OH absorption in the first quadrant of the Milky Way as seen by THOR
  The hydroxyl radical (OH) is present in the diffuse molecular and partially
atomic phases of the interstellar medium (ISM), but its abundance relative to
hydrogen is not clear. We aim to evaluate the abundance of OH with respect to
molecular hydrogen using OH absorption against cm-continuum sources over the
first Galactic quadrant. This OH study is part of the HI/OH/Recombination line
survey (THOR). THOR is a Karl G. Jansky Very Large Array large program of
atomic, molecular and ionized gas in the range 15{\deg}$\leq$l$\leq$67{\deg}
and |b|$\leq$1{\deg}. It is the highest-resolution unbiased OH absorption
survey to date towards this region. We combine the derived optical depths with
literature 13CO(1-0) and HI observations to determine the OH abundance. We
detect absorption in the 1665 and 1667 MHz transitions for continuum sources
stronger than $F_{\rm cont}\geq$0.1 Jy/beam. OH absorption is found against
$\sim$15% of these continuum sources with increasing fractions for stronger
sources. Most of the absorption is associated with Galactic HII regions. We
find OH and 13CO gas to have similar kinematic properties. The OH abundance
decreases with increasing hydrogen column density. The OH abundance with
respect to the total hydrogen nuclei column density (atomic and molecular
phase) is in agreement with a constant abundance for $A_V$ < 10-20. Towards the
lowest column densities, we find sources that exhibit OH absorption but no 13CO
emission, indicating that OH is a well suited tracer of the low column density
molecular gas. We present spatially resolved OH absorption towards W43. The
unbiased nature of the THOR survey opens a new window onto the gas properties
of the ISM. The characterization of the OH abundance over a large range of
hydrogen gas column densities contributes to the understanding of OH as a
molecular gas tracer and provides a starting point for future investigations.
