Intrinsic Ellipticity Correlation of SDSS Luminous Red Galaxies and Misalignment with their Host Dark Matter Halos

TL;DR

This study measures the intrinsic ellipticity correlation of luminous red galaxies from SDSS and compares it with simulations to understand galaxy-halo misalignment, revealing a significant but imperfect alignment that impacts weak lensing measurements.

Contribution

It provides the first detailed measurement of LRG ellipticity correlations and constrains the misalignment angle distribution between galaxies and their dark matter halos.

Findings

Ellipticity correlation is detected up to 30 h^{-1} Mpc.

Central LRGs are preferentially aligned but not perfectly with their halos.

Misalignment angle has a Gaussian distribution with a width of about 35 degrees.

Abstract

We investigate the orientation correlation of giant elliptical galaxies by measuring the intrinsic ellipticity correlation function of 83,773 luminous red galaxies (LRGs) at redshifts 0.16 -- 0.47 from the Sloan Digital Sky Survey. We have accurately determined the correlation up to 30 $h^{-1}$Mpc. Luminosity dependence of the ellipticity correlation is also detected although the error bars are large, while no evidence is found for its redshift evolution between $z=0.2$ and $z=0.4$. Then we use a cosmological $N$-body simulation to examine misalignment between the central LRGs and their parent dark matter halos. Central and satellite galaxies are assigned to simulated halos by employing a halo occupation distribution model for the LRGs. The ellipticity correlation is predicted to have the same shape as but an amplitude about 4 times higher than our observation if the central LRGs are perfectly aligned with their host halos. This indicates that the central LRG galaxies are preferentially but not perfectly aligned with their host halos. With the assumption that there is a misalignment angle between a central LRG and its host halo which follows a Gaussian distribution with a zero mean and a width $\sigma_\theta$, we obtain a tight constraint on the misalignment parameter, $\sigma_\theta={35.4}^{+4.0}_{-3.3}$ deg. This type of intrinsic ellipticity correlation, if not corrected, can lead to contamination at 5% level to the shear power spectrum in weak lensing surveys of limiting magnitude $R_{AB}=24.5$ if the source central galaxies follow the same misalignment distribution as the LRGs.