On the bar formation mechanism in galaxies with cuspy bulges

TL;DR

Numerical simulations demonstrate that Milky Way-like galaxies with cuspy bulges can develop bars through standard instability mechanisms, with bar properties consistent with observed features like the Hercules stream.

Contribution

This study shows that bar formation in galaxies with cuspy bulges can occur via classical instability, even with inner Lindblad resonances, and compares live versus rigid halo effects.

Findings

Bar formation occurs despite inner Lindblad resonance.

Bar pattern speed aligns with the Hercules stream observations.

Bar growth timescales suggest recent formation in Milky Way-like galaxies.

Abstract

We show by numerical simulations that a purely stellar dynamical model composed of an exponential disc, a cuspy bulge, and an NFW halo with parameters relevant to the Milky Way Galaxy is subject to bar formation. Taking into account the finite disc thickness, the bar formation can be explained by the usual bar instability, in spite of the presence of an inner Lindblad resonance, that is believed to damp any global modes. The effect of replacing the live halo and bulge by a fixed external axisymmetric potential (rigid models) is studied. It is shown that while the e-folding time of bar instability increases significantly (from 250 to 500 Myr), the bar pattern speed remains almost the same. For the latter, our average value of 55 km/s/kpc agrees with the assumption that the Hercules stream in the solar neighbourhood is an imprint of the bar--disc interaction at the outer Lindblad resonance of the bar. Vertical averaging of the radial force in the central disc region comparable to the characteristic scale length allows us to reproduce the bar pattern speed and the growth rate of the rigid models, using normal mode analysis of linear perturbation theory in a razor thin disc. The strong increase of the e-folding time with decreasing disc mass predicted by the mode analysis suggests that bars in galaxies similar to the Milky Way have formed only recently.