GALAXIES

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GALAXIES
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• From the background radiation, we know that the
  Universe was smooth (to within one part in 104)
  at an age of 1/2 million years.
• Yet, from this uniformity,
• atoms, molecules, dust, stars, galaxies,
  clusters of galaxieseven life, formed.
• HOW?

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• Even in a smooth medium, random fluctuations
  occur
• From the largest recognized (superclusters of
  galaxies), it appears that the largest
  fluctuations had masses
• Matter was affected by gravitational forces (but
  not the photons)

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• Gravitation enhances density fluctuations
• Pressure inhibits density fluctuations
• Things with large mass and/or cold T are
  gravitationally dominated.

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• As density fluctuations increased, and material
  cooled, the big chunks became SELF-GRAVITATING
• i.e., gravitational forces gtgt pressure forces
• ? CONTRACTION
• So, the first structures in the Universe were
• large clouds of H and He, separated by
• equally large voids.

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JEANS LENGTH
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Pressure

• Mechanical p F/A
• The same idea occurs in a hot/dense gas
• Relationship between pressure, density, and
  temperature is called an Equation of State

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Equation of State for an Ideal gas
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• As the large primordial cloud contracts, smaller
  fluctuations within it lead to
• SELF-GRAVITATION
• FRAGMENTATION
• CLUSTER OF GALAXIES
• GALAXIES

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• Turbulence, shock waves both existed in the media
  ? Chaotic Processes
• Some portions of the collapsing galaxy-sized
  clouds may have become dense enough to allow
  further collapse ? star formation

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• Star formation then
• Heated the cloud from nuclear processes
• Produced heavy elements
• Two competing processes occur at the same time in
  different regions of the cloud
• Break Up
• Merging
• Regardless, gravitational forces operated
• Most galaxies formed in groups

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• Details of large structure of clusters,
  superclusters, etc., are still controversial
• Need to determine relative roles of
• Shock waves ? Spherical
• Angular Momentum ? Pancake-like structure
• Gravitational Fields ? More Uniform Distribution

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Classifications of Galaxies (By Shape)

• SPIRALS

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SPIRALS
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BARRED SPIRALS
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Bars of stars run through the nuclear bulges of
barred spiral galaxies.
Type SBa
Type SBb
Type SBc
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ELLIPTICAL
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Hubble devised a system for classifying galaxies
according to their appearance
ELLIPTICALS
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TTunHubble devised a system for classifying
galaxies according to their appearance
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SPIRALS

• Disk Components ? flat
• Contains
• Young stars
• Gas and dust
• concentrated into spiral arms
• The spiral arms contain younger (and more
  luminous) stars HII regions.
• Light contrast gtgt density contrast
• The shape and rotational velocity of the galaxy
  are dictated by the mass distribution

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BARRED SPIRALS

• Spiral arms start at the ends of a luminous bar
  of stars instead of at the nucleus
• Everything else (star populations, gas and dust
  content, etc.) similar to normal spirals
• Computer simulations of normal spirals undergo
  periodic and reversible excursions into barred
  spirals

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ELLIPTICALS

• Very little gas or dust ? star formation mostly
  completed
• Either all gas has been used up in forming stars
  or all remaining gas been lost through encounters

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SPIRALS
Like the Milky Way Old and young
Stars Considerable gas and dust
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ELLIPTICAL
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S0 GALAXIES

• Shaped like E0, but distribution like spirals.
  However, no gas and dust content
• Perhaps they were normal spirals whose gas and
  dust were stripped off during collision with
  other galaxies
• Evidence they occur most frequently at the
  center of rich clusters of galaxies, where
  encounters are most common

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Hubble devised a system for classifying galaxies
IRREGULAR GALAXY to their appearance
LMC
IRREGULAR
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IRREGULARS

• Do not fit into the Hubble Sequence, since they
  have no well-defined shapes.
• They are often near very massive galaxies
• e.g. LMC, SMC ? Tidal forces disrupt them
• Forces introduce kinetic energy ? disrupt star
  formation
• They contain more gas and dust than spirals

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OTHER GALAXIES OUTSIDE HUBBLE CLASSIFICATION

• RADIO GALAXIES
• Emit much of their energy in the radio range
• Often radio emission comes from a pair of
  sources, opposite each other, and up to millions
  of light-years from the optical galaxy? Great
  explosion
• Emission mechanism SYNCHOTRON RADIATION
• Electron moving at speeds near c, interacting
  with magnetic fields

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Radio Galaxies.
Cygnus A
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SEYFERT GALAXIES

• Brighter than normal galaxies
• 10-100 times more energetic than giant
  ellipticals
• Emit energy at all ?
• ? - ? - UV - V - IR Radio
• The bulk of the energy comes from a region
• lt 1 ly across

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Active Galaxies bridge the gap between normal
galaxies and quasars.

