Angular ellipticity correlations in a composite alignment model for elliptical and spiral galaxies and inference from weak lensing

Abstract

We investigate a physical, composite alignment model for both spiral and elliptical galaxies and its impact on cosmological parameter estimation from weak lensing for a tomographic survey. Ellipticity correlation functions and angular ellipticity spectra for spiral and elliptical galaxies are derived on the basis of tidal interactions with the cosmic large-scale structure and compared to the tomographic weak lensing signal. We find that elliptical galaxies cause a contribution to the weak-lensing dominated ellipticity correlation on intermediate angular scales between 40 and 400 before that of spiral galaxies dominates on higher multipoles. The predominant term on intermediate scales is the negative cross-correlation between intrinsic alignments and weak gravitational lensing (GI-alignment). We simulate parameter inference from weak gravitational lensing with intrinsic alignments unaccounted; the bias induced by ignoring intrinsic alignments in a survey like Euclid is shown to be several times larger than the statistical error and can lead to faulty conclusions when comparing to other observations. The biases generally point into different directions in parameter space, such that in some cases one can observe a partial cancellation effect. Furthermore, it is shown that the biases increase with the number of tomographic bins used for the parameter estimation process. We quantify this parameter estimation bias in units of the statistical error and compute the loss of Bayesian evidence for a model due to the presence of systematic errors as well as the Kullback-Leibler divergence to quantify the distance between the true model and the wrongly inferred one.

Tightening weak lensing constraints on the ellipticity of galaxy-scale dark matter haloes

“…The good agreement of our measurements with the current model predictions provides an important consistency check for galaxy formation models. This is also of direct relevance for cosmic shear measurements (e.g., Schrabback et al 2010;Heymans et al 2013;Jee et al 2016;Troxel et al 2018;Hamana et al 2020;Asgari et al 2020;Hildebrandt et al 2020), which can be biased low due to shape-shear alignments (e.g., Hirata & Seljak 2004;Joachimi et al 2015;Tugendhat & Schäfer 2018). These are caused by alignments of galaxies with their surrounding dark matter haloes and large-scale structure environments, which shear the images of background galaxies.…”

“…Obtaining observational constraints on f h at the mass scale of galaxies is also of interest in the context of large upcoming cosmological weak gravitational lensing surveys (e.g., Laureijs et al 2011;LSST Science Collaboration 2009). Physical alignments between galaxies and their surrounding large-scale structure introduce shape-shear correlations (e.g., Hirata & Seljak 2004;Joachimi et al 2015), which constitute a major source of systematic uncertainty when constraining cosmological parameters with cosmological weak lensing surveys (e.g., Schäfer & Merkel 2017;Tugendhat & Schäfer 2018, in this context they are also referred to as 'gravitational-intrinsic' (GI) alignments). Their impact must therefore be carefully corrected for using theoretical modelling (e.g., Bridle & King 2007) and calibrations from simulations and observations (e.g., Tenneti et al 2016;Chisari et al 2017;Hilbert et al 2017;Piras et al 2018;Bate et al 2020) in order to not degrade the constraining power of these surveys.…”

Statistical separation of weak gravitational lensing and intrinsic ellipticities based on galaxy colour information

“…It is notable that the shapes of elliptical galaxies and lensing measure tidal field components perpendicular to the line of sight and are proportional to the magnitude of the tidal shears, but that in contrast spiral galaxies reflect with their shapes the tidal field orientation including line of sight-components. For details on the derivation of angular shape correlation functions and E/B-mode ellipticity spectra we refer to Capranico et al (2013), Schaefer & Merkel (2015) and Tugendhat & Schaefer (2018). Contributions to the ellipticity correlations by weak gravitational lensing C γ i j ( ) (grey bands, shown separately from linear structure formation in light grey and nonlinear structure formation in dark grey), C e,GI i j ( ) (green lines), C s,II i j ( ) (blue lines) and C e,II i j ( ) (red lines).…”

Environmental dependence of ellipticity correlation functions of intrinsic alignments

“…The intrinsic shapes of galaxies can be described by the tidal shearing-model [13,22], and the tidaltorquing model [6,7,42,43,46,47] for elliptical and spiral galaxies, respectively. In the tidal shearing-model an elliptical galaxy is virialised system which consists out of stars which are characterized by a constant velocity dispersion.…”

“…The intrinsic shape correlations arise due to the gravitational interaction of the galaxies with the ambient large-scale structure [4-12, for reviews]. Intrinsic alignments have been studied extensively both with analytical methods [see [13][14][15][16] and simulations [e.g. [17][18][19].…”