Correlations between multiple tracers of the cosmic web

Friday, Tracey (2020) Correlations between multiple tracers of the cosmic web. Doctoral thesis, University of Central Lancashire.

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The large-scale structure of the Universe is a `cosmic web' of interconnected clusters, filaments, and sheets of matter. This work comprises two complementary projects investigating the cosmic web using correlations between three different tracers: the cosmic microwave background (CMB), supernovae (SNe), and large quasar groups (LQGs). In the first project we re-analyse the apparent correlation between CMB temperature and SNe redshift reported by Yershov, Orlov and Raikov. In the second we investigate for the first time whether LQGs exhibit coherent alignment.
Project 1: The cosmic web leaves a detectable imprint on CMB temperature. Evidence presented by Yershov, Orlov and Raikov showed that the WMAP/Planck CMB pixel-temperatures at supernovae (SNe) locations tend to increase with increasing redshift. They suggest this could be caused by the Integrated Sachs-Wolfe effect and/or by residual foreground contamination.
We assess the correlation independently using Planck 2015 SMICA R2.01 data and, following Yershov et al., a sample of 2,783 SNe from the Sternberg Astronomical Institute. Our analysis supports the prima facie existence of the correlation but attributes it to a composite selection bias caused by the chance alignment of seven deep survey fields with CMB hotspots. These seven fields contain 9.2% of the SNe sample (256 SNe). Spearman's rank-order correlation coefficient indicates the correlation present in the whole sample (p-value = 6.7 x 10-9) is insignificant for a sub-sample of the seven fields together (p-value = 0.2) and entirely absent for the remainder of the SNe (p-value = 0.6). We demonstrate the temperature and redshift biases of these deep fields, and estimate the likelihood of their falling on CMB hotspots by chance is  6.8% – 8.9% (approximately 1 in 11 – 15). We show that a sample of 7,880 SNe from the Open Supernova Catalogue exhibits the same effect, and conclude that it is an accidental but not unlikely selection bias.
Project 2: The cosmic web influences structure formation and evolution; galaxy and quasar spins correlate with their host large-scale structures. We investigate for the first time whether the LQGs hosting the quasars themselves exhibit coherent alignment. We use the position angle (PA) of 71 LQGs in the redshift range 1.0  z  1.8 as the tracer here. We find that their PAs do not follow a random distribution (p-values 0.008  p  0.07). Their distribution is bimodal with modes at the centre of the A1 region of   52 ± 2, 137 ± 3. The median location of the modes at all 71 LQG locations is   45 ± 2, 136 ± 2. We find that the LQG PAs are correlated, specifically aligned and orthogonal, with a maximum significance of  0.8% (2.4 ) at typical angular (comoving) separations of  30 ( 1.6 Gpc).
This finding suggests an explanation for the Gpc-scale coherent orientation of quasar polarization, first reported by Hutsemékers in 1998, and since widely studied. Our results are remarkably close to the radio polarization angles of   42, 131 reported by Pelgrims and Hutsemékers. The origin of these correlations is still an open question, but is often attributed to quasars' intrinsic alignment. More recently, evidence has emerged that quasar polarization vectors are preferentially parallel and orthogonal to LQG axes. Our results integrate these two findings.
The statistical significance of our results is marginal, and we cannot exclude the LQG correlation being a chance anomaly. However, spatial coincidence between our LQG sample and regions of quasar polarization alignment, and the similarity between LQG PAs and radio polarization angles, suggest an interesting result. The tendency of LQGs to be either aligned or orthogonal warrants further investigation, and we suggest future work to evaluate this intriguing correlation further.

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