Subarcsecond international LOFAR radio images of Arp 220 at 150 MHz: A
  kpc-scale star forming disk surrounding nuclei with shocked outflows
  We analyse new observations with the International Low Frequency Array
(LOFAR) telescope, and archival data from the Multi-Element Radio Linked
Interferometer 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 33
GHz. We present an image of Arp 220 at 150 MHz with resolution
$0.65''\times0.35''$, sensitivity 0.15 mJy beam$^{-1}$, and integrated flux
density $394\pm59$ mJy. More than 80% of the detected flux comes from extended
($6''\approx$2.2 kpc) steep spectrum ($\alpha=-0.7$) emission, likely from star
formation in the molecular disk surrounding the two nuclei. We find elongated
features extending $0.3''$ (110 pc) and $0.9''$ (330 pc) from the eastern and
western nucleus respectively, which we interpret as evidence for outflows. The
extent of radio emission requires acceleration of cosmic rays far outside the
nuclei. We find that a simple three component model can explain most of the
observed radio spectrum of the galaxy. When accounting for absorption at 1.4
GHz, Arp 220 follows the FIR/radio correlation with $q=2.36$, and we estimate a
star formation rate of 220 M$_\odot\text{yr}^{-1}$. We derive thermal fractions
at 1 GHz of less than 1% for the nuclei, which indicates that a major part of
the UV-photons are absorbed by dust. International LOFAR observations shows
great promise to detect steep spectrum outflows and probe regions of thermal
absorption. However, in LIRGs the emission detected at 150 MHz does not
necessarily come from the main regions of star formation. This implies that
high spatial resolution is crucial for accurate estimates of star formation
rates for such galaxies at 150 MHz.
