PHYS 278 Advanced Astronomy

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PHYS 278 Advanced Astronomy
Lecture 8
Astronomy at IR sub-mm wavelengths
Dr.Quentin A Parker
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Infrared observations

• The 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 frequency is usually in the range 10-100Hz
• The need for non-silicon based detectors
  normal CCDs 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 (left) and 2MASS Near-Infrared View of
the Galactic Center Visible image courtesy of
Howard McCallon
Visible, near-infrared (2MASS), and mid-infrared
(ISO) view of the Horsehead Nebula. Image
assembled by Robert Hurt.
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UKIRT The world's largest telescope dedicated
to infrared astronomy
UKIRT is sited in Hawaii near the summit of Mauna
Kea

• 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 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

MICHELE 10-20um imaging and long-slit grating
spectroscopy. Echelle spectroscopy and
imaging/spectro-polarimetry also available.
Michelle is based upon a SiAs 256x256 pixel
array UIST1-5um imaging and long-slit grism
spectroscopy with R1500-3500. Also
cross-dispersed and integral-field spectroscopy,
and imaging- and spectro-polarimetry. Uses an
1024x1024 InSb array CGS41-5um 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. UFTI1-2.5um 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
comparison shows a single survey frame with 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 stars
  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
Images from JCMT www site

• 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
  D15m).
• 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.

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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 100mK 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
• a Far Infrared Absolute Spectrophotometer (FIRAS)
  to compare the spectrum of the cosmic microwave
  background radiation with a precise blackbody,
• a Differential Microwave Radiometer (DMR) to map
  the cosmic radiation sensitively, and
• 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 are
• overplotted with a 2.725K Planck function. The
  data agrees with the model with
• an rms accuracy of 1 part in 20,00

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Black-body radiation
When 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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The entire sky viewed at 60 microns

• 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.

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WMAP Wilkinson Microwave Anisotropy probe
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

• 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?

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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.