M31 is 2 million Lightyears
away
– M100 is 55 million Light years
away.
The Redshift and Expansion of the Universe
• Early
century
astronomers noted that the
spectra from most galaxies
was shifted towards red
wavelengths
• Edwin Hubble (and others)
discovered that galaxies
that were farther away
(dimmer) had even more
pronounced redshifts!
• This redshift was
interpreted as a measure of
radial velocity, and it
became clear that the more
distant a galaxy is, the
faster it is receding!
20th
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The Hubble Law
• In 1920, Edwin Hubble
developed a simple
expression relating the
distance of a galaxy to its
recessional speed.
• V=H×d
– V is the recessional
velocity
– D is the distance to the
galaxy
– H is the Hubble Constant
(70 km/sec per Mpc)
• This was our first clue that
the universe is expanding!
Spiral Galaxies
• Spiral arms and a central bulge
• Type S
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Elliptical Galaxies
•
•
•
•
No spiral arms
Ellipsoidal shape
Smooth, featureless appearance
Type E
Irregular Galaxies
• Stars and gas clouds
scattered in random patches
• No particular shape
• Type Irr
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Barred Spirals
• Related to regular spiral
galaxies, barred spirals are
noted for their large bar of
stars across the central bulge
• Type SB
S-zero galaxies
• The arms of the spiral begin at
the end of the bars
• Recently discovered that the
Milky Way is a barred spiral!
• S0 galaxies are in the
shape of a disk, but
have no spiral arms
• It is likely that the gas
and dust have been
blown out of such
galaxies
• The lack of gas and
dust means that no new
stars can form, so there
are no spiral arms
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The Tuning Fork
• Edwin Hubble (busy guy!)
organized these different
galaxy types into a tuning fork
shaped diagram
• Ellipticals are labeled E0-E7
– E0 is almost perfectly spherical,
E7 is quite flattened
• Spirals are labeled Sa – Sd
– Sa galaxies have tightly wound
arms and a large central bulge
– Sd galaxies are loosely wound
and have a small central bulge
• Barred Spirals are labeled SBa
– SBd
– Same flow as the Spirals
Additions to the list…
• Dwarf galaxies (left) are difficult to
detect, and may be the building blocks
of larger galaxies
• Low Surface Brightness galaxies (above
left) are very large, yet very faint
galaxies that have very little new star
formation occurring
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Differences in Star and
Gas Content
• Ellipticals:
– Low in gas and dust, so contains
mostly older Pop II stars
– Contain very high temperature,
very low density clouds of gas
that cannot condense into stars.
• Spirals:
– Lots of gas and dust, so have
active regions of star formation
– Have both Pop II and younger
Pop I stars
• Irregulars:
– Many hot, young stars
– Large amounts of interstellar
matter
– Might be young galaxies
The nuclei of most spiral galaxies appear redder than their spiral
arms because of
a) young blue stars in the arms, and old red ones in the nuclei
b) emission nebulae and dust in the nuclei
c) receding nuclei and advancing spiral arms (Doppler shifts)
d) nuclear reactions
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A look back in time
• The Hubble Space Telescope
was pointed at a part of the sky
that looked empty, taking a 100hour exposure
• Very distant galaxies were
detected, some closer than others
• This technique allows us to see
galaxies at various stages of
formation
• These early galaxies tend to be
smaller than the Milky Way, and
to not fall into Hubble’s
classification scheme
Galactic Collisions
• Galaxies can collide, though not in the sense of a car accident!
• The galaxies pass through one another, and their immense
gravitational pull tears both galaxies apart!
• Eventually, a new elliptical galaxy will form…
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Galaxy collision and merger
Collision movies
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The Mice
• These two interacting galaxies are tidally
distorting each other.
Which of the following is least easily explainable as a result of
interaction between galaxies?
a) Some galaxies have long "tails" of stars.
b) Rich, regular clusters are dominated by central giant ellipticals.
c) Both spiral and elliptical galaxies are seen at very high redshift.
d) Some galaxies seems to be undergoing bursts of star formation.
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Rich and Poor Galaxy Clusters
• Rich clusters:
– Contain hundreds to
thousands of member galaxies
– Are roughly spherical, with
the largest galaxies near the
center
– Contain mostly elliptical and
type S0 galaxies
– Lots of hot gas and dust
• Poor clusters
– Contain only tens of galaxies
– Have a ragged, irregular
appearance
– More spiral and irregular
galaxies
Superclusters
• Clusters of clusters are called
superclusters
– Contain a few to many dozen
clusters of galaxies
– Can be Mpc across!
– The Local Group is part of the
Local Supercluster, shown at left.
• The Local Supercluster is heading
toward a region of space known
as the Great Attractor, where there
are a large number of massive
superclusters
• There may be super-superclusters!
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Large Scale Structure in the Universe
• Using modern technology,
astronomers have mapped the
location of galaxies and
clusters of galaxies in three
dimensions
• Redshift is used to determine
distance to these galaxies
• Galaxies tend to form long
chains or shells in space,
surrounded by voids
containing small or dim
galaxies
• This is as far as we can see!
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