angular two point correlation function of nvss catalog · 2015. 5. 8. · nvss catalog national...
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Angular two point correlation function of NVSS catalog
Is the large angle correlation cosmic ?
Songchen
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Motivation
Two-point angular correlation function from the Planck smica map (Copi, Craig J. et al. 2013)
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NVSS catalog
National Radio Astronomy Observatory (NRAO) Very Large Array(VLA) Sky Survey
VLA manly has 4 configurations (A,B,C,D) NVSS catalog use D́ ́and D́nC ́Frequency : 1.4 GHz Sky coverage : dec > -40° Number of sources : nearly 2×106
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Contaminations
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Contaminations
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Contaminations
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The ways to correct
Contamination Methods
Bright sources Mask out with 0.6 degree radius/ pixels
Foreground Cut galactic plane with |b| < X°
Strip effect a. Use only high flux threshold (S > 10mJy)
b. Normalize the number density to the strip mean
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The ways to correct
Contamination Methods
Bright sources Mask out with 0.6 degree radius/ pixels
Foreground Cut galactic plane with |b| < X°
Strip effect a. Use only high flux threshold (S > 10mJy)
b. Normalize the number density to the strip mean
Caution !?
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What other people found
C.Blake, P.G. Ferreira and J.Borrill 2004
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What other people found
J.Xia, M. Viel, C.Baccigalupi,G. Zotti and S.Matarrese 2010
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What other people found
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Dipole measurements
M. Rubart and D.J. Schwarz 2013
C. Blake and J. Wall 2002 High flux threshold (> 15mJy) : The dipole model is a good fit Low flux threshold : Dipole direction move to high declination
To subtract dipole correctly, we choose : S > 15mJy Or S > 25mJy
Our New Mask + Healpix
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Estimator
Angular two point correlation function based on Land & Szalay estimator:
Error:
Considering dipole subtraction,
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What we found
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What we found
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Our Mask
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Mask
1. Neighborhood of bright sources deviate from Poisson distribution
2. High noise pixels
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Mask
1. Neighborhood of bright sources deviate from Poisson distribution
2. High noise pixels
Condon 1998 Theoretical rms noise level at the center of an NVSS snapshot image
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Large scale correlation
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Zoom In
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Angular Power Spectrum
S> 15mJy Our Mask
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Suspicion
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Conclusion
The large scale correlation signal (l=4) is approximately 1
magnitude bigger than the Lambda CDM prediction
We highly suspect that this signal is caused by the survey noise, instead of large scale structures
The predicted large scale correlation signal is at 10-6 level. To detect it , one needs at least 107 sources.
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Thank you for your attention !
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Mask
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Mask
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Mask