The various contaminants to the SZ effect

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Lecture 3

• The various contaminants to the SZ effect
• Telescopes and how they deal with the various
  issues
• Single dishes (radiometers and bolometers)
• Examples (OCRA)
• Interferometers
• Examples (AMiBA)
• Future prospects (Planck etc)

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SZ Practicalities

• SZ is a tiny signal - requires sophisticated
  observing techniques
• Various sources of contamination and confusion,
  which observing techniques deal with in different
  ways
• Radio sources (galaxies, planets)
• Atmospheric emission, ground emission
• Primordial CMB fluctuations

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Radio Sources

• If a radio source is present in the field of a
  galaxy cluster, it will fill in the SZ
  decrement
• This could be true for sources in front of /
  behind the cluster, or indeed member galaxies
• Problem greater at low frequency most sources
  are steep spectrum
• Can choose to observe clusters with no sources -
  introduce bias
• Better to subtract effects
• No high-freq. radio surveys - further complication

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Atmosphere, Ground

• Atmosphere is warm - radiates.
• Time variable emission
• Ground also a source of thermal emission
• Varies with pointing angle or telescope
• Can minimise this using a ground shield
• Various ways exist of dealing with these
  contaminant signals

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Primordial CMB

• Primordial anisotropies look remarkably similar
  to the SZ effect on large angular scales (tens of
  arcminutes)
• Seem unsurprising that telescopes such as the VSA
  and CBI (built to observe the primordial CMB)
  suffer drastically from this type of
  contamination....
• .....We were still surprised!

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SZ Telescopes

• 3 main types of instrument
• Single dishes. May have bolometric or
  radiometric receivers. Receiver arrays are
  becoming more common.
• Interferometers - synthesise a large telescope
  using an array of small dishes
• Each has its own advantages and disadvantages
  when it comes to dealing with sources of
  contamination and confusion

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Single Dishes
OVRO 40m
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Simplest Case

• Measure signal received by single beam of solid
  angle ??
• Provides one pixel of information
• Records signal from the sky plus parasitic
  signals from the ground and atmosphere
• PG(TskyTgroundTatmosphere)
• Signals from ground and atmosphere are time
  variable

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Improvement Beam Switching

• Receiver with 2 beams (horn feeds), A and B
• Beam A positioned coincident with target
  (cluster) so measures
• PA Cluster Ground Atmosphere
• Beam B observes blank sky
• PB Ground Atmosphere
• Differenced signal measures cluster signal only.
• PA - PB Cluster

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Problems...

• Only spend half observing time on target, but
  uniform signals (ground, atmosphere, CMB) are
  subtracted
• Configuration is asymmetrical. No problem for
  the atmosphere, put potentially fails to remove
  ground emission
• Also susceptible to differences in receiver gain
  and beamshape
• Need revised strategy....

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ImprovementPosition Switching

• Start with A on target, B on blank sky. Then
  swap
• Switch in azimuth - comparable volumes of
  atmosphere
• Differenced signal twice cluster signal
• Better at removing ground emission, but beware
  radio sources!

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Extensions

• Array receivers
• Multiple detectors offer increased sensitivity
  and imaging capabilities
• Can apply similar, or more complex, switching
• LEAD - MAIN - TRAIL
• Observe background fields either side of field
  containing cluster
• Offers further subtraction possibilities

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Single Dish Pros Cons

• Large dishes are very sensitive, particularly
  when fitted with array receivers
• Array receivers required for imaging
• Have to employ switching schemes to suppress
  systematics
• Some limitations on cluster observability due to
  angular size vs switching strategy
• Difficult to deal with radio sources - may have
  to avoid cluster centres

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Detectors

• Radiometers
• Incident radiation produces a voltage
• Difficult to build at high freq., limited
  bandwidth
• Bolometers
• Thermal - measure temperature increase
• Sensitive to all wavelengths - often used at high
  frequency, and for multi-frequency instruments
• Technically challenging - require excellent
  cooling

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Example OCRA

• Torun 32-m telescope, Poland. 30GHz radiometric
  detectors
• Currently, 2-beam receiver - prototype for focal
  plane array (OCRA-p)
• Observed 4 well-known clusters (Lancaster et al
  2007) to verify strategy
• Now observing sensible sample of 18 clusters

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4 clusters Results
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18 clusters work in progress
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18 clusters work in progress
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Example OCRA

• Next phase, OCRA-F, under construction.
• 16 beam (8xOCRA-p)
• Has the ability to make SZ images, resolved for
  the first time
• Will also be capable of small blind surveys
• Set to be mounted on the telescope later this year

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OCRA collaboration
The other members of the OCRA team Mark
Birkinshaw, Aziz Alareedh, Peter Wilkinson, Ian
Browne, Stuart Lowe, Michael Peel, Richard
Davis, Richard Battye, Andrzej Kus, Marcin
Gawronski, Roman Feiler, Eugeniusz Pazderski,
Bogna Pazderska
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Bolometers ACBAR

