Subarcsecond LOFAR imaging of Arp299 at 150 MHz. Tracing the nuclear and
  diffuse extended emission of a bright LIRG
  We study for the first time the low-frequency ($\sim$150 MHz) radio
brightness distribution of Arp~299 at subarcsecond resolution, tracing in both
compact and extended emission regions the local spectral energy distribution
(SED) in order to characterize the dominant emission and absorption processes.
We analysed the spatially resolved emission of Arp 299 revealed by 150 MHz
international baseline Low-Frequency Array (LOFAR) and 1.4, 5.0, and 8.4 GHz
Very Large Array (VLA) observations. We present the first subarcsecond
(0.4"$\sim$100~pc) image of the whole Arp~299 system at 150~MHz. The high
surface brightness sensitivity of our LOFAR observations ($\sim$100
$\mu$Jy/beam) allowed us to detect all of the nuclear components detected at
higher frequencies, as well as the extended steep-spectrum emission surrounding
the nuclei. We obtained spatially resolved, two-point spectral index maps for
the whole galaxy: the compact nuclei show relatively flat spectra, while the
extended, diffuse component shows a steep spectrum. We fitted the radio SED of
the nuclear regions using two different models: a continuous free-free medium
model and a clumpy model. The continuous model can explain the SED of the
nuclei assuming a population of relativistic electrons subjected to
synchrotron, bremsstrahlung, and ionization losses. The clumpy model fits
assuming relativistic electrons with negligible energy losses, and thermal
fractions that are more typical of star-forming galaxies than those required
for the continuous model. Our results confirm the usefulness of combining
spatially resolved radio imaging at both MHz and GHz frequencies to
characterize in detail the radio emission properties of LIRGs from the central
100 pc out to the kiloparsec galaxy-wide scales.
