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We examine the effects of stellar feedback and bursty star formation onlow-mass galaxies ($M_{\rm star}=2\times10^6-5\times10^{10}{\rm M_{\odot}}$)using the FIRE (Feedback in Realistic Environments) simulations. While previousstudies emphasized the impact of feedback on dark matter profiles, weinvestigate the impact on the stellar component: kinematics, radial migration,size evolution, and population gradients. Feedback-driven outflows/inflowsdrive significant radial stellar migration over both short and long timescalesvia two processes: (1) outflowing/infalling gas can remain star-forming,producing young stars that migrate $\sim1{\rm\,kpc}$ within their first $100{\rm\,Myr}$, and (2) gas outflows/inflows drive strong fluctuations in theglobal potential, transferring energy to all stars. These processes produceseveral dramatic effects. First, galaxies' effective radii can fluctuate byfactors of $>2$ over $\sim200 {\rm\,Myr}$, and these rapid size fluctuationscan account for much of the observed scatter in radius at fixed $M_{\rm star}.$Second, the cumulative effects of many outflow/infall episodes steadily heatstellar orbits, causing old stars to migrate outward most strongly. Thisage-dependent radial migration mixes---and even inverts---intrinsic age andmetallicity gradients. Thus, the galactic-archaeology approach of calculatingradial star-formation histories from stellar populations at $z=0$ can beseverely biased. These effects are strongest at $M_{\rmstar}\approx10^{7-9.6}{\rm M_{\odot}}$, the same regime where feedback mostefficiently cores galaxies. Thus, detailed measurements of stellar kinematicsin low-mass galaxies can strongly constrain feedback models and test baryonicsolutions to small-scale problems in $\Lambda$CDM.

Breathing FIRE: How Stellar Feedback Drives Radial Migration, Rapid Size Fluctuations, and Population Gradients in Low-Mass Galaxies