Galaxy Alignments with Surrounding Structure in the Sloan Digital Sky Survey

TL;DR

This study analyzes galaxy alignments with surrounding large-scale structures using SDSS data, revealing significant parallel alignments for red galaxies but no such trend for blue galaxies, across various luminosities and densities.

Contribution

It provides the first detailed statistical analysis of galaxy alignment dependence on color, luminosity, and environment using SDSS data.

Findings

Red galaxies show significant parallel alignment with surrounding structures.

Luminous red galaxies have stronger alignment than fainter ones.

Blue galaxies show no significant alignment regardless of luminosity.

Abstract

Using data from the Sloan Digital Sky Survey (SDSS) Legacy Survey, we study the alignment of luminous galaxies with spectroscopic data with the surrounding larger-scale structure as defined by galaxies with only photometric data. We find that galaxies from the red sequence have a statistically significant tendency for their apparent long axes to align parallel to the projected surrounding structure. Red galaxies more luminous than the median of our sample ($M_r < -21.78$) have a mean alignment angle $\langle \Phi \rangle < 45^{\circ}$, indicating preferred parallel alignment, at a significance level $>4.5 \sigma$ on projected scales $0.1\,\mathrm{Mpc} < r_p \leq 7.5\,\mathrm{Mpc}$. Fainter red galaxies have $\langle \Phi \rangle < 45^{\circ}$ at a significance level $>4.3\sigma$ at scales $1\,\mathrm{Mpc} < r_p < 3\,\mathrm{Mpc}$. At a projected scale $r_p = 3.0\,\mathrm{Mpc}$, the mean alignment angle decreases steadily with increasing luminosity for red galaxies with $M_r \lesssim -22.5$, reaching $\langle \Phi \rangle = 40.49^{\circ} \pm 0.56^{\circ}$ for the most luminous one percent ($M_r \sim -23.57$). Galaxies from the blue sequence show no statistically significant tendency for their axes to align with larger-scale structure, regardless of galaxy luminosity. Galaxies in higher-density regions do not show a statistically significant difference in mean alignment angle from galaxies in lower-density regions; this holds true for the faint blue, luminous blue, faint red, and luminous red subsets.