Density‐Ellipticity Correlations, Galaxy‐Galaxy Lensing, and the Importance of Non‐Gaussianity in Intrinsic Alignment

Abstract

We compute both the extrinsic (lensing) and the intrinsic contributionsto the density-ellipticity correlation function, the latter by usingcurrent analytic theories of tidal alignment. The gravitational lensingcontribution has two components, one analogous to galaxy-galaxy lensing,and the other arising from magnification bias: gravitational lensinginduces a modulation of the galaxy density, as well as the ellipticity.On the other hand, the intrinsic alignment contribution vanishes, evenafter we take source-clustering corrections into account, which suggeststhat the density-ellipticity correlation might be an interestingdiagnostic with which to differentiate between intrinsic and extrinsicalignments. However, an important assumption commonly adopted by currentanalytic alignment theories is the Gaussianity of the tidal field. Theinevitable non-Gaussian fluctuations from gravitational instabilityinduce a nonzero intrinsic density-ellipticity correlation, which weestimate. We also argue that non-Gaussian contributions to the intrinsicellipticity-ellipticity correlation are often nonnegligible. This leadsto a linear scaling, rather than a quadratic scaling, as is commonlyassumed, with the power spectrum on sufficiently large scales. Finally,we estimate the contribution of the intrinsic alignment to low-redshiftgalaxy-galaxy lensing measurements (e.g., the SDSS) due to the partialoverlap between foreground and background galaxies, and we find that theintrinsic contamination is about 10%-30% at 10'. Uncertainties in thisestimate are discussed.