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PHYS 278 Advanced Astronomy

Astronomy at IR & sub-mm wavelengthsAstronomy at IR & sub-mm wavelengths

Lecture 8

Dr.Quentin A Parker

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Infrared observationsThe IR is the only other spectral region apart from the optical and submm that can be partially observed with ground based telescopes and has been relatively well covered in the 1-25μm range.

However, there are additional problems apart from atmospheric absorption with observing in the infrared:

• The presence of a strong varying thermal background including that from the telescope itself! – need to resort to cryogenic cooling of detector instrumentation with liquid Helium

• IR observations are dominated by the need for highly accurate sky background subtraction

• Hence the need for a chopping mirror to rapidly switch between source and background (blank sky) – offset is usually <1arcmin and oscillating frequency is usually in the range 10-100Hz

• The need for non-silicon based detectors – normal CCD’s are not sensitive to such long wavelength radiation

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Why observe in the InfraRed?

Mid and far-infrared observations can only be made by telescopes which can get above our atmosphere. These observations require the use of special cooled detectors containing crystals like germanium whose electrical resistance is very sensitive to heat.

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Optical/IR image comparisons

Visible, near-infrared (2MASS), and mid-infrared (ISO) view of the Horsehead Nebula. Image assembled by Robert Hurt.

Visible (left) and 2MASS Near-Infrared View of the Galactic Center Visible image courtesy of Howard McCallon

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UKIRT: The world's largest telescope dedicated to infrared astronomy

UKIRT is a 3.8m classical cassegrain telescope with a thin primary mirror. It has been the subject of a concerted upgrades programme since 1990.

Image stabilisation to <0."1 (in winds <45 mph) by a fast tip-tilt secondary mirror. Tip-tilt control by CCD Fast Guider, on G or K stars with V > 18.6 in dark conditions. Auto-focus using the fast guider; focus actively maintained using a thermal and

elastic model of the telescope. Active correction of primary figure and alignment by a lookup table. Active and passive dome ventilation and internal air circulation

UKIRT is sited in Hawaii near the summit of Mauna Kea

MICHELE: 10-20um imaging and long-slit grating spectroscopy. Echelle spectroscopy and imaging/spectro-polarimetry also available. Michelle is based upon a Si:As 256x256 pixel arrayUIST:1-5um imaging and long-slit grism spectroscopy with R~1500-3500. Also cross-dispersed and integral-field spectroscopy, and imaging- and spectro-polarimetry. Uses an 1024x1024 InSb arrayCGS4:1-5um long-slit grating spectrometer with R ~ 400-40,000 with a 256x256 InSb array IRCAM/TUFTI: 1-5um camera with 256x256 pixels; pixel scale 0.08". L-band imaging polarimetry also available.UFTI:1-2.5um camera with 1024x1024 HgCdTe; pixel scale 0.09“ and fov 92”. Imaging polarimetry and K-band 400 km/s FP IRPOL: UKIRT's polarimetry module.

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2MASS: 2micron all sky survey in the NIR

Basic 2MASS features:• Two Matching 1.3m Telescopes, optimized for efficient sky coverage • Sited at Mt. Hopkins (USA) and CTIO (Chile) • Equipped with imaging cameras with 3 infrared HgCdTe arrays of size 256 × 256 pixels • Covers the: J (1.25 µm), H (1.65 µm), and Ks (2.17 µm) IR bands • Camera Pixel Size is 2.0" x 2.0" • Tilting secondary motion during declination scanning freezes frames. • 1.3s exposures per image x 6 images per fields = 7.8s total integration time. • Sub-stepping in both the in-scan and cross-scan direction minimizes effects of under-sampling due to large pixels. • High sky coverage capability with a mapping rate of 70 sq. deg./band/night

The 2MASS project is jointly funded by NASA and the National Science Foundation.

There are certain classes of objects -- e.g., brown dwarfs, stars which have too little mass to sustain nuclear burning -- which are expected to emit almost exclusively at near-infrared wavelengths. A census of these objects requires a deep, large-area survey. Surveys naturally detect the nearest, and thus brightest and most easily studied, examples of any class of object.

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The camera field-of-view shifts by approximately 1/6 of a frame in declination from frame-to-frame. The comparison below illustrates the relationship between individual camera frames and survey tiles. The camera images each point on the sky six times for a total integration time of 7.8 seconds. The scan rate (and thus the frame-to-frame declination offset) and array orientation are set so that each of the six images of a given star occur at a different location relative to a pixel center..

This sub-pixel "dithering" improves the ultimate resolution of the co-added images relative to a single undersampled image with 2.0" pixels. The comparisonshows a single survey framewith the final co-added image product

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MM and sub-mm astronomy – the decade of discovery

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Sub-mm astronomy Molecular cloud star-forming regions give rise to centimeter- and

(sub)millimetre-wavelength spectral line emission from numerous molecules and continuous emission from dust grains. Various state-of-the-art telescopes are now available to astronomers for observations of molecules and dust that yield information on densities, temperatures, kinematics, magnetic fields, and chemical abundances in the emitting regions.

