WALLABY Pilot Survey & ASymba: Comparing HI Detection Asymmetries to the SIMBA Simulation

TL;DR

This study compares atomic hydrogen asymmetries in observed WALLABY galaxy data with simulated data from the SIMBA model, developing a new method to reduce noise and analyzing the significance of asymmetry differences.

Contribution

It introduces a novel Scanline Tracing method for creating mock HI datacubes and provides the first comparison of HI asymmetries between WALLABY observations and SIMBA simulations.

Findings

High 3D asymmetries often indicate interactions or tidal features.

WALLABY shows a higher incidence of large asymmetries than SIMBA, but not significantly.

The statistical power of such comparisons will improve with more data and higher resolution.

Abstract

An avenue for understanding cosmological galaxy formation is to compare morphometric parameters in observations and simulations of galaxy assembly. In this second paper of the ASymba: Asymmetries of HI in SIMBA Galaxies series, we measure atomic gas HI asymmetries in spatially-resolved detections from the untargetted WALLABY survey, and compare them to realizations of WALLABY-like mock samples from the SIMBA cosmological simulations. We develop a Scanline Tracing method to create mock galaxy HI datacubes which minimizes shot noise along the spectral dimension compared to particle-based methods, and therefore spurious asymmetry contributions. We compute 1D and 3D asymmetries for spatially-resolved WALLABY Pilot Survey detections, and find that the highest 3D asymmetries A3D>0.5 stem from interacting systems or detections with strong bridges or tails. We then construct a series of WALLABY-like mock realizations drawn from the SIMBA 50 Mpc simulation volume, and compare their asymmetry distributions. We find that the incidence of high A3D detections is higher in WALLABY than in the SIMBA mocks, but that difference is not statistically significant (p-value = 0.05). The statistical power of quantitative comparisons of asymmetries such as the one presented here will improve as the WALLABY survey progresses, and as simulation volumes and resolutions increase.