2025MNRAS.538.1816O

We propose a new method for extracting bulk motion gases in the disc of a galaxy from H I data cubes, offering improvements over classical techniques like moment analysis and line profile fitting. Our approach decomposes the line-of-sight velocity profiles into multiple Gaussian components, which are then classified into (underlying and dominant) bulk and non-bulk motion gases based on criteria such as H I surface density, velocity dispersion, kinetic energy, and rotation velocity. A 2D tilted-ring analysis is employed to refine the kinematical parameters of the galaxy disc, ensuring robust extraction of the bulk motion gases. We demonstrate the effectiveness of this method using the H I data cubes of NGC 4559 from the Westerbork Synthesis Radio Telescope Hydrogen Accretion in LOcal GAlaxieS (WSRT-HALOGAS) survey, distinguishing between bulk and non-bulk gas components. From this, we find that approximately 50 per cent of the H I gas in NGC 4559 is classified as non-bulk, possibly linked to processes such as stellar feedback. This work provides a robust framework for analysing H I kinematics of galaxies from high-sensitivity H I observations of galaxies like MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters (MeerKAT-MHONGOOSE) and FAST Extended Atlas of Selected Targets Survey (FAST-FEASTS) and allows us to best exploit the kinematic information of the complex gas dynamics within galaxy discs.