Intrinsic and extrinsic galaxy alignment
We show with analytic models that the assumption of uncorrelated intrinsic ellipticities of target sources that is usually made in searches for weak gravitational lensing arising from large-scale mass inhomogeneities ('field lensing') is unwarranted. If the orientation of the galaxy image is determined either by the angular momentum or by the shape of the halo in which it forms, then the image should be aligned preferentially with the component of the tidal gravitational field perpendicular to the line of sight. Long-range correlations in the tidal field will thus lead to long-range ellipticity–ellipticity correlations that mimic the shear correlations arising from weak gravitational lensing. We calculate the ellipticity–ellipticity correlation expected if halo shapes determine the observed galaxy shape, and we discuss uncertainties (which are still considerable) in the predicted amplitude of this correlation. The ellipticity–ellipticity correlation induced by angular momenta should be smaller. We consider several methods for discriminating between the weak-lensing (extrinsic) and intrinsic correlations, including the use of redshift information. An ellipticity–tidal-field correlation also implies the existence of an alignment of images of galaxies near clusters. Although the intrinsic alignment may complicate the interpretation of field-lensing results, it is inherently interesting as it may shed light on galaxy formation as well as on structure formation.
Additional Information© 2001 RAS. Accepted 2000 October 11. Received 2000 October 2; in original form 2000 May 31. We thank D. Bacon, R. Croft, R. Ellis, A. Heavens, M. Metzger, C. Metzler, A. Refregier, M. Santos and M. White for discussions, and we thank especially U.-L. Pen and M. Zaldarriaga for identifying an error in an earlier draft. This work was supported in part by NSF AST-9900866, AST-0096023, NASA NAG5-8506, and DoE DE-FG03-92-ER40701.
Published - L7.full.pdf
Submitted - 0005470