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Far IR Submillimeter Astronomy

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					Far-IR/Submillimeter Astronomy


               Astronomy 101
        Dr. C. Darren Dowell, Caltech
         Submillimeter Observatory
               11 October 2000
 http://www.submm.caltech.edu/~cdd/class
                   Outline
•   Kirchhoff‟s Law
•   Far-infrared/submillimeter emission
•   Atmospheric constraints, observatories
•   Detectors
•   Observing strategies
•   Bolometers
•   A submillimeter camera
             Kirchhoff‟s Law
• Absorptivity = emissivity
• Example 1: A 99% reflective mirror absorbs 1%
  of radiation incident on it. This means it must also
  emit with a spectrum given by:
   – F(n) = B(n) × 1%
• Example 2: Spacecraft radiators (directed toward
  cold outer space) are coated with black
  (absorptive) paint to cool efficiently.
Where is the far-IR/submillimeter?
• A useful definition:
   – Far-IR: l = 30 mm to 300 mm, which is unobservable
     from the ground
   – Submillimeter: l = 300 mm to 1 mm, partially available
     from high, dry mountains
• In frequency units, 31011 Hz to 11013 Hz
• Compare:
   – Visual wavelengths at l  0.5 mm  61016 Hz
   – Commercial FM radio at l  3 m  1108 Hz
 Sources of far-IR/submillimeter
            emission
• Continuum
  –   Blackbody emission from solar system objects and stars
  –   „Graybody‟ emission from dusty nebulae
  –   Free-free (bremsstrahlung) emission from ionized gas
  –   Synchrotron emission from relativistic electrons
      spiraling around magnetic fields
• Line
  – Rotational transitions of molecules
  – Electronic transitions in atoms (large prinicipal
    quantum number „n‟, fine structure)
Vela/Puppis Nebula seen by IRAS
 Dust in the Interstellar Medium
• Dust grains are made of silicate and graphite
  material, coated with ices in cold regions.
• There is a grain size distribution (more small
  grains, fewer large grains); an average size of
  0.1 mm gives the best fit in simple models.
• Dust is intermixed with H2 in molecular clouds,
  with M(dust)/M(H2)  0.01.
• The majority of dust emission is from nebulae
  with ongoing star formation, where the dust is
  heated by nearby stars.
Emission from a single dust grain
• F(n) = A Q(n) B(n,T) / D2
   –   F(n) = flux density (measured intensity)
   –   A = geometrical cross section = pr2
   –   Q(n) = emissivity (modification to cross section)
   –   B(n,T) = Planck function
   –   D = distance from observer to dust grain
• AQ(n) = emission cross section = absorption cross
  section (by Kirchhoff‟s Law)
   – Q(n)  1 at ultraviolet wavelengths.
   – Q(n) falls as l-2 at submillimeter wavelengths.
   – Nebulae which are visually opaque are usually
     transparent in the far-IR/submillimeter.
Comparison of optical/IR with
   far-IR/submillimeter
The Milky Way – An Edge-On
       Spiral Galaxy
The Andromeda Galaxy (M 31)
Continuum spectra of various objects
Far-IR/submillimeter emission lines
 • Spectral lines are responsible for 5 to 50%
   of total far-IR/submillimeter emission from
   nebulae.
   – Larger fraction for longer wavelengths; smaller
     fraction for shorter wavelengths.
   – Largest fraction for environments where young
     stars are evaporating gas off dust grains.
Submm. lines in the Orion Nebula
             Rotational transitions
• Start with classical mechanics:
   – I = m1m2R2/(m1+m2)                      m1
                                         C
   – J = Iw
   – E = Iw2/2                                    w
• Add quantum mechanics:                     R
   – J = {N(N+1)}1/2h/2p quantized
                                                  O   m2
   – n = DE/h, with DN = 1 most likely
• Then:
   – E = h2N(N+1)/8p2I
   – n = h(N+1)/4p2I {state N+1  state N}
   Rotational transitions of Carbon
              Monoxide
• CO molecule:
                                        C     12 amu
   – m1 = 12 amu = 2.0 × 10-26 kg
   – m2 = 16 amu = 2.7 × 10-26 kg
   – R = 1.1 Å = 1.1 × 10-10 m
                                     1.1 Å
• Plug in:
   – I = m1m2R2/(m1+m2) = 1.4 × 10-46 kg m2            O   16 amu
   – n = h(N+1)/4p2I = 121 GHz (N+1)
• Actually:
   – n = 115.271 GHz, 230.538 GHz, 345.796 GHz, …
       Atmospheric constraints
• Water vapor is the primary enemy of far-
  IR/submillimeter astronomy.
• Range l = 30 – 300 mm is unavailable from the
  ground. Other options:
   – Airplanes (40,000 ft.) – KAO, SOFIA
   – Balloons (120,000 ft.)
   – Satellites – IRAS, ISO, SIRTF
• 300 mm – 1 mm range is partially available from
  high, dry mountains (> 10,000 ft.) – Mauna Kea
  (CSO, JCMT, SMA); South Pole; Chile (ALMA)
• l > 1 mm – available from lower elevations
          Mauna Kea, Hawaii (13,000 ft.)




