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The EVLA and SKA pathfinder surveys

The EVLA and SKA pathfinder surveys. Jim Condon. NRAO, Charlottesville. The EVLA recycles VLA infrastructure. The VLA is a 1960’s design 27 x 25-meter VLA parabaloids Off-axis Cassegrain optics for > 1 GHz Many km of railroad tracks: 4 scaled arrays, 1‒ 36 km

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The EVLA and SKA pathfinder surveys

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  1. The EVLA and SKA pathfinder surveys • Jim Condon • NRAO, Charlottesville

  2. The EVLA recycles VLA infrastructure • The VLA is a 1960’s design • 27 x 25-meter VLA parabaloids • Off-axis Cassegrain optics for > 1 GHz • Many km of railroad tracks: • 4 scaled arrays, 1‒ 36 km • Selectable resolution and brightness sensitivity • Good imaging with nearly natural weighting • Multiwavelength spectra at constant angular resolution Leiden 2011 Feb 24

  3. The EVLA recycles VLA infrastructure • VLA was designed in the 1960’s • Advantages: • Already exists and paid for! • Works well 1 to 50 GHz • Sensitive and flexible • Disadvantages: • Small field of view • Many competing users Leiden 2011 Feb 24

  4. What’s new? 1. Receivers and feeds for continuous coverage from 1 to 50 GHz Feed Heaters X C L S Ku K Ka Q

  5. What’s new? 2. The ‘WIDAR’ correlator • The key element of the EVLA is its `WIDAR’ correlator – a 10 petaflop computer • Major capabilities: • 8 GHz/polarization maximum instantaneous bandwidth • Spectral dynamic range up to 58 dB • Extensive special modes • 64 independently tunable full polarization ‘spectral windows’, each of which effectively forms an independent ‘sub-correlator’ Leiden 2011 Feb 24

  6. Summary of wideband coverage: • For dual (RR,LL) polarization, with recirculation • For full polarization, spectral channels are 2 x wider. * With 10 second averaging Leiden 2011 Feb 24

  7. EVLA sensitivity • The table gives the 1-hour, 1-s point-source sensitivities, in continuum and line (1 km/sec)

  8. How can the EVLA contribute? The EVLA already exists, so it can be used now to develop imaging techniques on real data: • Wideband imaging • High-fidelity imaging The EVLA can support other surveys with • Reference fields • High-frequency survey? • CO spectroscopy • Continuum spectral indices • High angular resolution and position accuracy

  9. Wideband imaging • The EVLA will often be operating in ‘wideband’ mode, with large fractional bandwidths: Dn/n ~ 1. • Source structures change significantly over this range – cannot simply grid all channels on one transform plane. • The primary telescope beam scales with l-1 – introducing a ‘false’ spectral index. • Deconvolving all background sources – even beyond the main beam – is necessary to achieve full sensitivity. Leiden 2011 Feb 24

  10. High-fidelity imaging • The super-linear design of the EVLA should enable much higher DR imaging than before, so DR will be limited at virtually all bands by various primary-beam issues: • Systematic pointing offsets • Unsteady tracking • Noncircular beams (alt-az mount telescopes) • Polarization beams (special case of above). Leiden 2011 Feb 24

  11. 3C147 deep field @ 1440 MHz • Detailed testing underway with 12-station, full polarization, 4 subband initial configuration. • Most demanding testing is at L-band, short spacings. • 12 antennas, 110 MHz, 6 hours integration • Best image so far: 400,000:1 DR for 3C147. (850,000:1 to noise in corners). • Some artifacts visible – due to noncircular primary beams. • RMS noise in corners ~ 1 millionth of the peak. First Null 50% power Leiden 2011 Feb 24

  12. Smearing and W-Term errors Errors due to PB sidelobes Errors due to PB pointing 3C147: Residual errors in the full field Leiden 2011 Feb 24 • 3C147 field at L-Band • with the EVLA • Only 12 antennas used • Bandwidth: 128 MHz • ~7 hr. integration • Dynamic range: ~700,000:1

  13. High-Fidelity Imaging – Cygnus A • Cygnus A, observed with WIDAR last year at X band (3.6 cm), self-calibrated and imaged with the best modern methods, provided an image: • No better than that which Rick Perley made 25 years ago with the VLA • A noise level 1000 times the thermal noise! Leiden 2011 Feb 24

  14. What Went Wrong? • Image of the calibrator for Cygnus A • DR is ~650,000:1 • Noise is 6 mJy (thermal) 0.004% of peak! • What is wrong with Cyg A? • Clearly associated with imaging/self-calibration. • Many ideas – little progress… • Important problem that must be solved. Leiden 2011 Feb 24

  15. CO in submm galaxies Low-excitation molecular gas in active star forming galaxies at z~2, during the ‘epoch of galaxy assembly.’ These observations show more total gas, by a factor 2, than previously derived from higher order transitions and indicate that the gas is extended on scales ~ 16 kpc. They challenge many preconceptions on massive galaxy formation at high z. (Ivison et al 2011, MNRAS, in press). Leiden 2011 Feb 24

  16. Molecular gas in lensed LBGs • The Ka band has performed the first imaging of molecular gas in normal star forming galaxies at high redshift (LBGs) • Two strongly lensed were detected in CO1-0. • The gas masses, gas fractions, moderate CO excitation, and star formation efficiencies are comparable to low-redshift star-forming galaxies Riechers et al., 2011, ApJ,724, L153

  17. CO in the most distant starburst galaxy at z = 5.3 The EVLA and the PdBI found CO emission from the most distant known submm galaxy at z=5.3. These observations show the cold gas that fuels the star formation, implying extreme amounts (>1010 M☉) of dense gas in this forming elliptical galaxy. (Riechers et al. 2010, ApJ, 720, L131)

  18. Evolving Universe GN20 molecule-rich proto-cluster at z=4 (Daddi) CO 2-1 in 3 submm galaxies, all in 256 MHz band 0.3mJy z=4.055 4.051 4.056 0.4mJy 0.7mJy CO2-1 46GHz • SFR ~ 103 Mo/year • Mgas ~ 1011 Mo • Early, clustered massive galaxy formation 1000 km/s Leiden 2011 Feb 24

  19. EVLA as SKA pathfinder: counts of very faint sources stars Local visibility function ϕ(L) = L5/2ρ(L) yields counts normalized by static Euclidean counts ∆ log(L) ~ 1.2 evolution for all radio sources <z> ~ 0.8, ∆ log(z) ~ 0.5 over wide flux range (more sensitive survey ≠ “deeper” survey) AGN stars AGN

  20. Simple model counts ≈ Wilner et al. (2008)

  21. The EVLA as an SKA pathfinder:deeper knowledge through confusion

  22. Ultimate limits for radio continuum surveys • confusion: c~ .03 K at 1.4 GHz) instrumental natural • field of view Ω • dynamic range • noise 

  23. Summary: Examples of possible EVLA contributions • CO J=1-0 spectra of high-z galaxies, clusters, and quasars • Continuum radio spectra of individual sources and fields • All-sky 6 cm NVSS for spectra of stronger (more luminous) AGN • Sky density of faint sources • “Reference” field with higher resolution and sensitivity than EMU/WODAN surveys Leiden 2011 Feb 24

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