Water masers in Compton-thick AGN

Abstract

Context.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. Aims. 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 the subject of a forthcoming publication, a new luminous water maser has been detected in the lenticular (field) S0 galaxy IRAS 15480-0344, whose origin, associated with an accretion disc or a nuclear outflow/jet, needs to be assessed. Methods. 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. Results. The new detection in IRAS 15480-0344 is reported: a megamaser with a total single-dish isotropic luminosity of ∼200 L and a profile composed of two main features, a broad line with a full width to half maximum (FWHM) linewidth of ∼90 km s −1 and a narrow (FWHM < 1 km s −1 ) 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. Conclusions. The different line profiles and the spatial separation between the two features in the spectrum of IRAS 15480-0344 suggests a composite origin for the maser. The large linewidth and the strong spectral variability indicate that the broad component might originate from the interaction between a radio jet and ambient molecular clouds. Based on its small linewidth and the absence of high velocity features, instead, we favour an outflow origin for the narrow component. This scenario is consistent with the hypothesis of the presence of strong nuclear outflows recently invoked to explain the main characteristics of field S0 galaxies.

Subarcsecond imaging of the water emission in Arp 220

“…These so-called disk-masers are mostly found in Seyfert 2 and LINER galaxies (low ionization emission region, e.g., Kartje et al 1999; Kuo et al 2011, 2015). In other galaxies megamaser emission is caused by the interaction between the AGN jet and molecular clouds in the surrounding interstellar medium ( jet-masers , e.g., Mrk 348; Peck et al 2003; Castangia et al 2016). The maser lines in these sources are typically as featureless as what we find in Arp 220.…”

A new jet/outflow maser in the nucleus of the Compton-thick AGN IRAS 15480-0344

“…We re-observed some of the non-detected sources in order to improve their upper limit on the maser luminosity. These new observations lead to the discovery of a new luminous water maser in the lenticular (S0) galaxy IRAS 15480-0344 (hereafter IRAS15480) confirming the exceptionality of this sample (Castangia et al 2016). Indeed, with the detection of IRAS15480, the maser detection rate of the entire sample becomes 50%, making the CT AGN sample used here one of the most prolific samples for finding extragalactic water masers (Castangia et al in prep.).…”

“…The spectrum consists of two main features: a broad blueshifted component, with a full width at half maximum (FWHM) linewidth of ∼90 km s −1 and a narrow (FWHM<1 km s −1 ) one close to the systemic velocity of the galaxy ( Fig. 1; for details, see Castangia et al 2016). The total isotropic luminosity is ∼200 L .…”

“…Left panel: GBT spectrum of the H2 O maser emission in IRAS15480. The vertical lines mark the recessional velocity of the galaxy and the CO centroid velocity, 9084 and 9039 km s −1 , respectively(Castangia et al 2016). Right panel: Zoom of the narrow emission feature.…”

VLBI observations of the H2O gigamaser in TXS 2226-184

“…In NGC 1052, high resolution observations indicate that the maser clouds are most likely located foreground to the jet in a circumnuclear torus (or disk), thus amplifying the continuum seed emission from the jet knots (Sawada-Satoh et al 2008). Recently, the detection and interferometric followup with the Very Long Baseline Array (VLBA) of a megamaser in the lenticular S0 galaxy IRAS15480-0344 (130 Mpc distance) has been reported by Castangia et al (2016). Also in this case, an association of (part of) the H 2 O maser emission with a nuclear jet or outflow is proposed (Castangia et al 2019).…”