Pattern Speeds of Bars and Spiral Arms From H-alpha Velocity Fields

TL;DR

This study applies the Tremaine-Weinberg method to H-alpha velocity fields of ten barred spiral galaxies to measure pattern speeds and corotation radii, finding good agreement with photometric and simulation data.

Contribution

It demonstrates that H-alpha emission can reliably be used with the Tremaine-Weinberg method to determine bar pattern speeds in spiral galaxies.

Findings

Corotation radius closely related to bar length.

Pattern speeds consistent with numerical simulations.

Higher velocity dispersion in circumnuclear regions with secondary bars.

Abstract

We have applied the Tremaine-Weinberg method to 10 late-type barred spiral galaxies using data cubes, in H-alpha emission, from the GHAFAS and FANTOMM Fabry-Perot spectrometers. We have combined the derived bar (and/or spiral) pattern speeds with angular frequency plots to measure the corotation radii for the bars in these galaxies. We base our results on a combination of this method with a morphological analysis designed to estimate the corotation radius to bar-length ratio using two independent techniques on archival near infrared images, and although we are aware of the limitation of the application of the Tremaine-Weinberg method using ionised gas observations, we find consistently excellent agreement between bar and spiral arm parameters derived using different methods. In general, the corotation radius, measured using the Tremaine-Weinberg method, is closely related to the bar length, measured independently from photometry and consistent with previous studies. Our corotation/bar-length ratios and pattern speed values are in good agreement with general results from numerical simulations of bars. In systems with identified secondary bars, we measure higher H-alpha velocity dispersion in the circumnuclear regions, whereas in all the other galaxies, we detect flat velocity dispersion profiles. The excellent agreement between the Tremaine-Weinberg method results and the morphological analysis and bar parameters in numerical simulations, suggests that although the H-alpha emitting gas does not obey the continuity equation, it can be used to derive the bar pattern speed.