Inflowing gas onto a compact obscured nucleus in Arp 299A: Herschel
  spectroscopic studies of H2O and OH
  Aims. We probe the physical conditions in the core of Arp 299A and try to put
constraints to the nature of its nuclear power source. Methods. We used
Herschel Space Observatory far-infrared and submillimeter observations of H2O
and OH rotational lines in Arp 299A to create a multi-component model of the
galaxy. In doing this, we employed a spherically symmetric radiative transfer
code. Results. Nine H2O lines in absorption and eight in emission as well as
four OH doublets in absorption and one in emission, are detected in Arp 299A.
No lines of the 18O isotopologues, which have been seen in compact obscured
nuclei of other galaxies, are detected. The absorption in the ground state OH
doublet at 119 {\mu}m is found redshifted by ~175 km/s compared to other OH and
H2O lines, suggesting a low excitation inflow. We find that at least two
components are required in order to account for the excited molecular line
spectrum. The inner component has a radius of 20-25 pc, a very high infrared
surface brightness (> 3e13 Lsun/kpc^2), warm dust (Td > 90 K), and a large H2
column density (NH2 > 1e24 cm^-2). The outer component is larger (50-100 pc)
with slightly cooler dust (70-90 K). In addition, a much more extended
inflowing component is required to also account for the OH doublet at 119
{\mu}m. Conclusions. The Compton-thick nature of the core makes it difficult to
determine the nature of the buried power source, but the high surface
brightness indicates that it is either an active galactic nucleus and/or a
dense nuclear starburst. The high OH/H2O ratio in the nucleus indicates that
ion-neutral chemistry induced by X-rays or cosmic-rays is important. Finally we
find a lower limit to the 16O/18O ratio of 400 in the nuclear region, possibly
indicating that the nuclear starburst is in an early evolutionary stage, or
that it is fed through a molecular inflow of, at most, solar metallicity.
