Water masers in Compton-thick AGN I. Detailed study of the new water
  megamaser in IRAS15480-0344
  A relationship between the water maser detection rate and large nuclear
column densities in AGN has often been cited in the literature. Indeed,
detailed studies of luminous water masers, typically associated with the
nuclear activity, allow us to investigate the innermost regions of AGN, with an
impact on the still debated Unified Model for this class of objects.We have
recently entertained a search for maser emission in a well-defined sample of
Compton-thick AGN aimed at investigating, on firm statistical bases, the
aforementioned relationship. While the survey is still ongoing, and is matter
of a forthcoming publication, a new luminous water maser has been detected in
the lenticular (field) S0 galaxy IRAS15480-0344, whose origin, associated with
an accretion disc or a nuclear outflow/jet, needs to be assessed. Multi-epoch
single-dish observations and VLBI measurements were performed to investigate
the distribution, spatial extension, and variability of the maser emission in
order to infer the main characteristics of the water megamaser. The new
detection in IRAS15480-0344 is reported: a megamaser with a total single-dish
isotropic luminosity of ~200 Lsun and a profile composed of two main features,
a broad line with a full width to half maximum (FWHM) linewidth of ~90 km/s and
a narrow (FWHM<1 km/s) one. We performed a follow-up to the detection with the
Very Long Baseline Array (VLBA) and confidently detected only the narrow
component, which is coincident with the nuclear radio continuum emission
detected with the Very Large Array at 8.4 GHz. A weak narrow feature has also
been detected in the velocity range of the broad feature and is located 15 pc
to the north-west with respect to the stronger component. Neither maser spot is
associated with the compact radio continuum sources derived from the same VLBA
dataset. [Abridged]