• 16-element bolometer mounted on VIPER
• Requires excellent observing conditions - located
  at the south pole
• Observing frequencies 150, 220 and 275GHz.
• Observed the decrement-null-increment (lecture 1)
• Blind survey complete, analysis underway

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Others worth noting

• Radiometers
• Nobeyama 45-m, Japan. 46 GHz
• OVRO 40-m / 5-m, USA. 30 GHz (Mason et al)
• Bolometers
• CSO 10.4-m (SuZIE), USA. 150, 220, 280, 350GHz
• JCMT 15-m (SCUBA), USA. 350GHz

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Interferometers
Ryle Telescope
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Interferometers

• Traditionally used to synthesise a large
  telescope with an array of small dishes, thus
  increasing resolution
• Useful in this context for suppressing
  systematics - various advantages over single dish
  experiments
• Can deal neatly with radio sources
• Can filter out contaminant signals

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Interferometry
? 2 Dishes, separated by a distance D
? Same resolution as a large dish of
diameter D
? Resolution ?/D
? But only samples this angular scale
Need many baselines for aperture synthesis
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Interferometry

• Measure the product (correlation) of the
  voltages of the two antennas
• A bit like reverse diffraction - the response for
  this interferometer will be cos2 fringes
  (remember Youngs slits?)
• Actually measures the Fourier transform of the
  sky (again, like diffracction)

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Real Interferometer

• In practice, have many pairs of antennas, usually
  referred to as baselines.
• n telescopes give n(n-1)/2 baselines.
• Baseline of length D is sensitive to angular
  scale ??/D (resolution)
• Long baselines sensitive to small scales
• Short baselines sensitive to large scales
• Evaluate response for each baseline in order to
  find response of whole telescope

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Radio Sources

• Short baselines (large angular scales) sensitive
  to SZ (large angular scales) radio sources (all
  angular scales)
• Long baselines (small angular scales) sensitive
  to radio sources (all angular scales)
• Measure source flux on long baselines and
  subtract from short baseline data
• Spatial Filtering - used to good effect by many
  experiments (Ryle, OVRO/BIMA)

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High res...
Low res, after source subtraction...
Colourscale - 30GHz Contours - 1.4GHz
Colourscale - Xrays Contours - SZ
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CMB Anisotropies

• The problem of contamination by primordial CMB
  anisotropies is most relevant for instruments
  working on large angular scales
• e.g. the VSA, also CBI
• Need multi-frequency experiments for spectral
  subtraction
• However, less important on smaller angular scales
  (i.e. only really affects low-redshift samples)

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- 30GHz, longest baseline 3m - Dedicated source
subtractor - Suffers badly with CMB - Some
clusters drowned out
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15 arcmin
Power in primordial anisotropies falls off with
decreasing angular scale
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Interferometer Pros Cons

• Can subtract signals from radio sources via
  spatial filtering
• Lack angular dynamic range - may be difficult to
  produce higher resolution images
• Problem of CMB contamination persists unless
  multi-frequency measurements are available (but
  only a large problem for a few instruments)

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Example - AMiBA
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Example - AMiBA

• Currently 7 60cm antenna, 90GHz, Hawaii.
  Baselines chosen for optimum sensitivity to SZ
  effect. Small enough scales for the primordial
  CMB to be suppressed.
• Radio sources are significantly less problematic
  at this frequency, although we currently have a
  large problem with ground emission....
• Huge potential as a survey instrument - will
  generate extensive cluster catalogues with well
  understood selection functions. Expected to find
  tens of clusters per square degree
• Will eventually also produce detailed images

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AMiBA Maps
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Simulated maps
?M1
?M0.3
All clusters found are too distant to be detected
by current X-ray or Optical telescopes
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AMiBA collaboration
Other team members Paul T.P. Ho, Ming-Tang Chen,
J.H. Proty Wu, Keiichi Umetsu, Mark Birkinshaw,
Chao-Te Li, Guo-Chin Liu, Kai-Yang Lin, Patrick
Koch, Yo-Wei Liao, Hiroaki Nishioka
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Next Generation

• Survey Instruments
• e.g. AMI, AMiBA, SZA, ACBAR, SPT, APEX
• Improved imaging / surveying
• e.g. OCRA, AzTEC

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The next big thing.... Planck

• CMB experiment - primordial anisotropies
  including polarisation
• Will solve cosmological paradigms
• Will also detect many thousands of SZ clusters -
  less deep than other studies but will survey the
  entire sky
• Launch.....2007?.....2008?...........

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Summary 3

• Measuring the SZ effect is difficult. Requires
  specialist techniques to eliminate various
  sources of contamination and confusion
• Two basic types of instrument - single dishes and
  interferometers. Single dish detectors may be
  radiometers or bolometers
• Plethora of survey instruments under
  construction. Will yield vast cluster
  catalogues and revolutionise the field.

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Exam hints!

• Short question
• You get to take notes in. There are not many
  equations. Write them down.
• Have a think about potential calculations based
  on the notes.
• Long question
• Students usually do far better on this part
• You will receive more marks if you can quote
  examples.