SAGAN-VI: When Jets Meet Filaments -- Environmental Imprints on the Growth of Giant Radio Galaxies

TL;DR

This study tests how the cosmic web’s filamentary environments influence the growth of giant radio galaxies (GRGs) by measuring their 3D distance to filament spines, jet-filament alignment, and lobe asymmetry (ALR), using SDSS-based filament data and LoTSS-era RG catalogues. GRGs and SRGs share similar filament occupancy, but GRGs preferentially align their jets at large angles to filament spines and display stronger lobe asymmetry, suggesting that growth to Mpc scales is facilitated by propagation into low-density, void-facing channels rather than proximity to filaments. The results imply that giant sizes arise from intrinsic fueling and jet dynamics modulated by anisotropic large-scale environments, providing a framework to map the thermodynamic and magnetic state of filaments with RGs. The work lays the groundwork for leveraging future facilities (e.g., SKA, LOFAR-2.0, 4MOST) to extract precise environmental diagnostics from radio galaxies embedded in the cosmic web.

Abstract

Giant radio galaxies (GRGs) represent the largest individual astrophysical structures, rivalling galaxy clusters in physical extent. Understanding how they attain such scales demands examining their large scale cosmic surroundings, particularly the under explored filament environment. We quantify the three dimensional (3D) distance of GRGs from the nearest filament spine; test how this distance correlates with their growth and formation of different morphological classes; assess whether their radio jets exhibit preferred orientations relative to filament axes; and examine how filament anisotropy from spine to periphery modulates radio morphology. We employed a filament catalogue from the SDSS together with the largest GRG catalogue currently available. For each source, we measured the comoving distance to the nearest filament spine, the projected jet spine orientation angle, and quantified lobe asymmetry via the arm length ratio (ALR). These metrics trace proximity, directionality, and the impact of filamentary environment on morphology. We then compared GRGs with a control sample of small radio galaxies (SRGs) to constrain the environmental factors that regulate the attainment of giant sizes. We validated the robustness of our results via bootstrap resampling and non parametric statistical tests. Our results show that GRGs and SRGs have similar filament occupancy. By contrast, GRGs preferentially display larger alignment angles relative to filament spines, while SRG orientations are consistent with a random distribution. GRGs further show enhanced morphological asymmetry, reflected in lower ALR values than SRGs. Attainment of giant sizes is not governed by proximity to filaments; rather, it correlates with jet filament alignment. Abridged.

Figures (11)

• Figure 1: The flowchart outlines the construction of the GRG sample, combining the Mostert2024 GRG catalogue with the filament catalogue of Tempel2014_fil_cat and excluding the sources found in the WH15 cluster catalogue. The "No data" category denotes sources lacking publicly available LoTSS data, preventing reliable morphological classification.
• Figure 2: The figure illustrates the process used to construct the SRG sample used for our analysis.
• Figure 3: The box-and-whisker plots present the distributions of distances from filaments for different morphological classes of GRGs: FRI, FRII, WAT, remnant sources, and complex. The black solid line within a box indicates the median of the distribution, the lower and the upper box boundaries represent the 25th percentile (Q1) and 75th percentile (Q3); therefore, the box shows the interquartile (IQR) range of the data. The whiskers denote the range of the distribution within 1.5 $\times$ IQR, while the circles represent the outliers. As the data are not symmetrically distributed, the whiskers are of unequal length, reflecting the variability above and below the median value. The asymmetry, therefore, indicates skewness or unequal variability in two directions.
• Figure 4: The plot shows cumulative distribution functions (CDFs) of the 3D distances to the nearest filament spine for GRGs (green) and SRGs (red). For each population, the solid curves correspond to sources within 2 Mpc of the spine and the dashed curves to sources within 5 Mpc.
• Figure 5: The figure shows the distribution of jet-filament alignment angles between the radio jet axis and the local filament spine (0$^{\circ}$ = parallel, 90$^{\circ}$ = perpendicular). Panels (a) and (b) present GRGs within 5 Mpc and 2 Mpc of filaments, with median alignments of 59$^{\circ}$ and 65$^{\circ}$, respectively. Panels (c) and (d) show the corresponding SRG distributions, both with a median of 42$^{\circ}$. The red dashed line shows the distribution for the uniformly oriented samples. The error bars denote 95% bootstrap (1000 samples) confidence intervals for the mean (blue circles) occupancy of the sources in each bin.