Intrinsic/Extrinsic Density-Ellipticity Correlations and Galaxy-Galaxy Lensing

Abstract

We compute both extrinsic (lensing) and intrinsic contributions to the (galaxy-)density-ellipticity correlation function, the latter done using current analytic theories of tidal alignment. The gravitational lensing contribution has two components: one is analogous to galaxy-galaxy lensing and the other arises from magnification bias -- that gravitational lensing induces a modulation of the galaxy density as well as ellipticity. On the other hand, the intrinsic alignment contribution vanishes, even after taking into account source clustering corrections, which suggests the density-ellipticity correlation might be an interesting diagnostic in differentiating between intrinsic and extrinsic alignments. {\it However}, an important assumption, commonly adopted by current analytic alignment theories, is the Gaussianity of the tidal field. Inevitable non-Gaussian fluctuations from gravitational instability induces a non-zero intrinsic density-ellipticity correlation, which we estimate. We also argue that non-Gaussian contributions to the intrinsic {\it ellipticity-ellipticity} correlation are often non-negligible. This leads to a linear rather than, as is commonly assumed, quadratic scaling with the power spectrum on sufficiently large scales. Finally, we estimate the contribution of intrinsic alignment to low redshift galaxy-galaxy lensing measurements (e.g. SDSS), due to the partial overlap between foreground and background galaxies: the intrinsic contamination is about 10 - 30 % at 10'. Uncertainties in this estimate are discussed.Comment: 14 pages, 5 figures, submitted to Ap