1000 mm                            300 mm
„Submillimeter Valley‟, Mauna Kea
Caltech Submm. Observatory – 10 m
Kuiper Airborne Observatory
    (1974-1995) – 0.9 m
A telescope in
an airplane (!)
On the Kuiper Airborne Observatory
   Stratospheric Observatory for
Infrared Astronomy (2002) – 2.5 m
Space Infrared Telescope Facility
        (2002) – 0.85 m
   Angular resolution in the far-
        IR/submillimeter
• Diffraction for a single telescope: q  1.2 l / D
   – Typical submillimeter telescope: D = 10 m,
     l = 800 mm  q = 20”
• Interferometry can solve angular resolution
  problem: q  1.2 l / B, where B is the separation
  of two telescopes
   – SMA: 1”
   – ALMA (2007): 0.1”
• There are slow atmospheric „seeing‟ (wavefront
  distortion) effects, but they can be corrected with
  sky monitors.
  Far-IR/submillimeter detectors
• „Incoherent‟ – light as a particle
  – Photoconductors – 1 photon raises 1 electron
    from valence band to conduction band; works
    only for l < 200 mm
  – Bolometers – photons raise the temperature of
    an absorber, which is measured by a thermistor
• „Coherent‟ – light as a wave
  – Heterodyne mixers – measure „beat frequency‟
    of cosmic radiation against a local oscillator
             Heterodyne mixers
• Basic idea: Illuminate mixer element with
  radiation from the sky, and also radiation from a
  transmitter („local oscillator‟).
• Beat frequencies get produced:
   – Vsky = V1 sin (w1t – f1)
   – VLO = V0 sin (w0t – f0)
   – Voutput = (Vsky + VLO)2 = C sin [(w1-w0)t – f] + high
     frequency terms which get filtered out
   – See Smith, p. 106, for exercise.
• Example: Line of interest at 345 GHz, LO at 344
  GHz  line appears at (345 – 344) = 1 GHz, a
  frequency which spectrometers can deal with.
 Difficulty of infrared/submillimeter
     astronomy from the ground
• Infrared astronomy from the ground has been
  likened to “observing in the day, with the
  telescope on fire”.
• Why? The atmosphere emits, and the telescope
  itself emits.
• Atmospheric emission is ~106 times brighter than
  the faintest source which can be detected in 1
  hour.
• Telescope emission can be minimized with a good
  design.
Optical telescope
Submillimeter telescope
         Observing strategies
• The terrestrial atmosphere absorbs heavily
  in the far-IR/submillimeter, so it must also
  emit. (Kirchhoff‟s Law)
  – Transmission = 60%  emissivity = 40%
  – T (atmosphere) = 270 K  lpeak  2900 mm / T
     10 mm  significant emission on the
    Rayleigh-Jeans side of the spectrum.
• The atmospheric emission changes as „cells‟
  of water vapor drift by.
• Constant sky subtraction is necessary.
    One sky subtraction approach –
           chopping mirror




• A mirror wobbles back and forth, causing the detector to
  view two different parts of the sky. (Smith, p. 128)
Another sky subtraction approach –
     differential radiometer
                    • One pixel is
                      „source + sky‟, and
                      two pixels are
                      „sky‟.
                    • This is similar to
                      using edge pixels
                      on a CCD to
                      subtract the sky,
                      but with a much
                      worse sky and
                      fewer pixels.
Bolometer – diagram of 1 „pixel‟
  radiation                           thermistor