Submillimeter receiver development is progressing at a rapid pace. Various groups are building heterodyne receivers for molecular spectroscopy and wideband bolometer systems for continuum mapping.

The late development of ground based sub-mm astronomy was due to the lack of key technologies earlier and the problems imposed by the earths atmosphere (absorption due principally to water vapour, some strong ozone absorption and contaminating sources of background noise

Sub-mm telescopes such as the JCMT on Mauna Kea Hawaii thus need to be at high altitudes to get above as much of the water vapour as possible

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Sub-mm astronomy

Wavelengths of 200microns to 1mm are most sensitive to cold gas and dust, with for example the blackbody emission of a 10 K source peaking at around 300microns.

Such cold material is associated with objects in formation such as the earliest evolutionary stages of galaxies, stars and planets. To understand the origins of these fundamental astronomical objects requires submillimetre observations which can trace molecular (H2) gas clouds in our own or other galaxies by using spectral lines from trace molecules or the continuum thermal emission from dust grains. Continuum observations have the advantage of wide bandwidths and great sensitivity.

Nearly all objects are optically thin, so submm images probe to the heart of crucial processes. Instead of looking at emission from a star’s surface or light scattering off a disk, as in the optical regime, it is possible to look directly at material accreting onto a protostar. Masses and geometries can then be determined in a less model-dependent way than in the optical/infrared.

Some of the coldest phenomena are only seen in the submm: e.g. large-scale gas outflows from young stars.

Emission from dust in primeval galaxies is redshifted into the submm regime

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JCMT The James Clerk Maxwell Telescope (JCMT) is the largest astronomical telescope in

the world designed specifically to operate in the submm wavelength region of the spectrum (dish diameter D~15m).

The JCMT operates in this technically challenging transition region between IR and radio techniques

Equipped with SCUBA a submm camera and a photometer. It has two arrays of bolometric detectors (or pixels); the Long-Wave (LW) array has 37 pixels operating in the 750 and 850 micron atmospheric transmission windows, while the Short-Wave (SW) array has 91 pixels for observations at 350 and 450 microns.

A bolometer is basically a thermometer. It measures changes in heat input from the surroundings converting it into a measurable quantity such as a current or voltage.

Each pixel has diffraction-limited resolution, corresponding to 7.5 arcseconds at 450 microns, and 14 arcseconds at 850 microns arranged as a close-packed hexagon. Both arrays have ~2.3 arc-minute diameter fov and can be used simultaneously (via a dichroic). In addition there are three pixels available for photometry in the transmission windows at 1.1, 1.35 and 2.0 mm, and these are located around the edge of the LW array.

Images from JCMT www site

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SCUBA was a massive advance in two technical areas:

It was designed to have a sensitivity limited by the photon noise from the sky and telescope background at all wavelengths, i.e. achieve background limited performance. This is achieved by cooling bolometric detectors to 100~mK using a dilution refrigerator, while limiting the background power by a combination of single-moded conical feedhorns and narrow-band filters. It was the realisation of the first large-scale array for submillimetre astronomy.This multiplex advantage means that SCUBA can acquire data thousands of times faster than the previous (single-pixel) instrument to the same noise level.

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Optical and sub-mm images of a galaxy cluster

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ALMA: Atacama large mm array

The Atacama Large Millimeter Array (ALMA) is the name for the merger of a number of major millimeter array projects into one global project: the European Large Southern Array (LSA), the U.S. Millimeter Array (MMA), and possibly the Japanese Large Millimeter and Submillimeter Array (LMSA). This will be the largest ground-based astronomy project of the next decade after VLT/VLTI, and, together with the Next Generation Space Telescope (NGST), one of the two major new facilities for world astronomy possibly coming into operation by the end of the next decade.

It will detect and study the earliest and most distant galaxies, the epoch of the first light in the Universe. It will also look deep into the dust-obscured regions where stars are born to examine the details of star and planet formation. In addition to these two main science drivers the array will make major contributions to virtually all fields of astronomical research. ALMA will be comprised of some 64 12-meter sub-millimetre quality antennas, with baselines extending up to 10 km. Its receivers will cover the range from 70 to 900 GHz. It will be located on the high-altitude (5000m) Zona de Chajnantor, east of the village of San Pedro de Atacama in Chile. This is an exceptional site for (sub)millimetre astronomy, possibly unique in the world.

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ALMA THE MOVIE –coming to a dry arid plain near you

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Function of a sub-mm wave radio telescope receiver

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Sub-mm technology

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Millimetre c.f. Optical

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Cosmic microwave background (CMB) radiation

The cosmic microwave background radiation is the light left over from the Big Bang. The whole Universe is bathed in this afterglow light. This is the oldest light in the Universe and has been traveling across the Universe for 14 billion years. The patterns in this light across the sky encode a wealth of details about the history, shape, content, and ultimate fate of the Universe.

