The Very Small Array: Latest Results and Future Plans
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The Very Small Array: latest results and future plans Keith Grainge Astrophysics Group, Cavendish Laboratory, Cambridge Rencontres de Moriond, 31 March 2004 OVERVIEW The Very Small Array (VSA) • Current primordial observations • The S-Z effect • Planned upgrades to the instrument • Future science at high ` • Summary • 1 THE VSA CONSORTIUM Cavendish Astrophysics Group Roger Boysen Tony Brown Chris Clementson Mike Crofts Jerry Czeres Roger Dace Keith Grainge (PM) Mike Hobson (PI) Mike Jones Rüdiger Kneissl Katy Lancaster Anthony Lasenby Klaus Maisinger Ian Northrop Guy Pooley Vic Quy Nutan Rajguru Ben Rusholme Richard Saunders Richard Savage Anna Scaife Jack Schofield Paul Scott (Former PI) Clive Shaw Anze Slosar Angela Taylor David Titterington Elizabeth Waldram Brian Wood Jodrell Bank Observatory Colin Baines Richard Battye Eddie Blackhurst Pedro Carreira Kieran Cleary Rod Davies Richard Davis Clive Dickinson Yaser Hafez Mark Polkey Bob Watson Althea Wilkinson Instituto de Astrofísica de Canarias Jose Alberto Rubiño-Martin Carlos Guiterez Rafa Rebolo Lopez Pedro Sosa-Molina Ricardo Genova-Santos 2 THE VERY SMALL ARRAY (VSA) Sited on Observatorio del Teide, Tenerife • 14 antennas 91 baselines • ) Observing frequency ν = 26 36 GHz; bandwidth ∆ν = 1:5 GHz • − Compact configuration: 143 mm horns; ` = 150 800; ∆` 90 (without • − ≈ mosaicing); Sept 2000 – Sept 2001 Extended configuration: 322 mm horns; ` = 300 1800; ∆` 200 (without • − ≈ mosaicing); Sept 2001 – present 3 COMPARISON WITH OTHER CMB EXPERIMENTS Different systematics from “single-dish” experiments: • – negligible pointing error – negligible beam uncertainty Different from other CMB interferometers (DASI, CBI): • – RT survey and dedicated source subtraction 2-element interferometer remove effect of extragalactic point sources ) – tracking elements rather than comounted can apply fringe rate filter to remove unwanted signals. ) 4 MEM VSA MAPS OF CMB ANISOTROPIES Colour scale runs from 200 200 µK; typical thermal noise level 25 µK. • − − − Consistent with Gaussian random field (Savage et al.;Smith et al.) • 5 VSA POWER SPECTRUM OF CMB ANISOTOPIES Combined power spectrum from: • – Two 3-field and one 2-field compact array mosaics (total sky area = 101 sq deg). – Three 7-field and four 3-field extended array mosaics (total sky area = 81 sq deg). – Dickinson et al. (astro-ph0402498) 6 COMPARISON OF VSA AND WMAP VSA 33-field mosaic power spectrum with source residual correction 8000 6000 ) 2 K µ ( π /2 l 4000 +1) C l ( l 2000 200 400 600 800 1000 1200 1400 Multipole, l Consistent calibration – measurement of dipole anisotropy transferred to Jupiter. • 7 8 VSA CLUSTER MAPS A1795 Coma A2142 A478 Primordial CMB contaminates SZ observations. • 9 GAS FRACTION ESTIMATIONS Measure Gas Fraction in Clusters measure Baryon Fraction of Universe ) Standard Method SZ Method – X-ray deprojection + constant T – Fit King model to visibilities ρgas(r) ρgas(r) ) ) – Hydrostatic Equilibrium – Hydrostatic Equilibrium fg ) Mr tot(r) – Different systematics ) ; fg – Will highlight clumping of gas ) Better method combines SZ and Lensing – Lensing total mass ! – SZ gas mass independent of its distribution ! Have weak lensing observations of six VSA clusters. Lancaster et al. in prep. 10 THE FUTURE:THE SUPER-EXTENDED VSA 1 m Paraboloid focus Mirror (moulded carbon fibre) Support struts (carbon fibre tube) Want to measure high-` power spectrum with high sensitivity. • Composite mirror with metallised surface supported by extension of existing struts. • Replace front-end amplifiers with new devices: Tsys = 35K 25 K. • ! Fit a clone of the AMI correlator: ∆ν = 1.5GHZ 6GHz. • ! Factor of 10 increase in observing speed • 11 SCIENCE WITH THE ENHANCED VSA 8000 6000 4000 2000 0 0 1000 2000 3000 8000 6000 4000 2000 0 0 1000 2000 3000 Single binned, predicted performance of enhanced VSA (using known observing efficiency).• Opportunity before 2008 for a high-` and small-∆` total-intensity experiment • beyond WMAP resolution, with low systematics and ability to subtract foregrounds Three clear goals: –• determine detailed structure of CMB power spectrum out to ` 2000 – determine overall power level of CMB power spectrum at ` > 2000' – high-resolution, high-sensitivity imaging of CMB anisotropies 12 Detailed structure of power spectrum out to ` 2000: ' Presence of further acoustic oscillations • Form of damping tail physics of recombination, mechanism for fluctuation damping • ) Form of primordial fluctuation index ns(k) inflation dynamics, test scale-invariance • ) Resolve existing parameter degeneracies, e.g. (Ωm; ΩΛ) and (ns; τ) • Set constraints on variable-α theories • 13 Power level at ` > 2000: 3000 2000 1000 0 500 1000 1500 2000 2500 3000 Investigate excess power observed by CBI to > 10σ • Determine contribution of integrated SZ effect: • – low frequency (30 GHz) observations – dedicated source subtraction – direct mapping rather than differencing If integrated SZ effect strong 2% constraint on σ8 (currently controversial) • ) ± 14 High-resolution, high-sensitivity imaging of CMB anisotropies: Deep 3-field mosaic (5 deg2) 1.5 µK per 5-arcmin beam (3 Planck sensitivity) • ) × Search for small-scale structures such as topological defects (cosmic strings) • Sensitive tests for statistical non-Gaussianity • Mapping SZ effect in nearly massive clusters • Detect signature of primordial magnetic field • 15 SUMMARY The VSA has measured the CMB power spectrum from ` = 150 1600. • − Post-WMAP, there is still a huge amount to be learnt at high `. • – Measure 4th, 5th.... peaks — break degeneracies (∆` critical). – Investigate primordial fluctuation index (nrun?; reject Harrison-Zel’dovich spectrum?) – Investigate CBI excess at ` > 2000. – High-sensitivity searches for topological defects. The VSA can be enhanced to meet these challenges and has the advantage of • excellent rejection of systematics. 16 FRINGE-RATE FILTERING VSA has tracking rather than co-mounted antennas. • can filter out signals that do not come from the part of the sky being observed ) VSA can observe during the day. • VSA makes direct images rather than difference maps. • 17 SOURCE SUBTRACTION FOR THE VSA The problem At 26 36 GHz expect contamination from (variable) extragalactic radio sources • − The solution Survey VSA fields in advance at 15 GHz with Ryle telescope • Simultaneously monitor these sources at 30 GHz with source subtractor 2-element • interferometer while making CMB observations with the main array No need to project out sources • 18.