Galaxies: Formation and Evolution

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GalaxiesFormation and Evolution
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What are galaxies? How did they form?

• A look at nearby galaxies
• The cosmic time machine
• The Big Bang expansion of the universe
• New techniques allow a remarkably detailed look
  at galaxies as they were billions of years ago.
• Galaxies have changed over the history of the
  universe.
• Fossil evidence from the Milky Way and nearby
  galaxies.
• The first stars and the formation of galaxies.

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Galaxies Today

• Spiral, elliptical, irregular
• Stellar and gas content linked to morphology
• Dwarf galaxies most common
• Dark matter dominates overall dynamics
• Disk and bulge components differ in motions and
  stellar properties

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The Cosmic Time Machine
The finite speed of light means that we always
see things after they have happeneda delay of 8
minutes for the Sun and about 12 billion years
for the most distant galaxies we can observe. In
other words,

• Astronomical telescopes cant help viewing the
  past.
• Distance and time are always mixed in
  astronomical observations.

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Expansion of the Universe

• Features in the spectra of galaxies are
  essentially always observed at wavelengths longer
  than the corresponding features in laboratories
  on Earth (the redshift).
• The cosmological redshift is not exactly a
  Doppler shift, but is linked to the expansion of
  space as light propagates as well as the
  gravitational field of the universe.
• Hubble found a redshift-distance relation that
  could be interpreted as a uniform expansion.
• Friedmann had shown that such an expansion was a
  solution to Einsteins equations.

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Lookback time

• Observable quantity is redshift z
• Need distance scale and cosmology to derive
  lookback time to a given redshift
• Distance has multiple definitions in an expanding
  Universe

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z shift/initial wavelength
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Redshift versus lookback time WMAP cosmology
Hubble constant 71 km/s/Mpc Flat
spacetime W(matter)0.27
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Galaxy evolution

• Episodic starburst, interactions, mergers
• Passive aging of elliptical galaxies
• Changes in spiralelliptical mix (Butcher-Oemler
  effect)

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Here and now Coma E/S0 members
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Then spirals in Abell 851 (HST)
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Gas in clusters of galaxies can sweep a spiral
clean
But thats a whole different PowerPoint
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Hubble Space Telescope

• 2.4m optics, now diffraction limited
• Detectors from 0.11-2 microns
• Lifetime 1990-2007?
• Upgrades/repairs in the past via shuttle
  servicing (not without risk)

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Chandra X-Ray Observatory

• Imaging optics, 0.5-arcsecond resolution
• High energy resolution for imaging data
• Very powerful probe of accretion history (that
  is, quasars and black-hole growth)
• Complemented by ESAs XMM-Newton

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8-10 meter optical/IR telescopes

• 2x10m, Keck Observatory, Mauna Kea
• 4x8m, ESO Very Large Tel., Paranal, Chile
• 2x8m, Gemini Observatory (Hawaii/Chile)
• 8m, Subaru (Natl. Astron. Obs of Japan)

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Multiobject spectroscopy (Gemini-N)
Direct image
Raw multislit spectra
Slit placement
Reduced spectrum (AGN at z3.35)
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Adaptive optics

• Use rubber mirror to correct for atmospheric
  distortion
• Brings 8-10m telescopes close to diffraction
  limit in best cases (especially near-IR)
• They now outperform HST for some observations
• Need natural or laser guide stars nearby

Gemini-N/Altair 0.26 to 0.06 FWHM 1.65 microns
(H)
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Submillimeter detectors

• Precise antennas
• Cryogenic detector arrays
• Interferometry
• This shows us the thermal emission from dust in
  galaxies at high redshift, even if their direct
  starlight is completely absorbed

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Stellar spectra the fossil record

• Chemistry of stellar surface reflects initial
  chemical makeup until late in its lifespan
• Stars orbits change only very slowly over time
• Makeup and motions of stars preserve a detailed
  record of our galaxys history
• Early stars formed with low heavy-element
  abundances and in a nearly spherical system

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The early galaxy bestiary

• Lyman-break galaxies
• Extremely red objects (EROs) - the oldest young
  galaxies and dusty environments
• Star-forming subgalactic objects
• Submillimeter galaxies
• Lyman a clouds
• Quasars and radio galaxies
• Absorption-line systems
• These often occur in groupings

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Lyman-Break Galaxies (LBGs)

• Galaxy spectra show a cutoff at 912 A due to
  absorption by neutral hydrogen
• This allows a straightforward multicolor
  selection (blue in two bands, missing shortward
  of that)
• Thousands of galaxies at zgt2.7 have now been
  found in this way

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The Lyman Break
300 nm 450 nm 606 nm 814 nm
The Lyman break from satellite UV observations of
a star-forming region in the nearby spiral M33
The brightest LBG in the Hubble Deep Field, a
clumpy galaxy at z3.21.
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Submillimeter-bright Galaxies

• Found at z2-3
• Most powerful early star-forming sites?
• Key on dust emission, not stars
• Clump with other high-redshift objects
• Many have buried quasar cores

Background ionized-gas plume in submm galaxy
ELAIS N2 850.4 at z2.4, from NASA Infrared
Telescope Facility, April 2003)
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Extremely Red Objects (EROs)

• May be either intrinsically red or reddened by
  dust absorption both kinds exist
• A way to seek the oldest galaxies at a particular
  redshift, a sensitive probe of when galaxy
  formation began in earnest

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Subgalactic Clumps

• Small size, blue color, Lyman a emission
• Active star formation, low metallicity
• Evidence for global winds escaping systems
• Exist in groupings with bright galaxies/AGN
• Are these the early units predicted by
  hierarchical schemes (and fitting dark-matter
  simulations)?

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Blue subgalactic objects versus nearby spiral
M101 at the same ultraviolet emitted wavelength
Size 1 kpc3000 light-yr Many are
double Comparable UV luminosity to bright
galaxies now
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The early Universe could be crowded(a group at
z2.4)
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Subgalactic objects
Radio galaxy
Quasars
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Quasars in the Early Universe

• Trace supermassive black holes and their growth
  by accretion
• Black holes today are ubiquitous in bright
  galaxies
• Quasars now seen to 0.5 Gyr after beginning, very
  common 10 Gyr ago
• Surrounding gas heavily processed by supernovae,
  even at highest redshifts

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Heavy elements in high-redshift quasars
HST composite, courtesy W. Zheng)
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Ingredients of a cosmic history

• Gravitational collapse and gas infall
• Star formation (a feedback process)
• Heavy-element production
• Winds and the intergalactic medium
• Growth of supermassive black holes
• The first stars a breed apart

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The First Stars (Population III)

• Formed of pure hydrogen/helium
• Very massive (80-300 solar masses)
• Hot, short-lived
• Energetic supernova explosions
• Enriched surrounding gas, disrupted parental gas
  clouds
• Enrichment led to normal star formation
• Enriched intergalactic gas as well

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New tools, new horizons

• James Webb Space Telescope
• Adaptive optics spreads in accessibility, field
  of view, and wavelength
• Atacama Large Millimeter Array (ALMA)
• High-dynamic-range simulations of galaxy
  formation
• Massive galaxy and star spectroscopic surveys

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James Webb Space Telescope (2010)
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Atacama Large Millimeter Array
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CELT/NOAO 30-meter design
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The giant OWL scans the skies
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Final Musings The Anthropic View

• We require particular physical laws and
  environmental conditions for life
• Galaxies provide a place for stars to form and
  chemically enrich their surroundings
• Their inception may require a first generation of
  uniquely massive stars
• Do we live in the unique Universe, or does it
  make sense to think of the Multiverse?

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