Cosmology with the 21cm line

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Seminar by Dr Gianni Bernardi (SKA) at AIMS, August 2013

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  • PSA-64, 32 antennas in a maximum-redundancy configuration, 30m between rows, 4m spacing within rows
  • Cosmology with the 21cm line

    1. 1. Cosmology with the 21cm line Gianni Bernardi SKA SA (RARG: O. Smirnov, G. Bernardi, T. Grobler, C. Tasse) Collaborators: (LOFAR-EoR, MWA-EoR, PAPER?) AIMS, August 14th 2013
    2. 2. The 21cm line is ideal to study the first billion years Dark Ages: no structures were formed, primordial fluctuations are imprinted in the HI gas Cosmic Dawn: first luminous structures (Pop III stars? Micro quasars?) are formed in the dark matter halos Reionization (EoR): luminous structures (galaxies, AGNs) re- ionize the IGM
    3. 3. mK 21 cm line cosmology
    4. 4. Courtesy A. Meisinger Evolution of fluctuations
    5. 5. Observational specs for 21cm line experiments: Frequency coverage: 30-200 MHz (6 < z < 35) Angular resolution: fluctuations  5 < θ < 30 arcmin  you need a radio interferometer imaging  up to < 1 arcmin  you need a radio interferometer Sensitivity: mK sensitivity is required to constrain most of the HI models (The VLA @ 74 MHz has an rms sensitivity of 26 K (1 hour)) Challenges: - correction of ionospheric distortions - calibration of time and frequency variable telescope response (beam) - subtraction of bright foregrounds (and their coupling with the instrumental response)
    6. 6. Interferometry in 1 slide
    7. 7. 2 Antennas 8 Image formation with N antennas
    8. 8. 3 Antennas 9 Image formation with N antennas
    9. 9. 4 Antennas 10 Image formation with N antennas
    10. 10. 5 Antennas 11 Image formation with N antennas
    11. 11. 6 Antennas 12 Image formation with N antennas
    12. 12. 7 Antennas 13 Image formation with N antennas
    13. 13. 8 Antennas 14 Image formation with N antennas
    14. 14. 8 Antennas x 6 samples 15 Image formation with N antennas
    15. 15. 8 Antennas x 30 samples 16 Image formation with N antennas
    16. 16. 8 Antennas x 60 samples 17 Image formation with N antennas
    17. 17. 8 Antennas x 120 samples 18 Image formation with N antennas
    18. 18. 8 Antennas x 240 samples 19 Image formation with N antennas
    19. 19. 8 Antennas x 480 samples 20 Image formation with N antennas
    20. 20. 21
    21. 21. We live in the era of exploration: current and future 21 cm experiments GMRT LOFAR PAPER MWA HERA - SKA
    22. 22. GB et al. 2009 ~2.3 arcmin resolution frequency: ~150 MHz peak flux ~ 2.8 Jy conversion factor: 1mJy/beam=4 K noise: 0.75 mJy/beam The key point to detect the 21cm signal is how well foregrounds can be removed! What do foregrounds look like?
    23. 23. GB et al. 2009 Statistical properties of foregrounds * 2noise 180 Y l b C X l X l b N N        Power law behavior with best fit amplitude A400= (0.0019 0.0003) K2, and best fit slope βI = -2.2 0.3: diffuse Galactic emission Flat power spectrum: residual point sources Power law behavior with best fit amplitude A700= (90 7) (mK)2 and best fit slope βIp = -1.52 0.16: diffuse polarized Galactic emission
    24. 24. Statistical properties of foregrounds: the “wedge” Pober et al. 2013
    25. 25. Do we know how to subtract foregrounds? How well? • Subtraction of Galactic diffuse emission and extragalactic radio sources: they are supposed to have smooth spectra compared to the 21 cm signal; Bowman, Morales & Hewitt 2009
    26. 26. Do we know how to subtract foregrounds? How well? • Subtraction of Galactic diffuse emission and extragalactic radio sources: they are supposed to have smooth spectra compared to the 21 cm signal; EoR + FG + noise Eor + noise EoR ~ 5 mK FG ~ 2 K noise ~ 50 mK How well does it work on data? Jelic, .., GB, et al. 2008
