Active Galaxies and Cosmology

Review for Astronomy 123 Midterm Exam 2

posted May 9, 2001, jeb
revised May 10, 2001, jeb

Outline

• Active Galaxies and Quasars
• Cosmology

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Active Galaxies and Quasars

Active galaxies
• a small percentage of all galaxies
• non-thermal spectrum

Seyfert Galaxy - spiral
• bright nucleus, sub-yearly variation -------> small source

Radio Galaxies
• synchrotron radiation (charged particles spiral in magnetic field)
• non-thermal spectrum (radio and infrared)
• radio emissions - thousand x normal
• types:
  • core-halo -----> bright core
    • region of radio emission smaller than visible
  • radio lobe ----> very large
    • region of radio emission much larger than visible
    • largest objects in the univers
• Example - Cygnus A lobe radio galaxy
  • 1 billion light years away
  • 1 million times the radio output of the Milky Way
• Other examples
  • Centaurus A - unusual dust
    • lobe radio galaxy
  • M87 - jet of gas
    • core - halo galaxy
    • 3,000,000,000 solar mass black hole at core

BL Lac
• star-like with rapid variability
• compact radio sources
• distant elliptical galaxies

Central Engine
• Super massive black hole - million/billion solar masses
• Accretion disk
• Jets of high-speed particles
  • synchrotron radiation



Quasars
• Quasi-Stellar Radio Sources
• First discovered as very intense radio sources from star-like objects
  • (most now are not so strong radio sources)
• Found to be moving away from us at tremendous velocities
  • large red shifts of the emission lines
    • redshift = ( observed wavelength - true wavelength ) / true wavelength
  • nearly the speed of light (from Doppler red-shift)
• Also, they are small (light-weeks)
  • rapid variability
• "Look-back time" tells us we're looking at the universe in the distant past
  • 1-10 billion years ago

Quasars --------> radio galaxies (galaxies in evolution?)

Gravitational Lenses
• Explained by Einstein's theory of General Relativity
• Einstein cross
  • multiple-imaged quasar
• Double quasars
• Blue arcs
  • very distant galaxies lensed by foreground galaxies
    • study dark matter

Cosmology

The study of the universe as a whole
• nature and evolution

Cosmological Principle - universe is homogeneous and isotropic
• universality of physical laws

Olbers's Paradox - why is the sky dark at night?
1. red-shift of starlight
2. finite age of universe

Big-Bang Theory - favored by astronomers
• supported by
  • Hubble's Law (1st evidence)
  • existence of quasars (early universe was different)
  • cosmic microwave background (CMB)
• age since big-bang 15 billion years, or so (CMB)
  • this depends on the density of matter
  • and the cosmological constant (or "dark energy")

Space-time nature of the universe
• 3 possibilities in Einstein's General Theory of Relativity
  • open
  • flat
  • closed
• Depends on density of matter (and "dark energy")
  
  Density of matter (and "dark energy") in the universe
  • Astronomers define a ratio of densities, ₀("Omega nought")
    • ₀ = actual density / critical density
      • the amount of mass needed to close the universe is call the critical density
  • We know ₀("Omega nought") is close to 1
  • Luminous matter explains ₀("Omega nought") = 0.02-0.05 ("a few percent" of critical density)
  • Dark matter is much more, bringing ₀("Omega nought") to 0.2, 0.3, or more
  • Dark energy completes closure of the universe, bringing ₀("Omega nought") to about 1.0
    
    
    summary of what we know about the components of ₀("Omega nought")
  
  

  density of matter	nature of universe	geometry of universe	future
  less matter than critical	OPEN	infinite and unbounded	expand forever
  critical density	FLAT	infinite and unbounded	expand forever
  more matter than critical	CLOSED	finite and unbounded	collapses in BIG CRUNCH

  
  

Steady-state theory
• was competing theory to Big Bang
• matter continuously created (1 atom/cm³/10¹⁵yrs)
  • violated conservation of energy and matter
  • (but only barely)
• assumes universe is unchanging in time
• Hubble's Law requires continuous creation
• ruled out by cosmic microwave background

Cosmic Microwave Background
• predicted by big-bang theory
  • the universe had a hot, dense past
• measured by Penzias and Wilson with radio telescope
• found 2.7 K background - nearly uniform in all directions
  • at all times
  • peak wavelength about 1 mm
  • matches blackbody spectrum
  • emerged from big bang fireball when the universe was 100,000 years old
• following big-bang, as universe expands, background radiation gets cooler
  • from background radiation we can determine motion of earth through universe
• COBE
  • small asymmetry due to the Earth's motion through space

Prediction of Big Bang Theory
• Expansion of the universe shifted the initially high energy radiation in the universe to longer and longer wavelengths
  • cosmological redshift
• The Big Bang Theory predicted that the universe should be hissing (static)
  • coming from all directions
  • follows "black body" curve
  • Temperature about 3 K (we now know it is 2.7 K)

Future of the universe
• Also depends on the matter density and the dark energy density ("the cosmological constant")

Two recent observations
• Boomerang has shown that the universe is very close to being perfectly flat
  • Boomerang measures fluctuations across the sky on the cosmic microwave background temperature
  • The character of these fluctuations depends on ₀("Omega nought")
  • Conclusion: ₀("Omega nought") is within 10% of 1.0
• The expansion of the universe appears to be accelerating
  • Supernovae at large redshift are fainter than they would be if the universe were expanding at a constant speed

Sample numerical problems

• Red shift problem 1
  
  
• Red shift problem 2