[Paper Review] Water masers in Compton-thick AGN I. Detailed study of the new water megamaser in IRAS15480-0344

P. Castangia, A. Tarchi+3 more|arXiv (Cornell University)|Jan 13, 2016

Galaxies: Formation, Evolution, Phenomena49 references3 citations

TL;DR

This study presents the first detailed analysis of a new 22 GHz water megamaser in the Compton-thick S0 galaxy IRAS15480-0344, detected via multi-epoch single-dish and VLBI observations. The maser exhibits a broad component (FWHM ~90 km s⁻¹) and a narrow component (FWHM <1 km s⁻¹), with the narrow feature spatially coincident with the nuclear radio continuum but not associated with compact radio cores, suggesting a jet/outflow origin for the narrow component and a jet-cloud interaction origin for the broad component.

ABSTRACT

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.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 matter of a forthcoming publication, a new luminous water maser has been detected in the lenticular (field) S0 galaxy IRAS15480-0344, whose origin, associated with an accretion disc or a nuclear outflow/jet, needs to be assessed. 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. The new detection in IRAS15480-0344 is reported: a megamaser with a total single-dish isotropic luminosity of ~200 Lsun and a profile composed of two main features, a broad line with a full width to half maximum (FWHM) linewidth of ~90 km/s and a narrow (FWHM&lt;1 km/s) 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. [Abridged]

Motivation & Objective

• To investigate the relationship between water maser detection and high nuclear column density in Compton-thick AGN.
• To determine the origin of a newly detected luminous water megamaser in the S0 galaxy IRAS15480-0344.
• To constrain the kinematics and spatial distribution of maser emission using multi-epoch single-dish and VLBI observations.
• To assess whether the maser emission arises from an accretion disc, a nuclear outflow, or a jet-cloud interaction.
• To evaluate the role of nuclear outflows in S0 galaxies in facilitating maser action despite low gas content.

Proposed method

• Conducted multi-epoch single-dish observations with the Green Bank Telescope (GBT) to monitor the spectral profile and variability of the maser emission.
• Performed Very Long Baseline Array (VLBA) interferometric observations to determine the spatial distribution and precise positions of maser spots.
• Analyzed the single-dish spectrum to identify two main features: a narrow line near systemic velocity and a broad blueshifted line.
• Cross-correlated maser positions with 8.4 GHz nuclear radio continuum emission from the Very Large Array (VLA) to assess spatial associations.
• Used the maser line width and velocity variability to infer dynamical conditions and potential physical origins.
• Evaluated the binding mass of the broad component using a Keplerian model assuming a 15 pc radius and 110 km s⁻¹ rotation velocity.

Experimental results

Research questions

• RQ1 What is the origin of the broad and narrow water maser components detected in IRAS15480-0344?
• RQ2 Is the maser emission associated with the accretion disc, a nuclear outflow, or a jet-cloud interaction?
• RQ3 How does the spatial distribution of the maser spots relate to the nuclear radio continuum and compact radio sources?
• RQ4 Can the kinematic properties of the maser lines constrain the physical conditions in the innermost regions of the AGN?
• RQ5 Why is the maser detection rate in S0 galaxies non-negligible despite their typically low gas content?

Key findings

• A luminous water megamaser with an isotropic luminosity of ~200 L⊙ was detected in IRAS15480-0344, a Compton-thick S0 galaxy at 130 Mpc.
• The single-dish spectrum revealed two main features: a narrow component with FWHM <1 km s⁻¹ near systemic velocity and a broad component with FWHM ~90 km s⁻¹ blueshifted from systemic.
• VLBI observations detected only the narrow maser component, which is spatially coincident with the nuclear 8.4 GHz radio continuum emission but not with any compact radio core.
• A weak narrow feature was detected 15 pc northwest of the main narrow component, with no associated compact radio source.
• The broad component's large linewidth and strong spectral variability suggest an origin in jet-cloud interactions, while the narrow component's low velocity dispersion supports an outflow origin.
• A Keplerian model implies a binding mass of ~5×10⁷ M⊙ for the broad component, consistent with black hole masses derived from disc-maser studies, though a disc origin cannot be fully excluded.