VLBI observations of the H2O gigamaser in TXS2226-184

Abstract

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.

13 pages, 10 figures, 3 tables, accepted for publication by Astronomy & Astrophysics

References in corpus (7)

• The SFXC software correlator for Very Long Baseline Interferometry: Algorithms and Implementation
• A gravitationally lensed water maser in the early Universe
• The Megamaser Cosmology Project.IX. Black hole masses for three maser galaxies
• Positional Coincidence of H2O Maser and a Plasma Obscuring Torus in Radio Galaxy NGC 1052
• Green Bank Telescope observations of the water masers of NGC 3079: accretion disk magnetic field and maser scintillation
• Measuring Magnetic Fields from Water Masers in the Synchrotron Protostellar Jet in W3(H${}_2$O)
• The innermost region of the water megamaser radio galaxy 3C403

Cited by in corpus (4)

• A general relativistic estimation of the black hole mass-to-distance ratio at the core of TXS 2226-184
• Estimating the mass-to-distance ratio for a set of megamaser AGN black holes by employing a general relativistic method
• Very Long Baseline Interferometry imaging of H2O maser emission in the nearby radio galaxy NGC 4261
• Searching Water Megamasers By Using Mid-infrared Spectroscopy (I): Possible Mid-infrared Indicators