VLBI observations of the H₂O gigamaser in TXS 2226-184

Abstract

Context. Outside the Milky Way, the most luminous H 2 O masers at 22 GHz, called 'megamasers' because of their extreme luminosity with respect to the Galactic and extragalactic H 2 O masers associated with star formation, are mainly detected in active galactic nuclei. In the case of the H 2 O maser detected in the nuclear region of the galaxy TXS 2226-184 for the first time the term 'gigamaser' was used. However, the origin of this very luminous H 2 O maser emission has never been investigated into details. Aims. We study the nature of the 22 GHz H 2 O gigamaser in TXS 2226-184 by measuring for the first time its absolute position at milliarcsecond resolution, by comparing the morphology and characteristics of the maser emission at the very long baseline interferometry (VLBI) scales after about 20 years, and by trying to detect its polarized emission. Methods. We observed the 22 GHz H 2 O maser emission towards TXS 2226-184 three times: the very first one 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 Very Long Baseline Array archival data at 22 GHz of TXS 2226-184 observed in 1998.40. Results. We detected six H 2 O 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 TXS 2226-184, we detected only one blue-shifted maser feature and it is the weakest one. All the H 2 O 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 H 2 O maser features with errors below 1 milliarcsecond. No linear and circular polarization was detected. Conclusions. We were able to associate the H 2 O maser features in TXS 2226-184 with the most luminous radio continuum clump reported in the literature. The comparison between the 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.

Searching Water Megamasers By Using Mid-infrared Spectroscopy (I): Possible Mid-infrared Indicators

“…Jet/outflow masers are useful to derive jet/outflows geometry as well as jet material speed (e.g. Castangia et al 2019;Surcis et al 2020). One of the major challenges for the utility of water masers is the small sample size currently available.…”

“…jet/outflow), which are not associated to accretion disks, constitute another interesting group for AGN sciences (e.g. Tarchi 2012 and references therein; Surcis et al 2020).…”

A general relativistic estimation of the black hole mass-to-distance ratio at the core of TXS 2226–184

“…The isotropic luminosity associated with this water maser is so large, L = 6100 ± 900 L (Koekemoer et al 1995), that it is called a "gigamaser". The distance to TXS 2226-184 is D = 103.8571 ± 0.2606 Mpc 3 as a result of applying Hubble's law to the reported systemic velocity V lsr,radio = 7270 ± 18.24 km s −1 in Taylor et al (2002) and Surcis et al (2020) and assuming H 0 = 70 km s −1 Mpc −1 Kuo et al (2018, these authors did not report uncertainties taken into account for the Hubble constant value). TXS 2226-184 has been optically classified as an elliptical galaxy (Koekemoer et al 1995).…”

“…In Surcis et al (2020), the authors made new observations of the H 2 O gigamaser in TXS 2226-184 with the VLBA (one epoch) and the EVN (two epochs). The authors detected six maser features in epoch 2017.45 (VLBA), one in epoch 2017.83 (EVN), and two in epoch 2018.44 (EVN).…”

“…So far, no estimate of the mass of central BHs in galaxies hosting a gigamaser has been made, leaving the question open of whether the high luminosity of the maser is related to a central BH mass of magnitude greater than 10 6 −10 7 M . New observations of the gigamaser made with the VLBA have been published in Surcis et al (2020) and, in principle, they give us the opportunity to investigate whether there is a connection between its intense luminosity and the mass of the central BH.…”