Limits on statistical anisotropy from BOSS DR12 galaxies using bipolar spherical harmonics

We measure statistically anisotropic signatures imprinted in three-dimensional galaxy clustering using bipolar spherical harmonics in both Fourier space and configuration space. We then constrain a well-known quadrupolar anisotropy parameter g(2M) in the primordial power spectrum, parametrized by P( k) = (P) over bar( k)[ 1 + Sigma(M) g(2M)Y(2M) ((k) over cap)], with M determining the direction of the anisotropy. Such an anisotropic signal is easily contaminated by artificial asymmetries due to specific survey geometry. We precisely estimate the contaminated signal and finally subtract it from the data. Using the galaxy samples obtained by the Baryon Oscillation Spectroscopic Survey Data Release 12, we find no evidence for violation of statistical isotropy, g(2M) for all M to be of zero within the 2 sigma level. The g(2M)-type anisotropy can originate from the primordial curvature power spectrum involving a directional-dependent modulation g(*)((k) over cap . (p) over cap)(2). The bound on g(2M) is translated into g(*) as -0.09 < g(*) < 0.08 with a 95 per cent confidence level when (p) over cap is marginalized over.