The Laniakea supercluster of galaxies

Abstract: Galaxies congregate in clusters and along filaments, and are missing from large regions referred to as voids. These structures are seen in maps derived from spectroscopic surveys that reveal networks of structure that are interconnected with no clear boundaries. Extended regions with a high concentration of galaxies are called 'superclusters', although this term is not precise. There is, however, another way to analyse the structure. If the distance to each galaxy from Earth is directly measured, then the peculiar velocity can be derived from the subtraction of the mean cosmic expansion, the product of distance times the Hubble constant, from observed velocity. The peculiar velocity is the line-of-sight departure from the cosmic expansion and arises from gravitational perturbations; a map of peculiar velocities can be translated into a map of the distribution of matter. Here we report a map of structure made using a catalogue of peculiar velocities. We find locations where peculiar velocity flows diverge, as water does at watershed divides, and we trace the surface of divergent points that surrounds us. Within the volume enclosed by this surface, the motions of galaxies are inward after removal of the mean cosmic expansion and long range flows. We define a supercluster to be the volume within such a surface, and so we are defining the extent of our home supercluster, which we call Laniakea. A key component of this paper is an accompanying movie that can be viewed (also in 3D) and downloaded at http://irfu.cea.fr/laniakea or http://vimeo.com/pomarede/laniakea

• The paper presents a refined method to delineate supercluster boundaries using peculiar velocity data instead of traditional redshift surveys.
• It employs the Wiener Filter algorithm and Bayesian inference on a dataset of over 8,000 galaxies to accurately reconstruct cosmic density and velocity fields.
• The study redefines the Laniakea Supercluster as a coherent region where galaxy flows converge toward major clusters like Virgo and the Great Attractor.

The Laniakea Supercluster: Redefining Our Cosmic Neighborhood

The study "The Laniakea Supercluster of Galaxies" by Tully et al. presents a refined methodology for delineating the structure of superclusters in the cosmos, particularly focusing on what they term the "Laniakea Supercluster." The authors provide a nuanced perspective of cosmic structure beyond traditional redshift surveys, leveraging peculiar velocity measurements to map out this vast galactic assembly.

Peculiar Velocities and Cosmic Structure

The central aspect of this work is the utilization of peculiar velocities to map the distribution of matter in the universe. Peculiar velocities are derived by subtracting the mean cosmic expansion from observed velocities, offering insights into gravitational perturbations and matter distribution. This approach contrasts with traditional redshift surveys, which require assumptions about the correlation between luminous and dark matter. Peculiar velocity studies shift the focus to more directly observable galaxy motions, although they are challenged by measurement uncertainties and require robust statistical and analytical tools, such as the Wiener Filter algorithm employed here.

Definition and Implications of the Laniakea Supercluster

The authors redefine a supercluster not as a random network of galaxies but as a region bounded by the convergence of galaxy flows. This method reveals the Laniakea Supercluster, encompassing several known clusters such as the Great Attractor and the Virgo Cluster within a volume of approximately 160 Mpc in diameter. This redefinition implies a more coherent view of supercluster structures, with boundaries set by dynamic interactions rather than arbitrary cutoffs in the galactic density.

By employing a reconstruction based on a comprehensive dataset of over 8,000 galaxies within the Cosmicflows-2 compilation, they reveal that local flows within Laniakea tend toward the Norma and Centaurus clusters, resembling water dividing at watershed boundaries. This basin-of-attraction approach aligns with the standard model of cosmology, reaffirming the gravitational instability framework while providing an empirical basis for mapping cosmic flow patterns.

Methodology and Analytical Framework

The study employs the Wiener Filter, a tool that alleviates some of the systematic errors and noise inherent in velocity data, to reconstruct the density and velocity fields on scales ranging from several Mpc to hundreds of Mpc. This method proves effective in regions where data is sparse, suggesting that cosmic flows reveal hidden structures, even those obscured by the Milky Way's galactic plane. Their approach is rigorous, utilizing Bayesian inference underpinned by a standard cosmological model, allowing for a coherent structure that integrates both local density-driven flows and broader tidal influences.

Theoretical and Practical Implications

This research posits significant implications for future studies of cosmic structures. First, it highlights the potential of peculiar velocity studies to complement and, in some cases, surpass traditional redshift surveys. The coherence and scale of Laniakea suggest a larger interconnectedness of cosmic structures than previously understood, raising questions about the interactions between local attractors and distant structures, such as the Shapley Concentration.

It also reaffirms the importance of comprehensive and accurate distance and velocity data, as peculiar velocities offer a unique lens through which to view the underlying matter distribution. More extensive datasets could further refine the mapping of cosmic flows and enhance our understanding of universe-scale gravitational dynamics.

Conclusion

The delineation of the Laniakea Supercluster represents a significant advancement in our conceptual and empirical understanding of superclusters. By employing peculiar velocities and advanced statistical methodologies, Tully et al. provide an insightful model that redefines how superclusters are identified and understood. Future refinements, assisted by more detailed cosmic flow data, could elucidate even larger structures in the universe, presenting a more interconnected cosmic web and advancing theoretical models of galaxy formation and evolution.