Subarcsecond international LOFAR radio images of the M82 nucleus at 118
  MHz and 154 MHz
  The nuclear starburst in the nearby galaxy M82 provides an excellent
laboratory for understanding the physics of star formation. This galaxy has
been extensively observed in the past, revealing tens of radio-bright compact
objects embedded in a diffuse free-free absorbing medium. Our understanding of
the structure and physics of this medium in M82 can be greatly improved by
high-resolution images at low frequencies where the effects of free-free
absorption are most prominent.
  The aims of this study are, firstly, to demonstrate imaging using
international baselines of the Low Frequency Array (LOFAR), and secondly, to
constrain low-frequency spectra of compact and diffuse emission in the central
starburst region of M82 via high-resolution radio imaging at low frequencies.
  The international LOFAR telescope was used to observe M82 at 110-126MHz and
146-162MHz. Images were obtained using standard techniques from very long
baseline interferometry. images were obtained at each frequency range: one only
using international baselines, and one only using the longest Dutch (remote)
baselines.
  The 154MHz image obtained using international baselines is a new imaging
record in terms of combined image resolution (0.3$"$) and sensitivity
($\sigma$=0.15mJy/beam) at low frequencies ($<327$MHz). We detected 16 objects
at 154MHz, six of these also at 118MHz. Four weaker but resolved features are
also found: a linear (50pc) filament and three other resolved objects, of which
two show a clear shell structure. We do not detect any emission from either
supernova 2008iz or from the radio transient source 43.78+59.3. The images
obtained using remote baselines show diffuse emission, associated with the
outflow in M82, with reduced brightness in the region of the edge-on
star-forming disk.
