Molecular gas and star formation in nearby starburst galaxy mergers

TL;DR

This study uses FIRE-2 simulations to analyze how giant molecular clouds evolve during galaxy mergers, revealing increased turbulence, less gravitational binding, and altered star formation efficiencies in starburst conditions.

Contribution

It provides new insights into GMC property changes during galaxy mergers, highlighting the role of stellar feedback and deviations from simple gravitational collapse models.

Findings

GMCs in mergers show 5-10 times higher surface density and velocity dispersion.

GMC virial parameters increase to 10-100 during starbursts.

Star formation rate correlates with increased virial parameter, indicating feedback-driven gas dispersal.

Abstract

We employ the Feedback In Realistic Environments (FIRE-2) physics model to study how the properties of giant molecular clouds (GMCs) evolve during galaxy mergers. We conduct a pixel-by-pixel analysis of molecular gas properties in both the simulated control galaxies and galaxy major mergers. The simulated GMC-pixels in the control galaxies follow a similar trend in a diagram of velocity dispersion ($\sigma_v$) versus gas surface density ($\Sigma_{\mathrm{mol}}$) to the one observed in local spiral galaxies in the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. For GMC-pixels in simulated mergers, we see a significant increase of factor of 5 - 10 in both $\Sigma_{\mathrm{mol}}$ and $\sigma_v$, which puts these pixels above the trend of PHANGS galaxies in the $\sigma_v$ vs $\Sigma_{\mathrm{mol}}$ diagram. This deviation may indicate that GMCs in the simulated mergers are much less gravitationally bound compared with simulated control galaxies with virial parameter ($\alpha_{\mathrm{vir}}$) reaching 10 - 100. Furthermore, we find that the increase in $\alpha_{\mathrm{vir}}$ happens at the same time as the increase in global star formation rate (SFR), which suggests stellar feedback is responsible for dispersing the gas. We also find that the gas depletion time is significantly lower for high $\alpha_{\mathrm{vir}}$ GMCs during a starburst event. This is in contrast to the simple physical picture that low $\alpha_{\mathrm{vir}}$ GMCs are easier to collapse and form stars on shorter depletion times. This might suggest that some other physical mechanisms besides self-gravity are helping the GMCs in starbursting mergers collapse and form stars.