Anisotropic Magnification Distortion of the 3D Galaxy Correlation. 1. Real Space
Jun, 2007
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Abstract: (arXiv)
It is well known that gravitational lensing, primarily via magnification bias, modifies the observed galaxy/quasar clustering. Such discussions have focused on the 2D angular correlation. Here and in a companion paper (Paper II) we show that magnification bias distorts the 3D correlation function and power spectrum in an interesting way: it makes them anisotropic. Magnification bias preferentially enhances the observed correlation in the line-of-sight (LOS) orientation, especially on large scales. For instance, at a LOS separation of ~100 Mpc/h, where the intrinsic galaxy-galaxy correlation is rather weak, the observed correlation can be enhanced by lensing by a factor of a few, even at a modest redshift of z ~ 0.35. This effect presents an interesting opportunity as well as challenge. The opportunity: this lensing anisotropy is distinctive, making it possible to separately measure the galaxy-galaxy, galaxy-magnification and magnification-magnification correlations, without measuring galaxy shapes. The anisotropy is distinguishable from the well known distortion due to peculiar motions, as will be shown in Paper II. The challenge: the magnification distortion of the galaxy correlation must be accounted for in interpreting data as precision improves. For instance, the ~100 Mpc/h baryon acoustic oscillation scale in the correlation function is shifted by up to ~3% in the LOS orientation, and up to ~0.6% in the monopole, depending on the galaxy bias, redshift and number count slope. The corresponding shifts in the inferred Hubble parameter and angular diameter distance, if ignored, could significantly bias measurements of the dark energy equation of state. Lastly, magnification distortion offers a plausible explanation for the well known excess correlations seen in pencil beam surveys.- 95.35.+d
- 98.65.Dx
- 98.80.Es
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