Beyond the Local Group -- I: Cosmic Filaments Govern the Spatial Alignments of Galaxy Pairs

TL;DR

This work tests whether LG-like galaxy pairs preferentially align with their local cosmic filaments, using SDSS data and a Bisous filament catalog to measure the angle between the pair connection and the local filament orientation. The fiducial sample shows a significant alignment with a mean angle of $\langle\theta\rangle=53.4^\circ$ versus an isotropic $57.3^\circ$, with about $10\sigma$ significance and a $7\%$ excess of pairs below the Local Group benchmark angle $39.4^\circ$. The alignment strength declines with distance to the filament spine: $\langle\theta\rangle$ drops to $22.14^\circ$ for $d_{ m fila}\le0.2$ Mpc and approaches the random value beyond $\sim1$ Mpc, with a notable bimodal distribution near the core indicating potentially distinct dynamical populations. These results support theoretical models of anisotropic accretion and tidal torques along filaments in $\Lambda$CDM and position the MW–M31 system as a representative filament-aligned pair, while highlighting the need for kinematic data and simulations to fully interpret the observed core bimodality and environmental dependence.

Abstract

Using a large observational sample from the Sloan Digital Sky Survey, we investigate the spatial alignment between galaxy pairs and their local cosmic filaments. Focusing on pairs with stellar masses and separations comparable to the Milky Way-Andromeda (MW-M31) system, we measure the angle between the pair connecting line and the orientation of the host filament, determined using a filament catalog constructed via the Bisous model. Our analysis reveals a statistically significant tendency of galaxy pairs to align their connecting lines along their host filaments, manifesting as an overall $\sim$7\% excess of alignment angles smaller than the MW-M31 case compared to a random distribution. Crucially, the strength of this alignment exhibits a strong dependence on the distance to the filament spine. Pairs located within 0.2 Mpc from the filament spine show the strongest alignment, while those beyond 1 Mpc display no significant alignment. Furthermore, we identify a bimodal distribution of alignment angles near filament cores, suggesting distinct dynamical populations potentially associated with infall and interaction processes. Our results provide robust observational support for theoretical models in which anisotropic accretion and tidal forces within the cosmic web drive galaxy pair evolution. They also position the MW-M31 system as a representative filament-aligned pair, offering insights into Local Group assembly. This study demonstrates the cosmic web's critical role in dictating pair orientations and motivates future work with kinematic data to unravel galaxy-environment interplay.

Figures (6)

• Figure 1: Illustration of the alignment angles between a galaxy pair and the local cosmic filament. The two circles represent the galaxies in the pair, connected by the pair connecting line. The arrows at each galaxy indicate the local filament orientations. The angles $\theta_1$ and $\theta_2$ denote the alignment angles between the pair connecting line and the filament orientations at each galaxy.
• Figure 2: Distributions of alignment angles for galaxy pairs of fiducial sample. The three panels display histograms of the angle between the galaxy pair connecting line and the filament orientation at the first galaxy (left), the angle between the pair connecting line and the filament orientation at the second galaxy (middle) , and the angle between the filament orientations at the two galaxies (right). All angles are restricted to the range 0$^\circ$ to 90$^\circ$. For a random distribution of orientations in three-dimensional space, the expected mean angle is approximately 57.3$^\circ$. The black solid lines in the left and middle panels indicate the mean alignment angle distribution expected from random trials, where the pair orientations are randomized while keeping the filament orientations fixed.
• Figure 3: Cumulative distribution functions (CDFs) of the alignment angles between galaxy pair connecting lines and their corresponding local filament orientations. The blue solid line shows the observed CDF for selected 3,087 LG-like galaxy pairs in SDSS. The blue shaded region denotes the 95% bootstrap confidence band by resampling the galaxy-pair sample with replacement 1000 times. The gray lines represent CDFs from 1,000 random realizations where pair connecting line are randomized but filament orientations are kept fixed, illustrating the null hypothesis of no alignment. The black dashed line is the mean CDF of these random samples. The vertical red dashed line marks the angle of $39.4^{\circ}$, corresponding to the MW-M31 pair in the local Universe. Blue and black dots indicate the fraction of observed and random pairs with alignment angles smaller than MW-M31 case, respectively.
• Figure 4: Histogram showing the distribution of the mean distances of galaxy pairs to their nearest filament spine. The x-axis represents the mean pair-filament distance, $d_{\rm fila}$, in megaparsecs (Mpc). The distribution peaks at approximately 0.4 Mpc and exhibits a long tail toward larger distances, indicating that most galaxy pairs reside close to filament.
• Figure 5: Similar as Fig. \ref{['fig:fig_all']}, but show the effect of distance ($d_{\rm fila}$) between galaxies pairs to filament spine. The blue curve shows the CDF for pairs with $d_{\rm fila} \leq 0.2$ Mpc, the red for $d_{\rm fila} \leq 0.5$ Mpc, the green for $d_{\rm fila} \leq 1.0$ Mpc, and the orange for $d_{\rm fila} \geq 1.0$ Mpc. The colored shaded regions denote the corresponding 95% bootstrap confidence bands for each curve, respectively. The legend lists the number of galaxy pairs in each subsample, and the confidence levels relative to randomized control samples, respectively. The 1,000 gray lines represent CDFs from random realizations, and the black solid line is the mean CDF of these random samples. The fraction of each subsample with alignment angles smaller than the MW-M31 case, namely $39.4^{\circ}$ shown in vertical red dashed line, is indicated by the corresponding points and annotated text, respectively.