WALLABY Pilot Survey: Characterizing Low Rotation Kinematically Modelled Galaxies

TL;DR

This work tackles the challenge of obtaining reliable kinematic models for low-velocity HI disks in the low-resolution, low-S/N regime. It develops and validates the WALLABY Kinematic Analysis Proto-Pipeline (WKAPP) to generate consistently modeled rotation curves across untargeted WALLABY detections, and benchmarks its performance against other algorithms, identifying a notable rate of false positives below an inclination of about $40^{\,\circ}$? Read as: ~$i\approx40^{\circ}$. Applying the framework to 11 detections with $V_{ m max} \le 50~\mathrm{km\,s^{-1}}$, the study shows that galaxies with $i > 40^{\circ}$ lie within $1$–$2\sigma$ of key scaling relations such as the baryonic Tully–Fisher relation, and that consistent kinematic and photometric inclinations correlate with closer adherence to these relations. Overall, the paper demonstrates both the challenges of low-velocity kinematic modelling and a practical framework for testing modelling codes and assembling a large, well-measured sample from untargeted HI surveys to inform the velocity function and related cosmological constraints.

Abstract

Many of the tensions in cosmological models of the Universe lie in the low mass, low velocity regime. Probing this regime requires a statistically significant sample of galaxies with well measured kinematics and robustly measured uncertainties. WALLABY, as a wide area, untargetted HI survey is well positioned to construct this sample. As a first step towards this goal we develop a framework for testing kinematic modelling codes in the low resolution, low $S/N$, low rotation velocity regime. We find that the WALLABY Kinematic Analysis Proto-Pipeline (WKAPP) is remarkably successful at modelling these galaxies when compared to other algorithms, but, even in idealized tests, there are a significant fraction of false positives found below inclinations of $\approx 40^{\circ}$. We further examine the 11 detections with rotation velocities below $50~\kms$ in the WALLABY pilot data releases. We find that those galaxies with inclinations above $40^{\circ}$ lie within $1-2~σ$ of structural scaling relations that require reliable rotation velocity measurements, such as the baryonic Tully Fisher relation. Moreover, the subset that have consistent kinematic and photometric inclinations tend to lie nearer to the relations than those that have inconsistent inclination measures. This work both demonstrates the challenges faced in low-velocity kinematic modelling, and provides a framework for testing modelling codes as well as constructing a large sample of well measured low rotation models from untargetted surveys.

Figures (1)

• Figure 1: The rotation curves (top panel) and surface density profiles (bottom panel) for the 11 low-velocity kinematic models. In each panel, the gray lines show the entire suite of WALLABY PDR kinematic models.