Large-Scale Intrinsic Alignment of Galaxy Images

TL;DR

This study measures the intrinsic alignment of galaxy images in SDSS data, finding strong alignment signals on scales up to 10 Mpc and comparing results with theoretical models, revealing insights into galaxy orientation correlations.

Contribution

First large-scale analysis of galaxy image intrinsic alignment using SDSS data, comparing observations with Lambda-CDM models, and examining dependencies on galaxy properties.

Findings

Strong intrinsic alignment detected up to 10 Mpc.

Alignment amplitude matches models when including position angle errors.

More luminous galaxies show slightly stronger alignment.

Abstract

We compute the two-point image correlation function for bright galaxies in the seventh data release of the Sloan Digital Sky Survey (SDSS) over angular scales 0.01' <= \theta <= 120' and projected separations 0.01 Mpc <= r <= 10 Mpc. We restrict our analysis to SDSS galaxies with accurate spectroscopic redshifts, and we find strong evidence for intrinsic alignment of the galaxy images. On scales greater than r ~ 40 kpc, the intrinsic alignment of the SDSS galaxy images compares well with the intrinsic alignment of galaxy images in a Lambda-CDM universe, provided we impose Gaussian-random errors on the position angles of the theoretical galaxies with a dispersion of 25 degrees. Without the inclusion of these errors, the amplitude of the two-point image correlation function for the theoretical galaxies is a factor of ~2 higher than it is for the SDSS galaxies. We interpret this as a combination of modest position angle errors for the SDSS galaxies, as well as a need for modest misalignment of mass and light in the theoretical galaxies. The intrinsic alignment of the SDSS galaxies shows no dependence on the specific star formation rates of the galaxies and, at most, a very weak dependence on the colors and stellar masses of the galaxies. At the ~3-sigma level, however, we find an indication that the images of the most luminous SDSS galaxies are more strongly aligned with each other than are the images of the least luminous SDSS galaxies.