Local Simulations of Spiral Galaxies with the TIGRESS Framework: I. Star Formation and Arm Spurs/Feathers

TL;DR

This study uses local 3D simulations with the TIGRESS framework to examine how spiral arms influence star formation and the formation of arm spurs/feathers, revealing that spiral arms concentrate star formation but do not significantly trigger it.

Contribution

First detailed local simulations of spiral galaxy arms showing their role in star formation concentration and spur/feather formation using the TIGRESS framework.

Findings

Over 90% of star formation occurs in spiral arms.

Spiral arms enhance the global star formation rate by less than a factor of 2.

Spurs/feathers form downstream from arms with magnetic fields aligned along their length.

Abstract

Spiral arms greatly affect gas flows and star formation in disk galaxies. We use local three-dimensional simulations of the vertically-stratified, self-gravitating, differentially-rotating, interstellar medium (ISM) subject to a stellar spiral potential to study the effects of spiral arms on star formation and formation of arm spurs/feathers. We adopt the TIGRESS framework of Kim & Ostriker (2017) to handle radiative heating and cooling, star formation, and ensuing supernova (SN) feedback. We find that more than 90% of star formation takes place in spiral arms, but the global star formation rate (SFR) in models with spiral arms is enhanced by less than a factor of 2 compared to the no-arm counterpart. This results from a quasi-linear relationship between the SFR surface density Sigma_SFR and the gas surface density Sigma, and supports the picture that spiral arms do not trigger star formation but rather concentrate star-forming regions. Correlated SN feedback produces gaseous spurs/feathers downstream from arms in both magnetized and unmagnetized models. These spurs/feathers are short-lived and have magnetic fields parallel to their length, in contrast to the longer-lived features with perpendicular magnetic fields induced by gravitational instability. SN feedback drives the turbulent component of magnetic fields, with the total magnetic field strength sublinearly proportional to Sigma. The total midplane pressure varies by a factor of ~10 between arm and interarm regions but agrees locally with the total vertical ISM weight, while Sigma_SFR is locally consistent with the prediction of pressure-regulated, feedback-modulated theory.