Water masers in Compton-thick AGN

I. Detailed study of the new water megamaser in IRAS 15480-0344

¹ INAF–Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy
e-mail: pcastang@oa-cagliari.inaf.it
² INAF–Osservatorio Astronomico di Brera, via Brera 28, 20121 Milan, Italy

Received: 12 August 2015
Accepted: 19 December 2015

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.