Galaxy flows within 8,000 km/s from Numerical Action methods (2201.12315v1)

Abstract: The trajectories since z=4 of systems of galaxies (`halos') with cz < 8,000 km/s are found through Numerical Action reconstructions. A set of 9,719 halos from a 2MASS group catalog and Cosmicflows-3 catalogs are given attention. Present distances are adjusted to minimize departures from observed redshifts. For those with the most precisely determined distances, compromises are made between distance and redshift agreement. $H_0$ is varied from 69 to 77 km s$^{-1}$ Mpc$^{-1}$ with $\Omega_m$ set by the baryon acoustic oscillation constraint from the Planck Satellite. A best fitting amplitude of the mass-to-light relation is found. A uniform density associated with the interhalo medium accounts for the matter not in halos. The solution paths provide the histories of the formation of the nearby large structures and depict how the voids emptied. Assuming no local over/underdensity, the best model has $H_0=73$ km s$^{-1}$ Mpc$^{-1}$ with nearly the same density arising from interhalo matter (IHM) as from halos. We examine local over/underdensities by varying the IHM density and find a valley of best fit models along $H_0 = 73.0 (1 + 0.165\delta)$ km s$^{-1}$ Mpc$^{-1}$. Friedmann models with distinct densities internal and external to the study region give a similar relationship. The fraction of matter in the IHM seen in n-body simulations roughly matches that in our $H_0=72$ scenario. Videos have been created to visualize the complexities of formation of large-scale structures. Standard n-body calculations starting from the first time-steps as tests of the NAM solutions, and continue until cosmic scale factor $a=2$ provide glimpses into the future.

• The paper uses numerical action methods to reconstruct 9,719 galaxy halo trajectories, optimizing the Hubble constant to 73 km/s/Mpc for best observational fit.
• The paper adjusts the mass-to-light ratio from 43 to 21.9, improving calibration of both luminous halos and the interhalo medium in the mass distribution.
• The paper demonstrates that including non-luminous matter and exploring density variations refines our understanding of local cosmic structure and galaxy flows.

Overview of "Galaxy Flows within 8,000 km/s from Numerical Action Methods"

This paper explores the reconstruction of galaxy trajectories with redshifts less than 8,000 km/s using numerical action methods (NAM). Specifically, the paper examines the motion of 9,719 galaxy halos obtained from the 2MASS group catalog and Cosmicflows-3 catalogs, aiming to accurately model peculiar velocities in the nearby universe. These motion models are calibrated by minimizing deviations between observed distances and redshifts, adjusting the Hubble constant $H_0$ between 69 and 77 km/s/Mpc, with constraints on $\Omega_m$ from Planck's baryon acoustic oscillations.

Key Findings

1. Model Calibration: The paper utilizes NAM to reconstruct galaxy velocities and positions, aiming for the best agreement with observational data by adjusting $H_0$. A local Hubble constant of $H_0 = 73 \text{ km/s/Mpc}$ is found optimal under the assumption of similar contributions to the overall density from both halos and an interhalo medium (IHM).
2. Mass-to-Light Ratio Adjustments: A critical aspect of the methodology includes calibrating the mass-to-light ratio, which was originally set at 43 for $H_0 = 100 \text{ km/s/Mpc}$. This paper identifies that a ratio of $M/L_K = 21.9$ is more consistent with an optimal $H_0$ of 73.
3. Matter Distribution: Galaxy flows were modeled with matter distribution divided into luminous halos and a non-luminous interhalo medium, helping to account for the total estimated density of matter. The paper's results are consistent with simulations suggesting significant mass exists outside traditional galaxy halos.
4. Implications of Over/Underdensity: The paper explores scenarios with different density configurations ($\delta$) and finds corridors of minimal $\chi^2$ for a range of hypothesized local over/underdensities. This exploration shows that the observed cosmic flow could suggest we live near a local underdensity unless rectified by a higher Hubble constant.
5. Structure and Dynamics: Observations reveal the Laniakea Supercluster and its dynamic influence over nearby cosmic structures. The paper also projects potential future developments of these structures over the next Hubble time, providing a nuanced view of large-scale structure dynamics during vast cosmic time scales.

Contributions to Cosmology

This research aids in refining models of the local cosmic web, enhancing our grasp over peculiar velocities in cosmology. It points out the necessity of accommodating non-linear dynamics in modeling such motions. Additionally, the paper underscores the need for incorporating dark matter distributed in filaments and voids beyond standard galaxy-bound halos.

Future Directions

The findings suggest further examination of the role of environment (e.g., voids and clusters) in modulating galaxy motion, with enhanced data from deep surveys offering better spatial coverage and precision in the zone of obscuration. Additionally, the integration of higher-fidelity simulations could improve the robustness of NAM models, offering predictions and potential validations for observed large-scale motions.

Through this examination of nearby galaxy expansions and velocities with NAM, the paper builds on and informs theoretical frameworks necessary for a detailed understanding of the universe's velocity field and the broader ΘCDM cosmological model adjustments.