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() = B() × 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: = 30 m to 300 m, which is unobservable

from the ground

– Submillimeter: = 300 m to 1 mm, partially available from high, dry mountains

• In frequency units, 31011 Hz to 11013 Hz• Compare:

– Visual wavelengths at 0.5 m 61016 Hz

– Commercial FM radio at 3 m 1108 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 m 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() = A Q() B(,T) / D2

– F() = flux density (measured intensity)– A = geometrical cross section = r2

– Q() = emissivity (modification to cross section)– B(,T) = Planck function– D = distance from observer to dust grain

• AQ() = emission cross section = absorption cross section (by Kirchhoff’s Law)– Q() 1 at ultraviolet wavelengths.– Q() falls as -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)

– J = I– E = I2/2

• Add quantum mechanics:– J = {N(N+1)}1/2h/2 quantized

– = E/h, with N = 1 most likely

• Then:– E = h2N(N+1)/82I

– = h(N+1)/42I {state N+1 state N}

m1

m2

R

C

O

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

– = h(N+1)/42I = 121 GHz (N+1)

• Actually:– = 115.271 GHz, 230.538 GHz, 345.796 GHz, …

12 amu

16 amu

1.1 Å

C

O

Atmospheric constraints• Water vapor is the primary enemy of

far-IR/submillimeter astronomy.• Range = 30 – 300 m is unavailable from the

ground. Other options:– Airplanes (40,000 ft.) – KAO, SOFIA– Balloons (120,000 ft.)– Satellites – IRAS, ISO, SIRTF

• 300 m – 1 mm range is partially available from high, dry mountains (> 10,000 ft.) – Mauna Kea (CSO, JCMT, SMA); South Pole; Chile (ALMA)

• > 1 mm – available from lower elevations

Mauna Kea, Hawaii (13,000 ft.)

1000 m 300 m

‘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: 1.2 / D– Typical submillimeter telescope: D = 10 m,

= 800 m = 20”

• Interferometry can solve angular resolution problem: 1.2 / 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 < 200 m

– 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 (1t – 1)– VLO = V0 sin (0t – 0)– Voutput = (Vsky + VLO)2 = C sin [(1-0)t – ] + 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 peak 2900 m / T

10 m 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 is intercepted, absorber heats, and temperature change is measured by thermistor.

weak thermal link

wires

cold bath at fixed temperature

absorber

thermistorradiation

Actual bolometers

A closer look…

An even closer look…

leg = thermal link, wire on top

absorber: 1 mm square

doped silicon thermistor (invisible)

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

linear region

turnover, due to self-heating

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 (r2/4d2)– 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 (/ > 0.01) detector from = 200 m – 1 mm.

SHARC II – a camera using bolometers

• SHARC II – Submillimeter High Angular Resolution Camera, 2nd generation

• For the Caltech Submillimeter Observatory• Observing at = 350 m• 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 = net power flow = AT14 – AT0

4 = A[T1

4 – T04]

P

T1 > T0

= 1 = aT0

= 1 = a

AT14

AT04

With one shield layer

• Equilibrium of shield: AT14a + AT0

4a = 2AT2

4

• P = AT24 + AT0

4(1-a) – AT04 =

A([T14 – T0

4]

P

T1

= 1 = aT0

= 1 = a

AT14

T2

= a 1

AT14a

AT14(1-a)

AT24 AT2

4

AT04(1-a)

AT04AT0

4a

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.