• Seyfert galaxies
• luminous, star-like nuclei with strong emission
  lines.

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This distant Seyfert Galaxy is likely two
galaxies undergoing collision.
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QUASARS

• Perhaps central nuclei of galaxies
• Very Bright
• Point-like sources (at times with jets)
• Highly red-shifted spectra
• 3C273 z 0.158
• Largest redshift to date z 5.3
• Compared to stars in our Galaxy
• z ? 0.0001
• and for galaxies
• z 0.01- 6.7 (10?)
• Variability on timescales as short as weeks ?
  energy producing region smaller or equal to a
  light-week

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Quasars and Active Galaxies
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Why?

• Suppose a light emitting region 1 light-year in
  diameter has a period of one year
• When the front of the source is brightest, we see
  the back of the source as it was 1 year before ?
  faint and vice versa
• Variability would wash out

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Size places a limit on how fast an object can
change brightness.
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A quasar emits a huge amount of energy from a
small volume.
Such rapid changes in brightness can only result
from changes small objects.
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• They are powerful emitters of
• Radio
• IR
• Optical
• UV
• X-rays
• and maybe ?-rays

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3C 273s spectral lines are greatly redshifted.
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PKS 2000-030 has a z 3.773 where La and Lb
should be in the UV!!
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Quasars are the ultraluminous centers of distant
galaxies.
The greater the redshift, the farther back in
time we are seeing it. There were numerous
quasars after the Big Bang, but in todays
universe, quasars have disappeared.
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Active Galaxies bridge the gap between normal
galaxies and quasars.

• Seyfert galaxies
• luminous, star-like nuclei with strong emission
  lines.
• BL Lacertae objects (BL Lacs)
• featureless spectrum with a brightness that can
  vary by a factor of 15 times in a few months.
• Most commonly known as a Blazar.

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Quasars, blazars, Seyferts, and radio galaxies
are active galaxies.
This radio galaxy M87 is like a dim, radio loud
quasar. The core shows thermal radiation central
whereas the jet shows polarized synchrotron
radiation from relativistic electrons.
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Active galaxies lie at the center of double radio
sources.
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Active galaxies lie at the center of double radio
sources.
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Quasars, blazars, Seyferts, and radio galaxies
are active galaxies.
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Black Holes(Engines of Active Galactic Nuclei?)

• Bright
• Small
• Variable
• E mc2
• Nuclear sources
• E 0.015mc2
• over entire lifetime of star
• Gravitational collapse
• E 0.1mc2

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• Liberated during the collapse
• Very fast
• Very small
• A black hole is an object which produces such
  intense gravitational field that even light
  becomes trapped

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• Notice that for uniform density,
• M? V ? R3
• So,
• R ? M1/3
• Whereas for black holes
• R ? M
• So for a black hole, the smaller the mass, the
  denser the object
• However, even for a black hole,
• R 3 ? 108 km
• Which is very small ? 1000 light-seconds.

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• Question
• If no light can escape a black hole, how can it
  power anything?
• Answer
• The energy is released as material is drawn into
  the black hole, and before it has fallen into it.

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• Matter (stellar or clouds) approaches a black
  hole
• intense tides tear it apart
• an accretion disk forms which constantly gets
  accelerated until it reaches relativistic (i.e.
  near c) speeds, releasing energy
• Matter is swallowed and forever disappears from
  view

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Supermassive black holes are the central
engines that power active galaxies.

• The Eddington limit describes how large a
  supermassive black hole must be to power an
  active galactic nucleus
• LEdd 30,000 (M/M?) L?
• LEdd is the maximum luminosity that can be
  radiated by accretion onto a compact object.
• M is the mass of the compact object.

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Supermassive black holes may be the central
engines that power active galaxies.
The rotation curve of M31 suggests a massive
compact object at the center.
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Unified model may explain active galaxies of
several different types.

• The general consensus is that the center of
  active galaxies contain a super massive black
  hole surrounded by a luminous accretion disk.
• Variations in the density of the disk will
  account for variations in brightness.
• Magnetic forces will cause jets of material to
  move outward.

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Jets of matter ejected from around a black hole
may explain quasars and active galaxies.
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From where you observe it might make all the
difference ...
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Local vs. Cosmological Origin of Quasar Redshifts

• The quasars are assumed to be extremely bright
  under the assumption that their redshift is
  produced by the expansion of the Universe
• Since v H0d
• for a very large v, d is very large

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Local vs. Cosmological Origin of Quasar Redshifts
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Of course, if the redshift were not cosmological,
the distance could be much smaller, and the
quasars intrinsic brightness much lower
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• Some people have suggested this
• Local origin of quasars
• Redshift due to SOMETHING ELSE

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Superluminal Expansion