The Green Bank Telescope Ronald Maddalena National Radio Astronomy Observatory

National Radio Astronomy Observatory  National Laboratory  Founded in 1954  Funded by the National Science Foundation

Telescope Structure and Optics

Large 100-m Diameter: High Sensitivity High Angular Resolution – wavelength / Diameter

GBT Telescope Optics  110 m x 100 m of a 208 m parent paraboloid Effective diameter: 100 m Off axis - Clear/Unblocked Aperture

Telescope Optics  High Dynamic Range  High Fidelity Images

Telescope Optics

Prime Focus: Retractable boom Gregorian Focus: 8-m subreflector - 6-degrees of freedom

Telescope Optics Rotating Turret with 8 receiver bays

Telescope Structure  Fully Steerable  Elevation Limit: 5º  Can observe 85% of the entire Celestial Sphere  Slew Rates: Azimuth - 40º/min; Elevation - 20º/min

Telescope Structure Blind Pointing: (1 point/focus) Offset Pointing: (90 min) Continuous Tracking: (30 min)

Telescope Structure

Active Surface Surface Deformations from Finite Element Model

Active Surface

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

Receivers ReceiverOperating RangeStatus Prime Focus 10.29—0.92 GHzCommissioned Prime Focus —1.23 GHzCommissioned L Band1.15—1.73 GHzCommissioned S Band1.73—2.60 GHzCommissioned C Band3.95—5.85 GHzCommissioned X Band8.2—10.0 GHzCommissioned Ku Band12.4—15.4 GHzCommissioned K Band18—26.5 GHzCommissioned Ka Band26—40 GHzPartially Commissioned Q Band40—50 GHzCommissioned W Band68—92 GHzUnder Construction Penn Array86—94 GHzUnder Construction

Backends

National Radio Quiet Zone

Science with the GBT

Current Science Projects

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

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 / cm 3 Place where stars & planets form The byproduct of the death of stars

Interstellar Medium Properties State of Hydrogen TemperatureDensities (H/cm 3 ) Percent Volume HII Regions & Planetary Nebulae Ionized5000 K0.5< 1% Diffuse ISMIonized1,000,000 K0.0150% Warm ISMAtomic3000 K0.330% Cold ISMAtomic300 K3010% Molecular Clouds Molecular< 30 K> 30010%

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 (< few x 10 6 years) stars. Near regions where they formed

Scientific Results - Imaging

Diffuse ISM – Galactic Center

Atomic Hydrogen Spectral-Line Radiation Discovered by Ewen and Purcell in Found in regions where H is atomic. 300 K, 30 H/cm 3 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, atoms in Milky Way so H atoms per second emit at 1420 MHz

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.

Doppler Affect Frequency Observed = Frequency Emitted / (1 + V/c)

Spectral-Line Radiation Milky Way Rotation and Mass For any cloud Observed velocity = difference between projected Sun’s motion and projected cloud motion. For cloud B The highest observed velocity along the line of site V Rotation = V observed + V sun *sin(L) R = R Sun * sin(L) Repeat for a different angle L and cloud B Determine V Rotation (R) From Newton’s law, derive M(R) from V(R)

Scientific Results – Milky Way Gas

Scientific Results – Milky Way

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 (< 100 K) Some molecules (e.g., H2) form on dust grains Most form via ion-molecular gas-phase reactions

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

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) H 2 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 H 2

 A few molecules (OH, H 2 O, …) maser Interstellar Molecules

Scientific Results - Molecules

Molecular Clouds  Discovered 1970 by Penzias, Jefferts, & Wilson and others.  Coldest (5-30 K), densest (100 –10 6 H atoms/cm 3 ) parts of the ISM.  Where stars are formed  50% of the ISM mass  A few percent of the Galaxy’s volume.  Concentrated in spiral arms  Dust Clouds = Molecular Clouds

Molecular Clouds  Discovered 1970 by Penzias, Jefferts, & Wilson and others.  Coldest (5-30 K), densest (100 –10 6 H atoms/cm 3 ) parts of the ISM.  Where stars are formed  50% of the ISM mass  A few percent of the Galaxy’s volume.  Concentrated in spiral arms  Dust Clouds = Molecular Clouds

Scientific Results – Lunar Radar

Scientific Results – Galaxy Formation

Scientific Results - Pulsars

Fastest Pulsar