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Satellite Galaxies are Radially Aligned Toward their Hosts

We use the fourth data release of the Sloan Digital Sky Survey to investigate the orientations of 4327 satellite galaxies with respect to their hosts. The orientation of the satellites is inconsistent with a random distribution at the 99.96% confidence level, and the satellites show a preference for radial alignment toward their hosts. Further, on scales < 50 kpc the major axes of the host galaxies and their satellites are preferentially aligned with each other. Phrased in the terminology of weak lensing, the images of the satellites have a mean shear of γ T = −0.030 ± 0.007, averaged over scales 10 kpc ≤ r P ≤ 70 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 account for between 10% and 15% of the objects that are identified as sources. In such studies, the radial alignment of the satellites will cause a reduction of the galaxy–galaxy lensing shear by of order 25% to 40%. Hence, the radial alignment of satellite galaxies toward their hosts is a potentially important effect for precision studies of galaxy–galaxy lensing, and argues strongly in favor of the use of accurate photometric redshifts in order to identify lenses and sources in future studies.