Hidden or missing outflows in highly obscured galaxy nuclei?
  Understanding the nuclear growth and feedback processes in galaxies requires
investigating their often obscured central regions. One way to do this is to
use (sub)millimeter line emission from vibrationally excited HCN (HCN-vib). It
has been suggested that the most intense HCN-vib emission from a galaxy is
connected to a phase of nuclear growth that occurs before the nuclear feedback
processes have been fully developed. We aim to investigate if there is a
connection between the presence of strong HCN-vib emission and the development
of feedback in (U)LIRGs. We collected literature and archival data to compare
the luminosities of rotational lines of HCN-vib, normalized to the total
infrared luminosity, to the median velocities of 119 {\mu}m OH absorption
lines, potentially indicating outflows, in a total of 17 (U)LIRGs. The most
HCN-vib luminous systems all lack signatures of significant molecular outflows
in the far-infrared OH absorption lines. However, at least some of the systems
with bright HCN-vib emission do have fast and collimated outflows that can be
seen in spectral lines at longer wavelengths. We conclude that the galaxy
nuclei with the highest L(HCN-vib)/L(IR) do not drive wide-angle outflows
detectable using the median velocities of far-infrared OH absorption lines. It
is possible that this is due to an orientation effect where sources which are
oriented in such a way that their outflows are not along our line of sight also
radiate a smaller proportion of their infrared luminosity in our direction. It
could also be that massive wide-angle outflows destroy the deeply embedded
regions responsible for bright HCN-vib emission, so that the two phenomena
cannot coexist. This would strengthen the idea that vibrationally excited HCN
traces a heavily obscured stage of evolution before nuclear feedback mechanisms
are fully developed.
