The Orientation of Satellite Galaxies: Evidence of Elongation in the Direction of the Host

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Abstract
We use the fourth data release of the Sloan Digital Sky Survey to investigate the orientations of 4289 satellite galaxies with respect to their hosts. The orientation of the satellites is inconsistent with a random distribution at the 99.94% confidence level, and the satellites show a preference for elongation in the direction of their hosts. Further, on scales < 50 kpc the major axes of the host galaxies and their satellites are preferentially aligned. Phrased in the terminology of weak lensing, the images of the satellites have a mean shear of gamma_T = -0.045 +/- 0.010, averaged over scales 10 kpc < r < 50 kpc. In a galaxy-galaxy lensing study where lenses and sources are separated solely on the basis of apparent magnitude, we estimate that on scales < 250 kpc satellite galaxies acount for between 10% and 15% of the objects that are identified as sources. In such studies, elongation of the satellites will cause a reduction of the galaxy-galaxy lensing shear by of order 25% to 40%. Hence, the elongation of satellite galaxies in the direction of their hosts is a potentially important effect for precision studies of galaxy-galaxy lensing and argues strongly in favor favor of the use of accurate photometric redshifts in order to identify lenses and sources in future studies.Comment: ApJ Letters, in press; title change, revised text includes preliminarly analysis of 2dFGRS satellites and additional null test

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