Barless flocculent galaxies: a dynamic puzzle

TL;DR

This study investigates why certain bright, barless galaxies with flocculent spiral structures lack bars, finding that low dark matter content and initial instability lead to their featureless, stable configurations.

Contribution

The paper demonstrates that low dark matter fractions and initial dynamical instability explain the absence of bars in certain flocculent galaxies, supported by kinematic data and N-body simulations.

Findings

Dark matter fraction within optical radii is less than 50%.

Initial instability in cold discs leads to featureless structures.

Barless galaxies may be descendants of high-redshift galaxies with high baryon fractions.

Abstract

We draw attention to the bright galaxies that do not show a bar in their structure but have a flocculent spiral structure. Using the THINGS' and HERACLES' kinematic data for four barless galaxies (NGC~2841, NGC~3512, NGC~5055, NGC~7331) we built their mass models including dark halos. We concluded that the fraction of the dark matter does not exceed 50\% within the optical radii of the galaxies. This is too little to explain the lack of a bar in these galaxies. In an attempt to understand the featureless structure of these galaxies we constructed several $N$-body models with an initially reduced content of dark matter. We concluded that, in addition to the low mass of the dark halo, the decisive factor that leads to a barless disc is the start from an initially unstable state. An isolated dynamically cold disc (with the Toomre parameter $Q < 0.5$) settled into rotational equilibrium passes trough the short stage of violent instability with fragmentation and formation of stellar clumps. After that, it evolves passively and ends up with a featureless structure. We assume that the barless flocculent galaxies studied in the present work may be descendants of galaxies at high redshifts with rotation curves which are consistent with the high mass fraction of baryons relative to the total dark matter halo.