Comparison between the Blue and the Red Galaxy Alignments Detected in the Sloan Digital Sky Survey

TL;DR

This study measures and compares the intrinsic alignments of blue and red galaxies in SDSS DR6, revealing different correlation behaviors and scales, which are important for weak lensing and structure formation understanding.

Contribution

It provides the first detailed comparison of blue and red galaxy alignments, showing distinct scaling relations and correlation scales for each galaxy type.

Findings

Red galaxies show linear correlation scaling up to 6 Mpc/h.

Blue galaxies exhibit quadratic scaling with correlation only at small scales.

Systematic errors are negligible in the correlation measurements.

Abstract

We measure the intrinsic alignments of the blue and the red galaxies separately by analyzing the spectroscopic data of the Sloan Digital Sky Survey Data Release 6 (SDSS DR6). For both samples of the red and the blue galaxies with axial ratios of b/a <= 0.8, we detect a 3 sigma signal of the ellipticity correlation in the redshift range of 0 <= z <= 0.4 for r-band absolute (model) magnitude cut of M_r <= -19.2 (no K correction). We note a difference in the strength and the distance scale for the red and the blue galaxy correlation eta_{2D}(r): For the bright blue galaxies, it behaves as a quadratic scaling of the linear density correlation of xi(r) as eta_{2D}(r) proportional to xi^{2}(r) with strong signal detected only at small distance bin of r <= 3 Mpc/h. While for the bright red galaxies it follows a linear scaling as eta_{2D}(r) proportional to xi(r) with signals detected at larger distance out to r~6 Mpc/h. We also test whether the detected correlation signal is intrinsic or spurious by quantifying the systematic error and find that the effect of the systematic error on the ellipticity correlation is negligible. It is finally concluded that our results will be useful for the weak lensing measurements as well as the understanding of the large scale structure formation.