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We analyse new observations with the International Low Frequency Array(LOFAR) telescope, and archival data from the Multi-Element Radio LinkedInterferometer Network (MERLIN) and the Karl G. Jansky Very Large Array (VLA).We model the spatially resolved radio spectrum of Arp 220 from 150 MHz to 33GHz. We present an image of Arp 220 at 150 MHz with resolution$0.65''\times0.35''$, sensitivity 0.15 mJy beam$^{-1}$, and integrated fluxdensity $394\pm59$ mJy. More than 80% of the detected flux comes from extended($6''\approx$2.2 kpc) steep spectrum ($\alpha=-0.7$) emission, likely from starformation in the molecular disk surrounding the two nuclei. We find elongatedfeatures extending $0.3''$ (110 pc) and $0.9''$ (330 pc) from the eastern andwestern nucleus respectively, which we interpret as evidence for outflows. Theextent of radio emission requires acceleration of cosmic rays far outside thenuclei. We find that a simple three component model can explain most of theobserved radio spectrum of the galaxy. When accounting for absorption at 1.4GHz, Arp 220 follows the FIR/radio correlation with $q=2.36$, and we estimate astar formation rate of 220 M$_\odot\text{yr}^{-1}$. We derive thermal fractionsat 1 GHz of less than 1% for the nuclei, which indicates that a major part ofthe UV-photons are absorbed by dust. International LOFAR observations showsgreat promise to detect steep spectrum outflows and probe regions of thermalabsorption. However, in LIRGs the emission detected at 150 MHz does notnecessarily come from the main regions of star formation. This implies thathigh spatial resolution is crucial for accurate estimates of star formationrates for such galaxies at 150 MHz.

Subarcsecond international LOFAR radio images of Arp 220 at 150 MHz: A kpc-scale star forming disk surrounding nuclei with shocked outflows