EOR Detection Strategies Somnath Bharadwaj IIT Kharagpur.
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Transcript of EOR Detection Strategies Somnath Bharadwaj IIT Kharagpur.
EOR Detection Strategies
Somnath Bharadwaj
IIT Kharagpur
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By Liz Pulliam Weston2 easy ways to detect the EOR 21-cm signal
Will be seen in Emission Ts > T
Varies with angle and frequency
Fluctuations caused by:
Variations in the neutral fraction
Fluctuations in gas density (Dark Matter Fluctuations)
Peculiar Velocities
The 21-cm EOR Signal
y
x
Radio Interferometric Arrays
Frequency MHz 153 235 325 610 1420
z 8.3 5.0 3.4 1.3 0
GMRT30 antennas 45m diameter
The 21-cm Signal
y
x
z
Interferometry and Visibilities
/ d
A visibility V(U,) records a single Fourier component of I(with angular wave number 2U
Angular multi-pole
Baseline Distribution
U
Visibility V(U,)
•21-cm Signal
•Noise (inherent to theobservation)
•Foregrounds from other astrophysical Sources
•Man-made RFI
Why bother about Visibilities?
FT
Why not try to detect the signal in the Image?
Noise in different visibilities is uncorrelated
Noise in different pixels of the image is correlated
Imaging Artifacts
Incomplete u-v coverage
The w term
l = cos m = cos
Small Field of View
FT
D
Relative Contributions
2 Easy Ways
Statistical Detection Statistical properties of the 21-cm signal are significantly different from those of foregrounds and noise. Use this to separate out the signal.
Ionized Bubble DetectionDevelop a template based on prior knowledge of
the expected signal. Use this to search for the signal buried in noise and foregrounds. Matched Filter.
Statistical Detection
Multi-frequency angular power spectrum
Jy2
K2
Visibility correlations
The Estimator
U1
U2
D/
V2 (U,)
U < D/
U >> U
Self Correlations avoided
0 ~ D
Begum, A. et al. 2006 MNRAS, 372, L33
500 pc
80 pc
The HI Signal
Bharadwaj, S.& Sethi, S.K. 2001, 22, 293Morales, M. F. & Hewitt, J. 2004, ApJ, 615,7; Bharadwaj, S. & Ali, Sk.S., 2005, MNRAS,356, 1519
10-7 Jy2
Decorrelation
14 hrs GMRT Observations
RA 01 36 46 DEC 41 24 23
153 MHz Observation
5 MHz Bandwidth
62.5 kHz resolution
Primary Beam FWHM ~4 deg.
Synthesized beam 28” x 23”
Noise 1.6 mJy/Beam
Ali, Sk. S. et al. 2008, MNRAS, 385, 2166
Visibility Correlations
Foregrounds
Santos, M.J. et al. 2005, ApJ,625,575; Di Matteo, T. et al., 2002,ApJ, 564,576Zaldarriaga, M. et al., 2004, ApJ, 608, 622
Foreground Removal
• Foregrounds are all continuum sources
• Emissions at ~1 MHz are expected to be highly correlated
• The HI signal decorrelates within 1 MHz
• In the image cube – fit and subtract a smooth polynomial in along each pixel
Jelic, V. et al. 2008, MNRAS, 389, 1319Bowman, J. D. et al. 2009, ApJ, 695, 183
Foreground Removal
• Advantages in working with visibilities
• Grid the visibilities V(U,- 3D grid
• Fit and subtract a smooth polynomial along at each U grid point
Liu, A. 2009, arxiv-0903.4890
McQuinn, M. et al. 2006, ApJ, 653,815; Morales, M.F. et al. 2006, ApJ,648, 767
Foreground Removal
• The foreground contributions to V2 (U,) is predicted to decorrelate less than 1% in the 5 MHz bandwidth of our observation
• The signal contribution decorrelates within 1 MHz
• Fit V2 (U,) with a smooth polynomial in and subtract this out
Measured Decorrelations
Three Visibility Correlation
• Probes the Bispectrum
• Significant as the HI distribution at reionization is expected to be quite non-Gaussian
• Non-zero only if three baselines form a closed triangle
• Decorrelates within
Bharadwaj, S. & Pandey, S.K. 2005, MNRAS, 358, 968
Ionized Bubble Detection
Datta, K.K. et al., 2007, MNRAS, 382, 809
Signal from an HII bubble
70 Jy
Bubble at center of field of view, phase factor and fall in amplitude if shifted
The Problem of Signal Detection
The Contaminants
• We treat the Noise, Foregrounds and the HI Fluctuations outside the bubble as independent random signals
• V(U,)=S(U,) + N(U,) + F(U,) + H(U,)
Matched Filter
V(U,)=S(U,) + N(U,) + F(U,) + H(U,)
The Variance
Dominated by Foregrounds
Exceeds the Signal
Foreground Removal necessary
The Filter
• Remove Foregrounds• Optimize Signal to Noise Ratio
Predictions
Depends on antennasand baseline coverage
Filter effectively removes foreground
Noise reduces withobserving time
HI fluctuations outside the bubble impose a fundamental restriction on the smallest bubble that can be detected
xHI=1
Z=8.5
1000 hrs, > 22 Mpc
2 Easy Ways to Detectthe EOR 21-cm Signal
Thank You