The NewHorizon Simulation -- To Bar Or Not To Bar

TL;DR

This study uses the NewHorizon simulation to analyze the evolution of galactic bars from redshift 1.3 to 0.25, revealing a very low bar fraction and highlighting challenges in simulating bar formation.

Contribution

It provides the first detailed analysis of bar fractions over redshift in cosmological simulations, identifying limitations in current models for bar formation in low-mass galaxies.

Findings

Bar fraction decreases from ~7% at z=1.3 to ~1% at z=0.25.

Only one galaxy is confirmed as strongly barred, hosted by the most massive disk.

Low bar fractions suggest dark matter dominance and bulge formation inhibit bar persistence.

Abstract

We use the NewHorizon simulation to study the redshift evolution of bar properties and fractions within galaxies in the stellar masses range $M_{\star} = 10^{7.25} - 10^{11.4} \ \rm{M}_{\odot}$ over the redshift range $z = 0.25 - 1.3$. We select disc galaxies using stellar kinematics as a proxy for galaxy morphology. We employ two different automated bar detection methods, coupled with visual inspection, resulting in observable bar fractions of $f_{\rm bar} = 0.070_{{-0.012}}^{{+0.018}}$ at $z\sim$ 1.3, decreasing to $f_{\rm bar} = 0.011_{{-0.003}}^{{+0.014}}$ at $z\sim$ 0.25. Only one galaxy is visually confirmed as strongly barred in our sample. This bar is hosted by the most massive disk and only survives from $z=1.3$ down to $z=0.7$. Such a low bar fraction, in particular amongst Milky Way-like progenitors, highlights a missing bars problem, shared by literally all cosmological simulations with spatial resolution $<$100 pc to date. The analysis of linear growth rates, rotation curves and derived summary statistics of the stellar, gas and dark matter components suggest that galaxies with stellar masses below $10^{9.5}-10^{10} \ \rm{M}_{\odot}$ in NewHorizon appear to be too dominated by dark matter to form a bar, while more massive galaxies typically have formed large bulges that prevent bar persistence at low redshift. This investigation confirms that the evolution of the bar fraction puts stringent constraints on the assembly history of baryons and dark matter onto galaxies.