The SEDIGISM survey: first data release and overview of the Galactic
  structure
  The SEDIGISM (Structure, Excitation and Dynamics of the Inner Galactic
Interstellar Medium) survey used the APEX telescope to map 84 deg^2 of the
Galactic plane between l = -60 deg and l = +31 deg in several molecular
transitions, including 13CO(2-1) and C18O(2-1), thus probing the moderately
dense (~10^3 cm^-3) component of the interstellar medium. With an angular
resolution of 30'' and a typical 1-sigma sensitivity of 0.8-1.0 K at 0.25 km/s
velocity resolution, it gives access to a wide range of structures, from
individual star-forming clumps to giant molecular clouds and complexes. The
coverage includes a good fraction of the first and fourth Galactic quadrants,
allowing us to constrain the large scale distribution of cold molecular gas in
the inner Galaxy. In this paper we provide an updated overview of the full
survey and the data reduction procedures used. We also assess the quality of
these data and describe the data products that are being made publicly
available as part of this first data release (DR1). We present integrated maps
and position-velocity maps of the molecular gas and use these to investigate
the correlation between the molecular gas and the large scale structural
features of the Milky Way such as the spiral arms, Galactic bar and Galactic
centre. We find that approximately 60 per cent of the molecular gas is
associated with the spiral arms and these appear as strong intensity peaks in
the derived Galactocentric distribution. We also find strong peaks in intensity
at specific longitudes that correspond to the Galactic centre and well known
star forming complexes, revealing that the 13CO emission is concentrated in a
small number of complexes rather than evenly distributed along spiral arms.
