Cross-correlating WMAP5 with 1.5 million LRGs: a new test for the ISW effect

Sawangwit, U.; Shanks, T.; Cannon, R. D.; Croom, S. M.; Ross, Nicholas P. and Wake, D. A. (2010). Cross-correlating WMAP5 with 1.5 million LRGs: a new test for the ISW effect. Monthly Notices of the Royal Astronomical Society, 402(4) pp. 2228–2244.



We present the cross-correlation of the density map of luminous red galaxies (LRGs) and the temperature fluctuation in the cosmic microwave background (CMB) as measured by the five-year Wilkinson Microwave Anisotropy Probe observations. The LRG samples were extracted from imaging data of the Sloan Digital Sky Survey (SDSS) Data Release 5 based on two previous spectroscopic redshift surveys, the SDSS LRG and the 2dF–SDSS LRG and QSO (2SLAQ) surveys designed to have average redshifts of z ≈ 0.35 and z ≈ 0.55. In addition, we have added a higher redshift photometric LRG sample based on the selection of the AAOmega LRG redshift survey at z ≈ 0.7. The total LRG sample thus comprises 1.5 million galaxies, sampling a redshift range of 0.2 < z<0.9 over ≈7600 deg2 of the sky, probing a total cosmic volume of ≈5.5 h−3 Gpc3.

First, we find that the new LRG sample at z ≈ 0.7 shows very little positive evidence for the Integrated Sachs–Wolfe (ISW) effect. Indeed, the cross-correlation is negative out to ≈1°. The standard Λ cold dark matter (ΛCDM) model is rejected at ≈2–3 per cent significance by the new LRG data. We then analyse the previous samples at z ≈ 0.35 and z ≈ 0.55. As found by other authors, these results appear consistent with the standard ISW model, although the statistical significance remains marginal.We also reproduce the same result for the magnitude-limited SDSS galaxy samples of Giannantonio et al. Taking the z ≈ 0.35 and z ≈ 0.55 LRG results in combination with the new z ≈ 0.7 sample, the overall result is now more consistent with a null detection than with the standard ΛCDM model prediction.

We then performed a new test on the robustness of the LRG ISW detections at z ≈ 0.35 and z ≈ 0.55. We made eight rotations through 360° of the CMB maps with respect to the LRG samples around the galactic pole. We find that in both cases, there are stronger effects at angles other than zero. This implies that the z ≈ 0.35 and z ≈ 0.55 ISW detections may still be subject to systematic errors which combined with the known sizeable statistical errors may leave the z ≈ 0.35 and z ≈ 0.55 ISW detections looking unreliable. We have further made the rotation test on several other samples where ISW detections have been claimed and find that they also show peaks when rotated. We conclude that in the samples we have tested, the ISW effect may be absent and we argue that this result may not be in contradiction with previous results.

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