ASET Colloquium; Tata Institute of Fundamental Research, June 10, 2005. The Square Kilometer Array:A global project in Radio Astronomy S. AnanthakrishnanNCRA-TIFR, Pune 411007
Radio astronomy has been crucial in discovering phenomena such as quasars, pulsars, superluminal motion and cosmic microwave background. • Using radio telescopes one canobserve synchrotron radiation, maser emissionas well as bremsstrahlung from thermal gas. • Radio waves penetrate dust / gas which absorbs & scatters in most other wavebands. • They provide Information on cosmic magnetic fields • Radio astronomy techniques provide the highest resolution images in all astronomy
Beck Thus Radio Astronomy provides unique information about the Universe • Non-thermal processes: quasars, radio galaxies, pulsars, masers... • Highest angular resolution: • VLBI • Penetrates dust and gas: Protostars Galactic nuclei • Tracer for Cosmic Magnetic fields
Large radio telescopes make discoveries! • Quasars and radio galaxies • Serendipity - 21cm HI line • Cosmological evolution of radio sources • Cosmic Microwave Background • Jets and super-luminal motions • Dark matter in spiral galaxies • Masers and megamasers - Mass of the blackhole in AGN NGC4258 • Pulsars - Gravitational radiation (pulsar timing) - First extra-solar planetary system
Radio Telescopes • Angular resolving power of a radio telescope is given by ~/D radians, where =wavelength and D is the aperture diameter. • To get arcsec resolutions, radio astronomers need a radio telescope which is a few hundred km in diameter! Since, this is not practical, the principle of radio interferometry is used, which is analogous to Michelson’s Optical interferometry.
The Ooty Radio Telescope Single frequency; = 92 cm
Earth Rotation Synthesis Radio Telescope As the Earth rotates, 2 antennas placed on the earth offer different projected baselines to a radio source in the sky. By combining many such baselines, one can synthesize a large aperture. Each pair of antennas functions like Young’s double slit, multiplying the sky brightness distribution by a sinusoidal response function. Thus, an interferometer measures one Fourier component of the radio image. N
Low cost 45-m diameter dish of GMRT achieved by a unique design ‘SMART’, which is Stretched Mesh Attached to Rope Truss.
SKA is the next major step in long-term advance of radio astronomy sensitivity.. VLA and GMRT are complementary but use 20th century technology. Need technology shift to progress !
~100x sensitivity of VLA ~1 square kilometre collecting area • study local galaxy dynamics in detail • detect galaxies at high redshift in HI and in synchrotron emission The original SKA vision: imaging galaxies in HI with <1” resolution NGC 4151 VLA 18 hours current state-of-the-art HI at 5 arcsec resolution (Image from Mundell et al.)
SKA’s basic specifications follow the original vision Huge change for radioastronomy • Sensitivity:50--100x VLA at same wavelength Brightness sensitivity ~1K • Frequency coverage: ~150 MHz to ~22 GHz Huge advantages for SKA • Field-of-view: >1 square degree • Max. Resolution: <0.1 arcsec to exceed • HST, NGST, ALMA It will become the world’s Premier Imaging Instrument !
…….. Specifications • Multibeam (at lower frequencies) • Need innovative design to reduce cost • International funding unlikely to exceed $1000m • 106 sq metre => $1000 / sq metre • cf VLA $10,000 / sq metre (50GHz) • GMRT $1,000 / sq metre (1GHz) • ATA $2,000/sq metre (11GHz)
Achieving the SKA vision… • Reduce overall cost per m2 of collecting area by a factor ~10 compared to current arrays While… • Maximising flexibility of design And… • Minimising maintenance/running costs Take advantage of massive industrial R&D in fibre optics and electronics industries (“Moore’s Law” to ~2015) for transport and handling of data Develop innovative new concepts for collectors
Phased array concept Basic idea: replace mechanicalpointing & beam forming by electronic means
Multi beams Element antenna pattern Station antenna patterns Synthesized beams 16 12 • Observing teams with their own beams • like particle accelerator, but can have all beams simultaneously 8 4
SKA 2017 VLA Future Sensitivity HST
To achieve this sensitivity we need: • HEMT receivers • wide band, cheap, small and reliable • Can build low noise systems with many elements • Focal plane arrays • Field of view • Interference rejection • adaptive nulling can work in single dishes and arrays • More computing capacity • computing power doubles every 18 months (Moore’s Law) • Software time scales are much longer • it becomes a capital cost !
