Testing Photometric and Kinematic Algorithms to predict Next-Generation Telescope Survey Statistics

Abstract

Reconciling the velocity function of dark matter halos in collisionless simulations with those derived observationally requires understanding how galaxies are embedded in their dark matter halos. Widefield neutral atomic hydrogen surveys with Square Kilometre Array pathfinders such as WALLABY on the ASKAP telescope will address this issue since they will resolve the kinematics of representative samples of galaxies beyond their optical disks. These statistical samples afford constructing a velocity function out of resolved galaxies, including those with distorted kinematics that recent hydrodynamical simulations suggest make up a significant fraction of nearby systems. To do so, however, the kinematics of the thousands of poorly resolved galaxies that will comprise the majority of WALLABY detections will need to be modelled using novel techniques, and we will require ancillary photometric inclination estimates. Several new codes have been developed to model kinematics, but few have been tested in the marginally resolved regime. We explore our ability to constrain photometric components using DiskFit and galaxies from the Spitzer Survey of Stellar Structure in Galaxies, determining that we can estimate ellipticity correctly to within δƐ=0.083. Then, using empirical scaling relations for disk galaxies in the local volume, we generate an extensive suite of resolved, WALLABY-like synthetic galaxy observations and use them to test how well galaxy kinematics can be recovered using the Fully Automated TiRiFiC 3D modelling code. We then simulate a mock WALLABY volume of resolved galaxy detections, use our model-fitting results to determine the reliability with which the kinematics of each can be recovered, and predict the resolved galaxy velocity function that WALLABY will measure. Our ability to model galaxies with fewer than five beams across their major axes significantly increases the number of resolved detections that could contribute to the sample. We discuss how the predicted WALLABY velocity function is impacted as the sample size grows to contain these more poorly resolved galaxies, and find that we can recover the velocity function down to 25 km/s, an improvement upon the predicted lower limit of 50 km/s based on the limitations of legacy algorithms.