Lecture 3

1. 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)

2. 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

3. 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

4. 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

5. 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!

6. 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

7. Single Dishes OVRO 40m

8. 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 P=G(Tsky+Tground+Tatmosphere) • Signals from ground and atmosphere are time variable

9. 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

10. 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....

11. Improvement:Position 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!

12. 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

13. 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

14. 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

15. 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

16. 4 clusters: Results

17. 18 clusters: work in progress

18. 18 clusters: work in progress

19. 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

21. 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

22. 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

23. Interferometers Ryle Telescope

24. 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

25. 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

26. 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 Young’s slits?) • Actually measures the Fourier transform of the sky (again, like diffracction):

27. 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

28. 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)

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

30. 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)

31. - 30GHz, longest baseline 3m - Dedicated source subtractor - Suffers badly with CMB - Some clusters ‘drowned out’

32. 15 arcmin Power in primordial anisotropies falls off with decreasing angular scale

33. 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)

34. Example - AMiBA

35. 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

36. AMiBA Maps

37. Simulated maps M=1 M=0.3 All clusters found are too distant to be detected by current X-ray or Optical telescopes

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

39. 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?...........

40. 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.