Far-IR/Submillimeter Astronomy: Exploring the Universe Beyond Visible Light

1. Far-IR/Submillimeter Astronomy Astronomy 101 Dr. C. Darren Dowell, Caltech Submillimeter Observatory 11 October 2000 http://www.submm.caltech.edu/~cdd/class

2. Outline • Kirchhoff’s Law • Far-infrared/submillimeter emission • Atmospheric constraints, observatories • Detectors • Observing strategies • Bolometers • A submillimeter camera

3. 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.

4. 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

5. 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)

6. Vela/Puppis Nebula seen by IRAS

7. 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.

8. 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.

9. Comparison of optical/IR with far-IR/submillimeter

10. The Milky Way – An Edge-On Spiral Galaxy

11. The Andromeda Galaxy (M 31)

12. Continuum spectra of various objects

13. 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.

14. Submm. lines in the Orion Nebula

15. Rotational transitions • Start with classical mechanics: • I = m1m2R2/(m1+m2) • J = Iw • E = Iw2/2 • Add quantum mechanics: • J = {N(N+1)}1/2h/2pquantized • n = DE/h, with DN = 1 most likely • Then: • E = h2N(N+1)/8p2I • n = h(N+1)/4p2I {state N+1  state N} m1 C w R O m2

16. Rotational transitions of Carbon Monoxide • CO molecule: • 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 • Plug in: • I = m1m2R2/(m1+m2) = 1.4 × 10-46 kg m2 • n = h(N+1)/4p2I = 121 GHz (N+1) • Actually: • n = 115.271 GHz, 230.538 GHz, 345.796 GHz, … 12 amu C 1.1 Å O 16 amu

17. 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

19. Mauna Kea, Hawaii (13,000 ft.) 1000 mm 300 mm

20. ‘Submillimeter Valley’, Mauna Kea

21. Caltech Submm. Observatory – 10 m

22. Kuiper Airborne Observatory (1974-1995) – 0.9 m

23. A telescope in an airplane (!)

24. On the Kuiper Airborne Observatory

25. Stratospheric Observatory for Infrared Astronomy (2002) – 2.5 m

26. Space Infrared Telescope Facility (2002) – 0.85 m

27. 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.

28. 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

29. 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.

30. 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.

31. Optical telescope

32. Submillimeter telescope

33. 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.

34. 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)

35. 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.

36. Bolometer – diagram of 1 ‘pixel’ radiation thermistor • Radiation is intercepted, absorber heats, and temperature change is measured by thermistor. wires absorber weak thermal link cold bath at fixed temperature

37. Actual bolometers

38. A closer look…

39. An even closer look… absorber: 1 mm square doped silicon thermistor (invisible) leg = thermal link, wire on top

40. 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.

41. Typical thermistor behavior

42. IV curves – a useful method for analyzing detector performance • An applied current is necessary to measure a resistance.

43. IV curve for a bolometer turnover, due to self-heating linear region

44. Now add radiation… operating current

45. 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

46. 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?

47. 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.

48. 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(?)

49. Goal: 2-dimension bolometer array

50. Cross section of SHARC II