Dancing in the dark: galactic properties trace spin swings along the cosmic web

Abstract: A large-scale hydrodynamical cosmological simulation, Horizon-AGN, is used to investigate the alignment between the spin of galaxies and the cosmic filaments above redshift 1.2. The analysis of more than 150 000 galaxies per time step in the redshift range 1.2<z<1.8 with morphological diversity shows that the spin of low-mass blue galaxies is preferentially aligned with their neighbouring filaments, while high-mass red galaxies tend to have a perpendicular spin. The reorientation of the spin of massive galaxies is provided by galaxy mergers, which are significant in their mass build-up. We find that the stellar mass transition from alignment to misalignment happens around 3.10^10 M_sun. Galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as active galactic nuclei is mandatory to quench in situ star formation in massive galaxies and promote various morphologies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping spins misaligned with cosmic filaments.

• The paper shows that low-mass blue galaxies align their spins parallel to filaments while high-mass red galaxies tend to orient perpendicularly.
• The paper employs Horizon-AGN hydrodynamical simulations to identify a transition near 3×10¹⁰ M☉ where mergers drive significant spin reorientations.
• The paper highlights the importance of AGN feedback in shaping galaxy evolution, urging simulations to account for baryonic processes for realistic cosmic alignments.

Galactic Spins and Cosmic Filamentary Alignments

The paper under examination explores the cosmic interplay between the angular momentum of galaxies and the large-scale structure of the universe, specifically focusing on the alignment between galactic spins and cosmic filaments. This investigation utilizes a robust suite of cosmological hydrodynamical simulations, namely the Horizon-AGN simulation, which probes galactic evolution and the role of environment in shaping galaxy properties. The study systematically analyzes the influences of cosmic filaments on the angular momentum vectors of galaxies from redshift 1.2 to 1.8.

Key Findings and Numerical Insights

The study reveals that low-mass blue galaxies have a statistically significant tendency for alignment of their spin axes parallel to nearby cosmic filaments, while high-mass red galaxies are more likely to have perpendicular spin orientations. This dichotomy is crucially influenced by the mass-dependent transition, found to occur at approximately $3 \times 10^{10}\, \text{M}_\odot$. The transition mass is supported by the mass-spin alignment predictions for dark matter halos, providing a consistent narrative across different cosmic entities.

Massive galaxies are shown to undergo significant spin reorientation due to mergers, which dominate their mass assembly histories. The role of mergers as a critical mechanism for altering galactic spin orientations emphasizes the impact of hierarchical galaxy formation processes fueled by cosmic accretion and merger events that utilize angular momentum conversion.

Implications and Theoretical Considerations

The findings challenge the classical tidal torque theory as the sole explanation for spin-filament alignments, highlighting the complexity of baryonic processes, such as feedback from active galactic nuclei (AGN), in shaping galaxy evolution. The requirement of AGN feedback to curtail in situ star formation in massive galaxies suggests the necessity for a comprehensive inclusion of such feedback mechanisms in simulations to achieve realistic galactic diversity.

The implication of these results extends beyond theoretical astrophysics. They offer observational testable predictions regarding the alignment of galactic axes with respect to the filamentary structure of the universe. Understanding this alignment could inform mass accretion histories, galaxy morphologies, and the role of feedback mechanisms in galaxy evolution.

Future Directions

Further research should explore the consistent patterns of spin alignments over longer cosmic periods and with other cosmic structures such as voids and sheets, expanding our understanding of the cosmic web. Additionally, refinement in hydrodynamic simulations to better resolve low-mass galaxies and incorporate diverse feedback processes could provide deeper insight into the alignment phenomena presented.

The exploration of intrinsic alignments within cosmic structures poses opportunities for reducing systematic uncertainties in weak gravitational lensing surveys, which are pivotal for precise cosmological parameter estimations. These insights may subsequently influence future theoretical models and interpretative frameworks concerning the dynamics of large-scale structures and galaxy evolution in the context of the expanding universe.

Overall, this paper illustrates a significant stride in connecting galaxy-scale properties with the vast cosmic architecture, offering nuanced insights into the underlying processes governing galaxy evolution driven by large-scale cosmic dynamics.