KiDS+VIKING-450 and DES-Y1 combined: Mitigating baryon feedback uncertainty with COSEBIs

TL;DR

This work demonstrates that COSEBIs can robustly extract cosmic shear information from KV450 and DES-Y1 by cleanly separating E/B modes on finite angular ranges and by being comparatively insensitive to baryon feedback at small scales. The authors show that including small-scale information with COSEBIs tightens constraints on $S_8$ and $\,Ω_m$, yielding $S_8=0.755^{+0.019}_{-0.021}$ for a flat $Λ$CDM model, in ~3.2σ tension with Planck. The analysis compares COSEBIs with traditional 2PCFs, discusses different redshift calibration setups, and confirms that COSEBIs mitigate baryon-model uncertainties while maintaining compatibility with prior cosmic shear results. A publicly available COSEBI pipeline integrated with CosmoSIS is provided to support robust, scalable cosmic shear analyses in future surveys.

Abstract

We present cosmological constraints from a joint cosmic shear analysis of the Kilo-Degree Survey (KV450) and the Dark Energy Survey (DES-Y1), conducted using Complete Orthogonal Sets of E/B-Integrals (COSEBIs). With COSEBIs we isolate any B-modes which have a non-cosmic shear origin and demonstrate the robustness of our cosmological E-mode analysis as no significant B-modes are detected. We highlight how COSEBIs are fairly insensitive to the amplitude of the non-linear matter power spectrum at high $k$-scales, mitigating the uncertain impact of baryon feedback in our analysis. COSEBIs, therefore, allow us to utilise additional small-scale information, improving the DES-Y1 joint constraints on $S_8=σ_8(Ω_{\rm m}/0.3)^{0.5}$ and $Ω_{\rm m}$ by $20\%$. Adopting a flat $Λ$CDM model we find $S_8=0.755^{+0.019}_{-0.021}$, which is in $3.2σ$ tension with the Planck Legacy analysis of the cosmic microwave background.

Figures (7)

• Figure 1: Comparison between $\xi_+$, $\xi_-$ and COSEBIs $E_n$. Left: Integrands are shown for three angular distances, $\theta$, for $\xi_+$, $\xi_-$ and three modes for $E_n$ defined over the $\theta$-range $[0.5', 300']$. All integrands are normalised by their maximum value. Right: Sensitivity of $\xi_\pm$ and $E_n$ to baryon feedback. Three baryon feedback cases from HMCode are shown with different feedback parameters, High: $A_{\rm bar}=1$, Mid: $A_{\rm bar}=2$ and Low: $A_{\rm bar}=3$. All curves are normalised with respect to the dark matter only case with a baryon feedback of $A_{\rm bar}=3.13$. The grey shaded regions show the error bars for KV450. The $\xi_\pm$ errors are shown for the 9 logarithmic bins used in the KV450 analysis. The arrows show which scales were used in the primary KV450 (H20) and DES-Y1 analyses (T18).
• Figure 2: COSEBIs measurements and their expected values for KV450. The E-modes are shown in the upper triangle and the B-modes in the lower triangle. Each panel corresponds to a redshift bin pair as indicated in its corner. Theoretically expected values are shown for the best fitting parameters for KV450 (red dashed) and its combination with DES-Y1 (solid purple). The COSEBI modes are discrete and the points are connected to each other purely for visual purposes. The error on the B-modes is calculated assuming that the only contribution is shape noise, while for the E-modes we take all Gaussian terms as well as the super sample covariance into account (see Appendix\ref{['app:covariance']}). Neighbouring COSEBI modes are correlated and the goodness-of-fit of the model cannot be established by eye (see Table\ref{['tab:bestfit']}).
• Figure 3: COSEBIs measurements and their expected values for DES-Y1. Similar to Fig.\ref{['fig:COSEBIsKV450']}, E and B-modes are shown on the upper and lower triangles, respectively. The redshift bin is indicated in the corner of each panel. Theoretically expected values for the best fitting parameters using DES-Y1 (red dashed) and the joint analysis with KV450 (solid purple) are shown as curves, although COSEBI modes are discrete. Both theoretical models fit the data well (see Table\ref{['tab:bestfit']}). As the data points are correlated the goodness-of-fit cannot be inspected visually.
• Figure 4: Constraints on $S_8$ and ${\Omega_{\rm m}}$. DES-Y1 analysis results with COSEBIs using the T18 setup (magenta), the H20 setup with bpz redshift distributions (cyan) and the H20 setup with DIR calibrated redshift distributions (orange). The grey contours belong to the fiducial analysis in troxel/etal:2018a and the red contours show Planck Legacy results (TT,TE,EE+lowE).
• Figure 5: KV450 analysis results with COSEBIs (green), DES-Y1 results with H20 setup and DIR spectroscopically calibrated redshifts (orange-yellow) and their joint analysis (pink). Red contours show the Planck legacy results for TT,TE,EE+lowE. Constraints on $\sigma_8$ and ${\Omega_{\rm m}}$ are shown in the left panel, while the right panel shows results for $S_8=\sigma_8({\Omega_{\rm m}}/0.3)^{0.5}$ and ${\Omega_{\rm m}}$. KV450 constraints for $S_8=0.737^{+0.036}_{-0.038}$, DES-Y1 $S_8=0.755\pm{0.023}$ and their joint constraint is $S_8=0.755^{+0.019}_{-0.021}$ which is in $3.2\sigma$ tension with the Planck constraints $S_8=0.834\pm 0.016$.