Kiloparsec-Scale Simulations of Star Formation in Disk Galaxies. I. The unmagnetized and zero-feedback limit

TL;DR

This study uses hydrodynamic simulations to explore how dense gas structures form in galactic disks, highlighting the roles of gravity, turbulence, and shear without magnetic fields or feedback, and their impact on star formation rates.

Contribution

It provides the first detailed kiloparsec-scale simulations of dense gas evolution in galactic disks without magnetic fields or stellar feedback, emphasizing the importance of these factors in star formation regulation.

Findings

GMC virial parameters stay above unity over multiple free-fall times.

Star formation rates are much higher than observed due to overproduction of dense clumps.

Magnetic support and feedback are likely necessary to match observed star formation efficiencies.

Abstract

We present hydrodynamic simulations of the evolution of self-gravitating dense gas on scales of 1 kiloparsec down to < parsec in a galactic disk, designed to study dense clump formation from giant molecular clouds (GMCs). These structures are expected to be the precursors to star clusters and this process may be the rate limiting step controling star formation rates in galactic systems as described by the Kennicutt-Schmidt relation. We follow the thermal evolution of the gas down to ~5K using extinction-dependent heating and cooling functions. We do not yet include magnetic fields or localized stellar feedback, so the evolution of the GMCs and clumps is determined solely by self-gravity balanced by thermal and turbulent pressure support and the large scale galactic shear. While cloud structures and densities change significantly during the simulation, GMC virial parameters remain mostly above unity for time scales exceeding the free-fall time of GMCs indicating that energy from galactic shear and large-scale cloud motions continuously cascades down to and within the GMCs. We implement star formation at a slow, inefficient rate of 2% per local free-fall time, but even this yields global star formation rates that are about two orders of magnitude larger than the observed Kennicutt-Schmidt relation due to over-production of dense gas clumps. We expect a combination of magnetic support and localized stellar feedback is required to inhibit dense clump formation to ~1% of the rate that results from the nonmagnetic, zero-feedback limit.