                                                   wires
absorber

                           weak thermal link
           cold bath at fixed temperature

• Radiation is intercepted, absorber heats, and
  temperature change is measured by thermistor.
Actual bolometers
A closer look…
An even closer
   look…
      absorber:
      1 mm square


 doped silicon thermistor
 (invisible)



 leg = thermal link,
 wire on top
        What is a thermistor?
• A thermistor is a resistor whose resistance
  varies with temperature.
• Thermistors can be made out of
  semiconductors. When the temperature
  increases, more electrons enter the
  conduction band, and therefore the
  resistance goes down.
Typical thermistor behavior
    IV curves – a useful method for
    analyzing detector performance




• An applied current is necessary to measure a resistance.
IV curve for a bolometer



             turnover, due
             to self-heating


      linear region
Now add radiation…




         operating current
 Limits to bolometer performance
• The kinetic energy of the electrons and atoms in a
  bolometer limit its performance:
   – Johnson noise – random voltage from fluctuations in
     the motion of electrons
   – Phonon noise – random bolometer temperature from
     fluctuations of energy flowing down the thermal link
• The colder, the better.
• NEP: Smallest power which is detected in a 1
  second integration; units W Hz-1/2 = W s1/2
• State of the art:
   – Bolometers at 0.3 K: NEP = 10-16 W s1/2
   – Bolometers at 0.1 K: NEP = 10-17 W s1/2
     A Lightbulb on the Moon?
• How much power can be collected by a 10 meter
  telescope from a 100 watt lightbulb at the distance
  of the Moon?
   – P = 100 watts (pr2/4pd2)
   – r = 5 m, d = 4 x 108 m
   – Therefore, P = 4 x 10-15 watts
• In principle, one could easily detect the light bulb
  with a bolometer at 0.3 K.
• Detecting the light bulb might be more difficult
  than our simple calculation would indicate. Why?
Bolometers – impartial detectors of
            radiation
 • Bolometers can detect radiation over a very broad
   range of the electromagnetic spectrum, from X
   rays to radio wavelengths
 • Wavelength is contrained to passband of interest
   by choice of absorber and by choice of filters in
   front of detector.
 • The bolometer is the superior broadband
   (Dl/l > 0.01) detector from l = 200 mm – 1 mm.
    SHARC II – a camera using
         bolometers
• SHARC II – Submillimeter High Angular
  Resolution Camera, 2nd generation
• For the Caltech Submillimeter Observatory
• Observing at l = 350 mm
• Goal: 12 × 32 = 384 bolometers – the most
  submillimeter pixels in the world, by a factor of 3
• Bolometers cooled to 0.3 K
• Started in 1997; first tests with 16 pixels in
  September 2000; finish in 2001(?)
Goal: 2-dimension bolometer array
Cross section of SHARC II
    Useful cryogens for far-IR cameras
• Liquid cryogens are used at their boiling point
  temperature, since they are colder than their
  surroundings. Water as a „cryogen‟ would
  provide a temperature of 373 K.

•   Liquid nitrogen (N2) – 77 K
•   Liquid helium (4He) at 1 atmosphere – 4.2 K
•   Liquid 4He at 10-3 atmosphere – 1.5 K
•   Liquid 3He at 10-3 atmosphere – 0.3 K
Assembling the
   camera
Assembling
the camera
    No Multi-Layer Insulation
                    P


            sAT14



                          sAT04
       T1 > T0                 T0
       e=1=a                 e=1=a

• P = net power flow = sAT14 – sAT04 =
  sA[T14 – T04]
       With one shield layer
                           P

      sAT14       sAT14a sAT04a             sAT04


              sAT14(1-a)       sAT04(1-a)


                 sAT24e        sAT24e
    T1                 T2                        T0
  e=1=a              e=a1                     e=1=a
• Equilibrium of shield: sAT14a + sAT04a =
  2sAT24e  T24 = T14/2 + T04/2
• P = sAT24e + sAT04(1-a) – sAT04 =
  sA(e/2) [T14 – T04]
Putting it on the
   telescope
  On the
telescope
Inaugural image – Orion Nebula core
 Key points – far-IR/submm. astronomy
• Most of the far-IR/submillimeter emission from the universe
  comes from dust. Line emission is mostly from rotational
  transitions of simple molecules.
• The far-IR/submillimeter is particularly useful for studying the
  formation of stars, from nearby nebulae to high-redshift galaxies.
• The water in the Earth‟s atmosphere absorbs most of far-
  IR/submillimeter radiation from space – the main motivation for
  airborne and spaceborne observatories.
• Far-IR/submm detectors include photoconductors, bolometers,
  and heterodyne mixers.
• Bolometers detect a small temperature rise from absorbed
  radiation.

				
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