    27. 27. • An interferometer never samples all the Fourier modes  PSF sidelobes corruption (k┴,k║); • Instrumental frequency response corrupts the foreground frequency smoothness (k║); • Telescope beams change with frequency and pointing direction (dipoles do not track the sky) and they can be different from each other (k┴,k║); • The ionosphere is no longer transparent (time and frequency dependent distortion & refraction) (k┴,k║); • RFI corrupts the sky signal (mostly ,k║); • Real foreground polarized signal can leak into total intensity due to polarized beams a/o imperfect polarization calibration (k║); The point is that instrument calibration and foregrounds are coupled  foreground properties are corrupted by the instrumental response
    28. 28. Sidelobes of bright sources far away from the target field GB et al. 2010
    29. 29. Instrumental spectral response Pober et al. 2013
    30. 30. 3C197.1: ~6.8 Jy Solutions every 10 sec after averaging the visibilities over ~230 channels rms residual: ~9.8 mJy Calibration accuracy: <0.2% 4C+46.17: ~6.2 Jy Solutions every 10 sec after averaging the visibilities over ~230 channels rms residual: ~6.2 mJy Calibration accuracy: <0.2% 6C B075752.1+501806: ~5.8 Jy Solutions every 10 sec after averaging the visibilities over ~230 channels rms residual: ~6.2 mJy Calibration accuracy: ~0.4% Ionospheric distortions GB et al. 2010
    31. 31. Results
    32. 32. Giant Metrewave Radio Telescope (GMRT) • Large collecting area; • Clever calibration strategy (pulsar on–off); • Stable and known beams; • Small field of view; • Severe RFI problems; Paciga et al. 2011, 2013
    33. 33. (Low Frequency ARray) LOFAR • Largest collecting area; • Complex elements (levels of dipole clustering)  element beams are inherently different from each; • Small field of view; • Active RFI environment (mitigated by high time and frequency resolution);
    34. 34. Deep imaging on 3C196 and NCP fields
    35. 35. Murchison Widefield Array (MWA) • Centrally condensed core to maximize power spectrum sensitivity for the EoR (but smaller collecting area); • Large field of view; • Minimum RFI contamination; • Analog signal paths; courtesy A. Offringa
    36. 36. Upper limits on the EoR at z~8.5 from the 32T prototype Dillon, …, GB, et al. 2013 Δk < 0.26 K @ 95% c.l.
    37. 37. Precision Array to Probe the Epoch of Reionization (PAPER) • Maximum redundant configuration (baselines length are equal to each other as much as possible), optimized for EoR power spectrum measurement; • The simplest design (beam stability, smoothness, minimal ionospheric impact); • Very large field of view; • Minimum RFI contamination; • Analog signal paths; • Smallest collecting area;
    38. 38. Upper limits on the EoR from PAPER 32 WSRT (GB et al., 2010) Courtesy J. Pober
    39. 39. What have we learned about reionization from current 21cm measurements? Xi ~ 0.5 The IGM must have been heated by X-rays (MXRBs a/o quasars)
    40. 40. Hydrogen Epoch of Reionization Array: HERA-576 http://reionization.org
    41. 41. Conclusions • The redshifted 21cm line promises to be a fantastic probe of the high-z Universe; • Steady progress towards the first detection of HI at z > 6; • Many challenges still to be overcome (calibration, foreground subtraction) – development required!; • The detection will open up the field for a characterization of the EoR and Dark Ages; • SKA low and HERA looking ahead; • Observations of the global sky signal represent a way to probe the cosmic dawn at z ~ 25-30; • HI intensity mapping!  BAOs at 0.1 < z < 6

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