Halo Spin Depends on The Distance to Cosmic Filament

We employ a semi-analytical methodology to estimate the dark matter halo spin of HI-rich galaxies in the Arecibo Legacy Fast Alfa Survey and investigate the relationship between halo spin and the proximity of galaxies to cosmic filaments. We exclude galaxies with low HI signal-to-noise ratios, those potentially influenced by velocity dispersions, and those affiliated with galaxy clusters/groups. Additionally, we apply a mass-weighting technique to ensure consistent mass distribution across galaxy samples at varying distances from filaments. Our analysis reveals, for the first time, a subtle yet statistically significant correlation between halo spin and filament distance in observational data, indicating higher spins closer to filaments. This suggests that the tidal forces exerted by filaments may impact the spin of dark matter halos.

💡 Research Summary

In this work the authors investigate whether the spin of dark‑matter halos, as inferred from the kinematics of HI‑rich galaxies, depends on the distance of those galaxies to the nearest cosmic filament. The study uses the Arecibo Legacy Fast ALFA (ALFALFA) survey, which provides HI spectra for roughly 31 500 extragalactic sources. From this parent catalogue the authors construct a clean sample of 3 280 galaxies by applying several stringent cuts: (i) HI signal‑to‑noise ratio ≥ 10, (ii) inclination angle derived from the optical axis ratio ≥ 50°, (iii) double‑horned HI line profiles (kurtosis > −1.0) to exclude dispersion‑dominated systems, and (iv) removal of all galaxies belonging to groups or clusters (group richness N = 1). These selections aim to minimise contamination from low‑quality kinematics, projection effects, and external tidal fields.

The rotation velocity V_rot for each galaxy is estimated from the HI line width (W_50) and the inclination angle (φ) via V_rot = W_50 / (2 sin φ). The authors adopt different intrinsic thickness parameters (q₀) for low‑mass (M_* < 10⁹ M_⊙) and higher‑mass galaxies to improve the inclination correction. Assuming an isothermal sphere halo and a thin exponential HI disc, the halo spin parameter λ_h is approximated by the semi‑analytic expression λ_h ≈ R_HI,d / V_rot^{3/2}. The disc scale length R_HI,d is derived from the observed relation between HI mass (M_HI) and the radius at which the HI surface density drops to 1 M_⊙ pc⁻² (r_HI), namely log r_HI = 0.51 log M_HI − 3.59. Combining this with the exponential surface‑density model yields R_HI,d for each object.

To quantify the large‑scale environment, the authors cross‑match their galaxies with a filament catalogue built from SDSS data using the Bisous algorithm. The distance to the nearest filament spine (d_gf) is taken as the environmental metric. Galaxies are split into three bins: d_gf ≤ 1 Mpc h⁻¹ (inside the filament), 1 < d_gf ≤ 3 Mpc h⁻¹ (filament outskirts), and d_gf > 3 Mpc h⁻¹ (field). Because halo spin correlates with stellar mass, the authors enforce comparable stellar‑mass distributions across the three bins. They do this by defining the d_gf ≤ 1 Mpc h⁻¹ bin as a reference, computing mass‑dependent weights w_x(M_) = f_ref(M_) / f_x(M_*), and then randomly resampling each bin according to these weights (the “mass‑weighting control”). After this procedure the stellar‑mass histograms of the three subsamples are statistically indistinguishable.

The main result is that the median spin values (expressed as log λ_h) differ systematically with filament distance: −2.16 ± 0.01 for the closest bin, −2.19 ± 0.01 for the intermediate bin, and −2.20 ± 0.01 for the farthest bin. The difference between the innermost and outermost bins is significant at roughly the 4σ level, and two‑sample Kolmogorov–Smirnov tests yield p‑values ≲ 10⁻⁴, confirming that the spin distributions are distinct. The authors explore possible systematic effects. First, they model the misalignment between optical and HI inclinations as a Gaussian with σ ≈ 20°, perform Monte‑Carlo resampling, and find that the spin‑distance trend persists, indicating that inclination errors do not erase the signal. Second, they incorporate the known scatter between gas spin and halo spin from hydrodynamical simulations (mean ratio 1.27, σ = 0.40) by defining a “practical” spin λ_h,p = λ_h / f_λ, where f_λ follows the quoted Gaussian. Even after this correction the trend remains, albeit slightly weakened, reinforcing the robustness of the observational finding.

The discussion acknowledges that the filament catalogue captures primarily ~2 Mpc‑thick backbones and may miss thinner tendrils or split massive filaments into multiple spines. Such incompleteness would add random noise, making the observed correlation a conservative estimate. Conversely, true massive filaments likely exert stronger tidal torques, suggesting that the real dependence could be even steeper. The authors also note that the semi‑analytic spin estimator, while widely used, carries uncertainties for individual galaxies; however, random errors tend to increase scatter and therefore dilute, not create, correlations. Their results are consistent with N‑body and hydrodynamical simulations that predict higher halo spins in regions of stronger tidal fields, especially near filament spines.

In summary, the paper provides the first observational evidence that dark‑matter halo spin, inferred from HI kinematics, is modestly but significantly higher for galaxies residing closer to cosmic filaments. By carefully controlling for stellar mass, removing cluster members, and testing for inclination and gas‑halo spin scatter, the authors demonstrate that large‑scale tidal fields associated with filaments can imprint measurable angular‑momentum signatures on galaxy halos. This work bridges the gap between theoretical predictions of tidal‑torque theory and observational data, and it sets the stage for future studies employing higher‑resolution HI surveys (e.g., WALLABY, SKA) and more refined filament‑finding algorithms to further elucidate the role of the cosmic web in shaping galaxy angular momentum.