  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.
