Galaxy orientation with the cosmic web across cosmic time

Abstract: This work investigates the alignment of galactic spins with the cosmic web across cosmic time using the cosmological hydrodynamical simulation Horizon-AGN. The cosmic web structure is extracted via the persistent skeleton as implemented in the DISPERSE algorithm. It is found that the spin of low-mass galaxies is more likely to be aligned with the filaments of the cosmic web and to lie within the plane of the walls while more massive galaxies tend to have a spin perpendicular to the axis of the filaments and to the walls. The mass transition is detected with a significance of 9 sigmas. This galactic alignment is consistent with the alignment of the spin of dark haloes found in pure dark matter simulations and with predictions from (anisotropic) tidal torque theory. However, unlike haloes, the alignment of low-mass galaxies is weak and disappears at low redshifts while the orthogonal spin orientation of massive galaxies is strong and increases with time, probably as a result of mergers. At fixed mass, alignments are correlated with galaxy morphology: the high-redshift alignment is dominated by spiral galaxies while elliptical centrals are mainly responsible for the perpendicular signal. These predictions for spin alignments with respect to cosmic filaments and unprecendently walls are successfully compared with existing observations. The alignment of the shape of galaxies with the different components of the cosmic web is also investigated. A coherent and stronger signal is found in terms of shape at high mass. The two regimes probed in this work induce competing galactic alignment signals for weak lensing, with opposite redshift and luminosity evolution. Understanding the details of these intrinsic alignments will be key to exploit future major cosmic shear surveys like Euclid or LSST.

Spin and Shape Alignments of Galaxies with the Cosmic Web

The study conducted by Codis et al. delves into the complex interactions between galaxies and the large-scale structure of the universe, otherwise known as the cosmic web. This research utilizes the Horizon-AGN cosmological hydrodynamical simulation to analyze the orientational dynamics of galaxies within this cosmic environment over time. The cosmic web is characterized by its filaments and walls, structures that guide matter flow and influence the formation and evolution of galaxies. The investigation focuses on understanding the alignment of galaxy spins and shapes with these cosmic structures, a topic of significant interest for both theoretical modeling and observational cosmology.

Methodology and Key Findings

The analysis leverages the DisPerSE algorithm to extract the cosmic web from simulation data, employing the persistent skeleton approach to reliably identify filaments and walls. Galaxy orientations are studied relative to these structures, with a particular emphasis on spin vectors and shapes defined by inertia tensors.

1. Spin Alignments:

   • Galaxies: The simulation reveals a notable transition in spin alignments with the cosmic web. Low-mass galaxies tend to align their spins with the filament axes, especially at high redshifts. Conversely, more massive galaxies exhibit spins perpendicular to the filament direction, a signal that strengthens at lower redshifts. This shift is linked to mass accretion processes, particularly mergers, which redirect the angular momentum of galaxies.
   • Dark Matter Haloes: Similarly, low-mass haloes show spin alignment with filaments, while high-mass haloes align their spins perpendicularly. This transition mass evolves with redshift, aligning with theoretical predictions from tidal torque theory.

2. Shape Alignments:

   • Galaxies: A stronger signal is found in terms of shape alignments, especially for massive galaxies, which are elongated along filaments and within walls. The shape alignments are hypothesized to result from tidal stretching due to cosmic web structures.

3. Influence of Morphology: The findings indicate that galaxy morphology plays a role in spin alignments. Elliptical galaxies predominantly contribute to the perpendicular spin signal, while spirals drive parallel alignment to filaments at high redshift. This behaviour suggests that intrinsic alignments are influenced by both mass and morphological characteristics.

4. Environmental Effects:

   • Centrals and Satellites: The study differentiates between central and satellite galaxies. Central galaxies exhibit stronger spin-filament alignment compared to satellites, whose alignment weakens as they are engulfed by larger halo structures.

Implications

These results underscore the significant impact of the cosmic web on the angular momentum acquisition and morphological evolution of galaxies. Understanding these alignments is crucial for developing accurate models of galaxy formation and evolution. Moreover, the study highlights the potential of leveraging large-scale cosmic structures to mitigate intrinsic alignment contamination in weak lensing surveys—an important consideration for upcoming cosmological experiments like Euclid and LSST.

Furthermore, the findings encourage the integration of environmental dependence into intrinsic alignment models, distinguishing between spin alignments of disk galaxies and shape alignments of ellipticals, each influenced differently by large-scale cosmic structures.

Future Prospects

While the Horizon-AGN simulation provides a robust platform for exploring these processes, further studies incorporating different hydrodynamical codes and varying physical prescriptions would be beneficial to evaluate the impact of baryonic physics on these alignments. Additionally, ongoing and upcoming observational campaigns could offer empirical tests of these theoretical predictions, potentially refining our understanding of the complex interplay between galaxies and their cosmological habitats. The nuanced portrayal of spin and shape alignments presented in this study represents a significant step forward in elucidating the intricate scaffolding upon which galaxies evolve.