It was first observed in 1965 by Arno Penzias and Robert Wilson at the Bell Telephone Laboratories in Murray Hill, New Jersey.

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COBE: Cosmic background Explorer

The COBE satellite was developed by NASA's Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early universe to the limits set by our astrophysical environment.

It was launched November 18, 1989 and carried three instruments:

1) a Far Infrared Absolute Spectrophotometer (FIRAS) to compare the spectrum of the cosmic microwave background radiation with a precise blackbody,

2) a Differential Microwave Radiometer (DMR) to map the cosmic radiation sensitively, and

3) a Diffuse Infrared Background Experiment (DIRBE) to search for the cosmic infrared background radiation.

Each COBE instrument yielded a major cosmological discovery:

N.B. The COBE datasets were developed by the NASA Goddard Space Flight Center under the guidance of the COBE Science Working Group and were provided by the NSSDC.

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Principal cosmological results from COBE

• FIRAS – Showed the cosmic microwave background (CMB) spectrum is that of a nearly perfect blackbody with a temperature of 2.725 +/- 0.002 K. These fluctuations are related to fluctuations in the density of matter in the early universe and thus carry information about the initial conditions for the formation of cosmic structures such as galaxies, clusters, and voids. The observations closely matched the predictions of the standard hot Big Bang model, showing that nearly all of the radiant energy of the Universe was released within the first year after the Big Bang.• FIRAS is a polarizing Michelson interferometer operated differentially with an internal reference blackbody, and calibrated by an external blackbody having an estimated emissivity of better than 0.9999.  It covers the wavelength range from 0.1 to 10 mm in two spectral channels separated at 0.5 mm and has approximately 5% spectral resolution.  A flared horn antenna aligned with the COBE spin axis gives the FIRAS a 7 degree field of view.  The instrument was cooled to 1.5 K to reduce its thermal emission and enable the use of sensitive bolometric detectors.  The FIRAS ceased to operate when the COBE supply of liquid helium was depleted on 21 September 1990, by which time it had surveyed the sky 1.6 times

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COBE

The spectrum of the CMB background from COBE. Measured data points areoverplotted with a 2.725K Planck function. The data agrees with the model withan rms accuracy of 1 part in 20,00

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Black-body radiationWhen the temperature of a blackbody radiator increases, the overall radiated energy increases and the peak of the radiation curve moves to shorter wavelengths. When the maximum is evaluated from the Planck radiation formula, the product of the peak wavelength and the temperature is found to be a constant.

This relationship is called Wien's displacement law and is useful for the determining the temperatures of hot radiant objects such as stars, and indeed for a determination of the temperature of any radiant object whose temperature is far above that of its surroundings.

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DMR

• DMR - The CMB was found to have intrinsic "anisotropy" for the first time, at a level of a part in 100,000. These tiny variations in the intensity of the CMB over the sky show how matter and energy was distributed when the Universe was still very young. Later, through a process still poorly understood, the early structures seen by DMR developed into galaxies, galaxy clusters, and the large scale structures seen today.

• The instrument consists of six differential microwave radiometers, two nearly independent channels that operate at each of three frequencies: 31.5, 53, and 90 GHz.  These frequencies were chosen to minimize the contamination from Galactic emission.  Each differential radiometer measures the difference in power received from two directions in the sky separated by 60 degrees, using a pair of horn antennas. Each antenna has a 7 degree (FWHM) beam.

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We see an S-shaped band of light shown by the red coloration, which if we folded the image around to make it a true globe, would stand out as a narrow band of light. This is the light produced by the warm dust grains that make up the material in the Solar Systems asteroid belt which is called Zodiacal Light. On a clear evening away from city lights, in the twilight sky, you can see the optical light from this interplanetary dust yourself. In the image above, we are seeing not the light of the sun reflected from the dust, but the light from the dust grains themselves as they glow from the heat they receive from the sun. The infrared light from Zodiacal dust is strongest at 12 and 25 microns in the so-called 'thermal' bands for dust at temperatures near 250 K. Astronomers have studied the infrared images of the Zodiacal dust and discovered several parallel bands due to old comets.

The entire sky viewed at 60 microns

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WMAP: Wilkinson Microwave Anisotropy probe

Will the universe expand forever, or will it collapse? Is the universe dominated by exotic dark matter? What is the shape of the universe? How and when did the first galaxies form? Is the expansion of the universe accelerating rather than

decelerating?

The Wilkinson Microwave Anisotropy Probe (WMAP) has mapped of the temperature fluctuations of the CMB radiation with much higher resolution, sensitivity, and accuracy than COBE. The new information contained in these finer fluctuations will shed light on several key questions in cosmology

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Colours indicate "warmer" (red) and "cooler" (blue) spots. The microwave light captured is from 379,000 years after the Big Bang, over 13 billion years ago

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The WMAP Angular power spectrum!

For detailed explanation – refer to paper handout….