VLBI observations of the H2O gigamaser in TXS2226-184
  Outside the Milky Way, the most luminous H2O masers at 22 GHz, called
'megamasers' because of their extreme luminosity with respect to the Galactic
and extragalactic H2O masers associated with star formation, are mainly
detected in active galactic nuclei. In the case of the H2O maser detected in
the nuclear region of the galaxy TXS2226-184 for the first time the term
'gigamaser' was used. However, the origin of this very luminous H2O maser
emission has never been investigated into details. We study the nature of the
H2O gigamaser in TXS2226-184 by measuring for the first time its absolute
position at mas resolution, by comparing the morphology and characteristics of
the maser emission at the VLBI scales after about 20 years, and by trying to
detect its polarized emission. We observed the maser emission towards
TXS2226-184 three times: the very first one with the VLBA (epoch 2017.45) and
the next two times with the EVN (epochs 2017.83 and 2018.44). The first two
epochs were observed in phase-reference mode, while the last epoch was observed
in full-polarization mode but not in phase-reference mode to increase the
on-source integration time. We also retrieved and analyzed the VLBA archival
data at 22 GHz of TXS2226-184 observed in 1998.40. We detected 6 H2O maser
features in epoch 2017.45 (VLBA), one in epoch 2017.83 (EVN), and two in epoch
2018.44 (EVN). All of them but one are red-shifted with respect to the systemic
velocity of TXS2226-184, we detected only one blue-shifted maser feature and it
is the weakest one. For the first time, we were able to measure the absolute
position of the H2O maser features with errors below 1 mas. No linear and
circular polarization was detected. We were able to associate the H2O maser
features in TXS2226-184 with the most luminous radio continuum clump reported
in the literature.
