Lecture 28: Normal and Active Galaxies
Astronomy 101/103
Terry Herter, Cornell University
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Lecture
Topics
  • Galaxy Properties
    • Classifying galaxies
  • Local Group
  • Clusters of galaxies
  • Active galaxies
  • What powers these things?

Galaxies
  • A galaxy is a collection of stars, gas and dust along w/ associated starlight, magnetic fields and cosmic rays.
  • Four broad categories:
    • E: elliptical
    • S: spiral (normal & barred)
    • S0: lenticular
    • I: irregular

Elliptical
Galaxies
  • Range from spherical to highly flattened
    • with designations E0 to E7
  • Contain old stars (Pop II)
  • Very little gas and dust
  • 1-200 kpc in diameter
  • Mostly found in clusters of galaxies
  • Average spectral type: K
  • 106 to 1013 Msun

Spiral
Galaxies
  • Flattened systems which have a thin disk
  • Display spiral structure.
  • Divided into barred (SB) and unbarred (S) spirals.
  • Further subdivided into classes a, b, and c; e.g. SBb, Sc, ... where:
    • a => large nuclear bulge & tightly wound spiral arms
    • c => small nuclear bulge & loosely wound spiral arms
  • Contain young (Pop I) and old (Pop II) stars
  • Copious amounts of gas and dust
  • 5-50 kpc in diameter
  • Found mostly in the "field" (outside clusters)
  • Average spectral type: A, F, G, K
  • 109 to 1011 Msun

Lenticulars
  • Similar to spiral galaxies in shape and color but no spiral arms
  • Flattened systems which are morphologically between ellipticals and spirals.

Irregulars
  • By definition, irregular in shape
  • Mostly young stars (Pop I)
  • Lots of gas and dust
  • 1-10 kpc in diameter
  • Found in the field (outside clusters)
  • Average spectral type: A, F
  • 108 to 1010 Msun

Hubble
Tuning
Fork Diagram



  • The classification scheme is strictly morphological (based upon physical appearance) and does not necessarily imply an evolutionary sequence.
  • The morphological transition is smooth, but a galaxy may stay the same type for its entire lifetime.

Other types
of Galaxies
  • Dwarfs
    • 106 to 108 stars
  • Peculiar
    • Exploding, Ring, Disrupted
  • Seyfert
    • Very Bright Nucleus
  • N
    • Extremely Bright Nucleus
  • Interacting
    • Tidal Effects, Tails (pairs)
  • QSO
    • Collapsed Nuclei?

"Nearby"
Galaxies
  • LMC, SMC:
    • D ~ 50 kpc
  • M31 (Andromeda):
    • D ~ 700 kpc
  • Virgo Cluster:
    • D ~ 20,000 kpc = 20 Mpc

The Local
Group



Galaxy
Statistics
  • ~1/3 of all spirals are barred spirals.
  • There are "Field" and "Cluster" galaxies.
  • Ellipticals are most common in clusters.
  • Spirals are most common in the "field."

Clusters of
Galaxies
  • The Local Group is just outside the Virgo Cluster
  • Poor clusters have ~ 10 galaxies
  • Rich clusters have ~10,000 galaxies
  • Size of rich clusters: ~10 Mpc
  • Millions of clusters in the universe

Superclusters
  • Clusters of clusters
  • The Local Group is part of the Virgo Supercluster
  • Large scale structures containing many clusters have been found.
  • Does clustering continue forever in the universe?


Links to
Galaxy
Images

Active
Galaxies


Galaxy

Luminosity (LMW*)

Normal

< 10

Seyfert

0.5 - 50

Radio

0.5 - 50

Quasar

100 - 5,000

* where LMW = Luminosity of Milky Way = 2x1010 Lsun

 

Seyfert
Galaxies
  • Spectral lines don't resemble normal stars.
    • Highly ionized heavy elements, e.g. iron!
    • Lines very "wide" => tremendously hot (>108 K) or rapidly rotating (~1000 km/s)
  • Nearly all emission comes from the galactic nucleus (a small central region).
    • ~ 104 times brighter than the center of our galaxy
  • Most energy emitted in infrared and radio parts of the spectrum.
    • Look very much the same as normal galaxies in the visual.
  • Emitted energy varies with time!
    • => Compact source of energy
    • => Emitting region < 1 lyr across
    • Some emit more light than the Milky Way!

Radio
Galaxies
  • Very bright in the radio
    • Cosmic rock stations?
  • Core-Halo radio galaxies
    • Most emission from a very small core
      (< 1 parsec across)
  • Extended (or Lobe) radio galaxies
    • Emissions extend hundreds of kiloparsecs!

Core-Halo
Radio
Galaxies


Extended
Radio
Galaxies



Links to
Radio
Galaxies
  • Cygnus A
  • Cygnus A close-up
  • Hercules A
  • 3C75, Multi-Jets
  • Jet in NGC 6251
  • Jet in M87

These links are not yet enable. You'll find images some images at the Space Telescope Science Institute - Interesting Astronomy links page.


Quasi-Stellar
Objects (QSOs)
  • Galaxies with extremely luminous sources
    • Look like stars in photographs.
    • Some evidence of faint "parent" galaxy
  • Energy originates in a very small region.
  • QSOs are variable
  • Can see to great distances
    • Most distant objects seen in the universe
  • Such a source at 50 pc would appear as bright as the Sun!

 

  • Quasar: quasi-stellar radio source
    • a subset which is a strong radio emitter

What Powers
These?
  • We need to produce up to 5,000 times the luminosity of the Milky Way yet within a region ~1 pc in size !!!!
  • Leading theory is a black hole with an accretion disk:
    • Material gains energy as it falls towards the black hole.
    • The gas heats up, and radiates energy.

Accretion Disk
Model



Energetics
  • Material needs to keep flowing onto the black hole to power the source.
  • 1 Msun / decade => ~ 10 LMW
  • 10,000 LMW => 100 Msun / year!!
  • Large black holes (108 - 109 Msun) are needed to fit the models, otherwise the accretion disks blow themselves apart.
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