  We investigate the star formation-feedback cycle in cosmological galaxy
formation simulations, focusing on progenitors of Milky Way (MW)-sized
galaxies. We find that in order to reproduce key properties of the MW
progenitors, such as semi-empirically derived star formation histories and the
shape of rotation curves, our implementation of star formation and stellar
feedback requires 1) a combination of local early momentum feedback via
radiation pressure and stellar winds and subsequent efficient supernovae
feedback, and 2) efficacy of feedback that results in self-regulation of the
global star formation rate on kiloparsec scales. We show that such
feedback-driven self-regulation is achieved globally for a local star formation
efficiency per free fall time of $\epsilon_{\rm ff}\approx 10\%$. Although this
value is larger that the $\epsilon_{\rm ff}\sim 1\%$ value usually inferred
from the Kennicutt-Schmidt (KS) relation, we show that it is consistent with
direct observational estimates of $\epsilon_{\rm ff}$ in molecular clouds.
Moreover, we show that simulations with local efficiency of $\epsilon_{\rm
ff}\approx 10\%$ reproduce the global observed KS relation. Such simulations
also reproduce the cosmic star formation history of the Milky Way sized
galaxies and satisfy a number of other observational constraints. Conversely,
we find that simulations that a priori assume an inefficient mode of star
formation, instead of achieving it via stellar feedback regulation, fail to
produce sufficiently vigorous outflows and do not reproduce observations. This
illustrates the importance of understanding the complex interplay between star
formation and feedback and the detailed processes that contribute to the
feedback-regulated formation of galaxies.
