The Radio Milky Way and The Green Bank Telescope

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The Radio Milky Way and The Green Bank Telescope

• Ronald MaddalenaNational Radio Astronomy
  Observatory

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National Radio Astronomy Observatory

• National Laboratory
• Founded in 1954
• Funded by the National Science Foundation

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Telescope Structure and Optics
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Telescope Structure and Optics

• Large 100-m Diameter
• High Sensitivity
• High Angular Resolution wavelength / Diameter

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GBT Telescope Optics

• 110 m x 100 m of a 208 m parent paraboloid
• Effective diameter 100 m
• Off axis - Clear/Unblocked Aperture

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Telescope Optics

• High Dynamic Range
• High Fidelity Images

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Telescope Optics
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Telescope Optics
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Telescope Optics
Prime Focus Retractable boom Gregorian Focus
8-m subreflector - 6-degrees of freedom
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Telescope Optics
Rotating Turret with 8 receiver bays
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Telescope Structure

• Fully Steerable
• Elevation Limit 5º
• Can observe 85 of the entire Celestial Sphere
• Slew Rates Azimuth - 40º/min Elevation - 20º/min

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Telescope Structure
Blind Pointing (1 point/focus) Offset
Pointing (90 min) Continuous Tracking
(30 min)
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Telescope Structure
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Active Surface
Surface Deformations from Finite Element Model
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Active Surface
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Active Surface

• Main Reflector 2209 actuated panels with 68 µm
  rms.
• Total surface rms 400 µm

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Receivers
Receiver Operating Range Status
Prime Focus 1 0.290.92 GHz Commissioned
Prime Focus 2 0.9101.23 GHz Commissioned
L Band 1.151.73 GHz Commissioned
S Band 1.732.60 GHz Commissioned
C Band 3.955.85 GHz Commissioned
X Band 8.210.0 GHz Commissioned
Ku Band 12.415.4 GHz Commissioned
K Band 1826.5 GHz Commissioned
Ka Band 2640 GHz Partially Commissioned
Q Band 4050 GHz Commissioned
W Band 6892 GHz Under Construction
Penn Array 8694 GHz Under Construction
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Backends
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National Radio Quiet Zone
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National Radio Quiet Zone
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Science with the GBT

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Current Science Projects
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Milky Way

• Our Home Galaxy
• Projected image on the night sky is the Milky Way
• Dust in the Interstellar Medium obstructs our
  optical view.
• Need Radio observations to peer through the
  dust
• Our perspective is from a star in the outer Milky
  Way.
• Serves as a nearby example of the 100 billion
  other galaxies

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Interstellar Medium The Material Between the
Stars

• Constituents
• Gases
• Hydrogen (92 by number)
• Helium (8)
• Oxygen, Carbon, etc. (0.1 )
• Dust Particles
• 1 of the mass of the ISM
• Average Density 1 H atom / cm3
• Place where stars planets form
• The byproduct of the death of stars

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Interstellar Medium Properties
State of Hydrogen Temperature Densities (H/cm3) Percent Volume
HII Regions Planetary Nebulae Ionized 5000 K 0.5 lt 1
Diffuse ISM Ionized 1,000,000 K 0.01 50
Warm ISM Atomic 3000 K 0.3 30
Cold ISM Atomic 300 K 30 10
Molecular Clouds Molecular lt 30 K gt 300 10
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HII Regions

• Isolated regions where H is ionized.
• UV from hot (20,000 50,000 K), blue stars
  produces ionization.
• HII Regions
• Formed around young, massive, short-lived (lt
  few x 106 years) stars.
• Near regions where they formed

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Scientific Results - Imaging
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Scientific Results - Imaging
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Scientific Results - Imaging
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Diffuse ISM Galactic Center
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Diffuse ISM Galactic Center
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Atomic HydrogenSpectral-Line Radiation

• Discovered by Ewen and Purcell in 1951.
• Found in regions where H is atomic.
• 300 K, 30 H/cm3
• Spin-flip (hyperfine) transition
• Electron protons have spin
• In a H atoms, spins of proton and electron may be
  aligned or anti-aligned.
• Aligned state has more energy.
• Difference in Energy h v
• v 1420 MHz
• An aligned H atom will take 11 million years to
  flip the spin of the electron.
• But, 1067 atoms in Milky Way so 1052 H atoms per
  second emit at 1420 MHz

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Spectral-Line Radiation- What do they tell us?

• Width of line ? Motion of gas within the region
• Height of the line ? Maybe temperature of the gas
• Area under the line ? Maybe number of atoms in
  that direction.

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Doppler Affect

• Frequency Observed Frequency Emitted / (1
  V/c)

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Spectral-Line RadiationMilky Way Rotation and
Mass

• For any cloud
• Observed velocity difference between projected
  Suns motion and projected cloud motion.
• For cloud B
• The highest observed velocity along the line of
  site
• VRotation Vobserved Vsunsin(L)
• R RSun sin(L)
• Repeat for a different angle L and cloud B
• Determine VRotation(R)
• From Newtons law, derive M(R) from V(R)

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Scientific Results Milky Way Gas
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Scientific Results Milky Way Gas
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Interstellar Molecules

• Hydroxyl (OH) first molecule found with radio
  telescopes (1964).
• Molecule Formation
• Need high densities
• Lots of dust needed to protect molecules for
  stellar UV
• But, optically obscured need radio telescopes
• Low temperatures (lt 100 K)
• Some molecules (e.g., H2) form on dust grains
• Most form via ion-molecular gas-phase reactions

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Interstellar Molecules Ion-molecular gas-phase
reactions

• Starts with a cosmic ray that ionizes a H atom
• All exothermic reactions
• Charge transfer
• Two-body interactions

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Interstellar Molecules

• About 90 of the over 129 interstellar molecules
  discovered with radio telescopes.
• Rotational (electric dipole) Transitions
• Up to thirteen atoms
• Many carbon-based (organic)
• Many cannot exist in normal laboratories (e.g.,
  OH)
• H2 most common molecule
• No dipole moment so no rotational transition at
  radio wavelengths.
• Only observable in UV (rotational) or Infrared
  (vibrational) transitions from space.
• Use CO, the second most common molecule, as a
  tracer for H2

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Interstellar Molecules

• A few molecules (OH, H2O, ) maser

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Scientific Results - Molecules
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Molecular Clouds

• Discovered 1970 by Penzias, Jefferts, Wilson
  and others.
• Coldest (5-30 K), densest (100 106 H atoms/cm3)
  parts of the ISM.
• Where stars are formed
• 50 of the ISM mass
• A few percent of the Galaxys volume.
• Concentrated in spiral arms
• Dust Clouds Molecular Clouds

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Molecular Clouds

• Discovered 1970 by Penzias, Jefferts, Wilson
  and others.
• Coldest (5-30 K), densest (100 106 H atoms/cm3)
  parts of the ISM.
• Where stars are formed
• 50 of the ISM mass
• A few percent of the Galaxys volume.
• Concentrated in spiral arms
• Dust Clouds Molecular Clouds

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Scientific Results Lunar Radar
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Scientific Results Galaxy Formation
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Scientific Results - Pulsars
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