VLBI observations of the H2O gigamaser in TXS 2226-184

Abstract

Context. 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 TXS 2226-184, the term gigamaser was used for the first time. However, the origin of this very luminous H2O maser emission has never been investigated in detail. Aims. We study the nature of the 22 GHz H2O gigamaser in TXS 2226-184 by measuring its absolute position for the first time at milliarcsecond resolution, by comparing the morphology and characteristics of the maser emission on very long baseline interferometry (VLBI) scales after about 20 years, and by trying to detect its polarized emission. Methods. We observed the 22 GHz H2O maser emission toward TXS 2226-184 three times: the very first time with the NRAO Very Long Baseline Array (VLBA, epoch 2017.45) and the next two times with the European VLBI Network (EVN, epochs 2017.83 and 2018.44). The first two epochs (2017.45 and 2017.83) were observed in phase-reference mode, while the last epoch (2018.44) 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 TXS 2226-184 observed in epoch 1998.40. Results. We detected six H2O maser features in epoch 2017.45 (VLBA), one in epoch 2017.83 (EVN), and two in epoch 2018.44 (EVN). All but one are red-shifted with respect to the systemic velocity of TXS 2226-184; we detected only one blue-shifted maser feature and it is the weakest one. All the H2O maser features but the blue-shifted one are composed of two components with very different linewidths. For the first time we were able to measure the absolute position of the H2O maser features with errors below 1 milliarcsec. No linear and circular polarization was detected. Conclusions. We were able to associate the H2O maser features in TXS 2226-184 with the most luminous radio continuum clump reported in the literature. The comparison between epochs 1998.40 and 2017.45 reveals a difference in the morphology and velocity of the maser features that can be justified accounting for maser variability.