2D Bayesian automated tilted-ring fitting of disc galaxies in large H i galaxy surveys: 2dbat

Abstract

We present a novel algorithm based on a Bayesian method for 2D tilted-ring analysis of disk galaxy velocity fields. Compared to the conventional algorithms based on a chi-squared minimisation procedure, this new Bayesian-based algorithm suffers less from local minima of the model parameters even with highly multi-modal posterior distributions. Moreover, the Bayesian analysis, implemented via Markov Chain Monte Carlo (MCMC) sampling, only requires broad ranges of posterior distributions of the parameters, which makes the fitting procedure fully automated. This feature will be essential when performing kinematic analysis on the large number of resolved galaxies expected to be detected in neutral hydrogen (Hi) surveys with the Square Kilometre Array (SKA) and its pathfinders. The socalled '2D Bayesian Automated Tilted-ring fitter' (2dbat) implements Bayesian fits of 2D tilted-ring models in order to derive rotation curves of galaxies. We explore 2dbat performance on (a) artificial Hi data cubes built based on representative rotation curves of intermediate-mass and massive spiral galaxies, and (b) Australia Telescope Compact Array (ATCA) Hi data from the Local Volume Hi Survey (LVHIS). We find that 2dbat works best for well-resolved galaxies with intermediate inclinations (20 • < i < 70 • ), complementing three-dimensional techniques better suited to modelling inclined galaxies.

WALLABY – an SKA Pathfinder H i survey

“…H I is an ubiquitous tracer of large-scale structure in the Universe (e.g., Koribalski et al 2004;Meyer et al 2004;Springob et al 2005;Papastergis et al 2013;Kleiner et al 2017;Hong et al 2019), unimpeded by foreground dust and stars (Staveley-Smith et al 2016). It is also an excellent tracer of the total mass in rotating disk galaxies (e.g., Bosma 1981aBosma , 1981bde Blok et al 2008;Ott et al 2012;Oh et al 2018), the gas content of galaxies, and the gaseous structures in between galaxies. H I is an excellent probe of galaxy environments, allowing us to investigate the faint outer discs of galaxies, extended tidal features (such as tails, bridges, filaments) and dwarf companions (Hibbard et al 2001;Sancisi et al 2008;Heald et al 2011;Serra et al 2012a;Bosma 2017;Koribalski et al 2018).…”

“…Furthermore, the H I kinematics of galaxies, which are typically measured with high accuracy (several km s −1 ), can be used to derive their rotation curves as well as noncircular motions, if present. The shape, amplitude and extent of galaxy rotation curves allow detailed modelling of a galaxy's overall mass distribution as a function of radius (e.g., van Albada et al 1985;Carignan and Freeman 1985;de Blok et al 2008;Oh et al 2008Oh et al , 2018.…”

WALLABY early science − V. ASKAP H i imaging of the Lyon Group of Galaxies 351

“…This makes the fitting subjective, and thus the analysis is usually performed in an interactive manner. Given the large number of resolved galaxies that are expected to be detected in WALLABY, automated softwares such as 2D Bayesian Automated Tilted-ring (2DBAT; Oh et al 2018b) and 3D Fully Automated TiRiFiC (FAT; Kamphuis et al 2015) are being developed.…”

“…It then fits a 2D kinematic model that is constructed using the 2D tilted-ring parameters together with the Einasto halo rotation velocity on the input velocity field. We refer the readers to Oh et al (2018b) for a detailed description of 2DBAT. The final fits yield four reasonable rotation curves out of the 14 galaxies.…”

An H i study of the collisional ring galaxy NGC 922

“…The radial variation of the position angle is prominent in the approaching side of the observed velocity field and especially on the western side of the H I disc. As an additional check, we used a different package to fit the rotation curve of NGC 922, namely the 2D Bayesian Automated Tilted-ring fitter of disc galaxies in large H I galaxy surveys (2DBAT; Oh et al 2018). This new software uses the Bayesian Markov chain Monte Carlo (MCMC) technique to optimise the tilted-ring fit as opposed to the traditional chi-square minimization procedure in ROTCUR.…”