InP devices 12mm 3mm
Radio Frequency Interference • The Challenge • Sensitivity to increase (100x) • current regulations will be inadequate • Whole of radio spectrum needed • 2% of spectrum is reserved for Radio Astronomy • early Universe studies require “whole” spectrum, but only to “listen”, and only from a few locations. • LEO telecom satellites a new threat • no place on Earth free frominterference from sky • OECD task force on Radio Astronomy
Forte satellite: 131MHz Terrestrial Interference FORTÉ satellite: 131 MHz
source ci Ai Receiver- A/D 8 8 1 1 DBF D/A ci Ai adaptive algorithm spectrum analyser Positioner RFI () () Control Interference excision
Object Oriented Software • AIPS++ • Astronomical Image Processing System • C++, scripting, GUI’s, libraries, toolkits and applications • Designed by a team of astronomers and programmers
SKA proposals • 6 proposals have been presented for the SKA design • - Array of Cylindrical reflectors : Australia • Array of large reflectors : Canada : LAR; • China : KARST • - Planar phased array : Europe : THEA • Array of small dishes : India : PPD; • USA : Hydroform dishes • Costs are between 1- 2 G$ • International comparison: • A modern bridge: 5 G$ • 100 km Highway: 2 G$
Indian Design: 12-m PPD Antenna PPD consists of a central hub of diameter 4 m 24 elastically bent stainless steel tubes with 8 mm wall thickness and 40 mm O.D. an outer circumferential ring to hold the elastically bent radial tubes of 40 mm O.D. an intermediate ring of 40 mm O.D. 103 stations of 9 x 9 PPD 8343 dishes. Frequency up to 8 GHz.
200km An example of a SKA configuration Not a single 1 km square aperture ! a wide range of baselines
Science with the SKA • The Universe in the Dark Ages • redshifted HI • Star formation • epoch of (re-)ionization • Cosmology and Large Scale Structure • Gravitational Lensing • Gamma Ray bursters • AGN - VLBI • Stellar radio astronomy • Pulsars • Solar system • SETI
Evolution of star formation rates • Starburst galaxies e.g. M82 • Radio reveals starburst region through dust • VLBI resolves expanding supernovae • Infer star birth rate from death rate more directly than other means • Calibrate integrated radio continuum SFR at high z • SKA can do this at any redshift Cosmological history of star formation M82 optical M82 VLA+ MERLIN+VLBI
Avery Meiksen Epoch of reionization
Imaging Normal Galaxies at high z…a basic goal of SKA In continuum HI & CO SKA sensitivity radio image of any object seen in other wavebands Not effected by dust obscuration Resolution advantage cf. ALMA, NGST, HST Radiometric redshifts Continuum Neutral Hydrogen H2O CO
Frail & Kulkarni VLA 8GHz Scintillates if < 10 as Calibrate of field SNR’s Only 1 GRB strong enough in 4 years Many days integration InterstellarScintillation
for surveys and transient events in 106 galaxies ! SKA 6cm HST SKA’s 1o field-of-view SKA 20 cm 15 Mpc at z = 2 ALMA
To summarize the present status • Netherlands: LOFAR (phased array R&D) • Canada: Large Adaptive Reflector (flat panels, tethered balloon) • China:KARST (Array of Arecibo's) • US consortium: ATA (300 x 5m dishes, 1-10 GHz) • US NRAO: VLA upgrade - paths to SKA? • Australia: Cylindrical reflector 0.3 - 5 GHz • India:Lower cost dishes with fine meshes.
SKA International Steering Committee • 18 members representing 11 countries • 6 European (UK, Germany, Netherlands, Sweden, Italy, Poland) • 6 United States • 2 Canada • 2 Australia • 1 China • 1 India • MOU signed IAU Manchester August 2000 • New membership requests • Russia, Sth Africa, Japan