The HI/OH/Recombination line survey of the inner Milky Way (THOR)
  Context: The past decade has witnessed a large number of Galactic plane
surveys at angular resolutions below 20". However, no comparable
high-resolution survey exists at long radio wavelengths around 21cm in line and
continuum emission. Methods: Employing the Very Large Array (VLA) in the
C-array configuration and a large program, we observe the HI 21cm line, four OH
lines, nineteen Halpha radio recombination lines as well as the continuum
emission from 1 to 2GHz in full polarization over a large part of the first
Galactic quadrant. Results: Covering Galactic longitudes from 14.5 to 67.4deg
and latitudes between +-1.25deg, we image all of these lines and the continuum
at ~20" resolution. These data allow us to study the various components of the
interstellar medium (ISM): from the atomic phase, traced by the HI line, to the
molecular phase, observed by the OH transitions, to the ionized medium,
revealed by the cm continuum and the Halpha radio recombination lines.
Furthermore, the polarized continuum emission enables magnetic field studies.
In this overview paper, we discuss the survey outline and present the first
data release as well as early results from the different datasets. We now
release the first half of the survey; the second half will follow later after
the ongoing data processing has been completed. The data in fits format
(continuum images and line data cubes) can be accessed through the project
web-page http://www.mpia.de/thor. Conclusions: The HI/OH/Recombination line
survey of the Milky Way (THOR) opens a new window to the different parts of the
ISM. It enables detailed studies of molecular cloud formation, conversion of
atomic to molecular gas, and feedback from HII regions as well as the magnetic
field in the Milky Way. It is highly complementary to other surveys of our
Galaxy, and comparing different datasets allows us to address many open
questions.
