Kinematic clues to bar evolution for galaxies in the local universe: why the fastest rotating bars are rotating most slowly

TL;DR

This study analyzes barred spiral galaxies to understand how dark matter halos influence bar rotation speeds, revealing that some fast-appearing bars are actually slow rotators due to dark matter effects.

Contribution

It provides the first systematic observational evidence that dark matter halos slow down bar rotation, challenging previous classifications of bar speed based solely on R values.

Findings

Bars with R < 1.4 can be slow rotators in normalized speed.

Dark matter halos significantly slow down bar rotation over Gyr timescales.

Longer bars can be slow rotators due to braking effects.

Abstract

We have used Spitzer images of a sample of 68 barred spiral galaxies in the local universe to make systematic measurements of bar length and bar strength. We combine these with precise determinations of the corotation radii associated with the bars, taken from our previous study which used the phase change from radial inflow to radial outflow of gas at corotation, based on high resolution two-dimensional velocity fields in H{\alpha} taken with a Fabry-P\'erot spectrometer. After presenting the histograms of the derived bar parameters, we study their dependence on the galaxy morphological type and on the total stellar mass of the host galaxy, and then produce a set of parametric plots. These include the bar pattern speed versus bar length, the pattern speed normalized with the characteristic pattern speed of the outer disk versus the bar strength, and the normalized pattern speed versus R, the ratio of corotation radius to bar length. To provide guide-lines for our interpretation we used a recently published simulations, including disk and dark matter halo components. Our most striking conclusion is that bars with values of R < 1.4, previously considered dynamically fast rotators, can be among the slowest rotators both in absolute terms and when their pattern speeds are normalized. The simulations confirm that this is because as the bars are braked they can grow longer more quickly than the outward drift of the corotation radius. We conclude that dark matter halos have indeed slowed down the rotation of bars on Gyr timescales.